Compendium 12
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Questions and Answers

What is the process of cytokinesis and when does it occur during cell division?

Cytokinesis is the process of dividing the cytoplasm to form two distinct daughter cells, which begins during anaphase and continues through telophase.

Describe the role of centrioles in mitosis.

Centrioles are involved in organizing microtubules during mitosis, helping to form the mitotic spindle that separates chromosomes.

Explain how chromatin and chromosomes are related in the context of the cell cycle.

Chromatin is the loose, uncondensed form of DNA found during interphase, which condenses into chromosomes during mitosis.

How do skin cells demonstrate the importance of mitosis in tissue regeneration?

<p>Skin cells undergo constant mitosis to regenerate from the basal layer, facilitating repair and replacement in response to abrasion or injury.</p> Signup and view all the answers

Outline the stages of mitosis and briefly describe what happens in each stage.

<p>Mitosis involves prophase (chromosomes condense), metaphase (chromosomes align), anaphase (chromatids separate), and telophase (nuclei reform).</p> Signup and view all the answers

What is the significance of crossing over during meiosis?

<p>Crossing over during meiosis enhances genetic diversity by allowing homologous chromosomes to exchange DNA segments.</p> Signup and view all the answers

Describe the outcome of meiosis compared to mitosis in terms of the number and genetic makeup of daughter cells.

<p>Meiosis produces four genetically different haploid gametes, while mitosis produces two genetically identical diploid daughter cells.</p> Signup and view all the answers

What occurs during telophase 2 of meiosis?

<p>During telophase 2, new nuclei form around the separated chromosomes, resulting in the completion of meiosis.</p> Signup and view all the answers

What are the four phases of mitosis and briefly describe one of them?

<p>The four phases of mitosis are prophase, metaphase, anaphase, and telophase. For example, during prophase, chromatin condenses into chromosomes and the nuclear membrane begins to break down.</p> Signup and view all the answers

What is the role of centrioles during mitosis?

<p>Centrioles help in organizing the mitotic spindle which separates the chromatids during anaphase.</p> Signup and view all the answers

Describe the process of cytokinesis and its significance.

<p>Cytokinesis is the division of the cytoplasm of a cell, resulting in the formation of two daughter cells. It is significant because it completes the cell division process after mitosis.</p> Signup and view all the answers

How do chromatin and chromosomes differ in structure?

<p>Chromatin is a relaxed form of DNA that is complexed with proteins, while chromosomes are tightly coiled structures formed during cell division that consist of two sister chromatids.</p> Signup and view all the answers

Explain how mitosis contributes to tissue regeneration.

<p>Mitosis allows for the production of two identical daughter cells, facilitating the replacement of damaged or dead cells during tissue regeneration.</p> Signup and view all the answers

What is a karyotype and how is it obtained?

<p>A karyotype is a map of an individual's chromosomes, obtained by staining and photographing cells during metaphase.</p> Signup and view all the answers

What are homologous chromosomes, and why are they important?

<p>Homologous chromosomes are pairs of chromosomes, one inherited from each parent, that contain the same genes at the same loci. They are important for genetic diversity and proper meiosis.</p> Signup and view all the answers

What occurs to chromatin during DNA replication?

<p>During DNA replication, chromatin unwinds and each strand is replicated, resulting in two identical strands of DNA for each chromosome.</p> Signup and view all the answers

Define 'allele' and explain its relevance to genetics.

<p>An allele is a different form of the same gene found at a specific locus on homologous chromosomes. They are relevant because they contribute to genetic variation in traits.</p> Signup and view all the answers

What is the significance of having 23 pairs of chromosomes in humans?

<p>Having 23 pairs of chromosomes ensures that humans possess a diploid number of 46, which maintains genomic stability and enables sexual reproduction.</p> Signup and view all the answers

What happens to chromatin during prophase?

<p>Chromatin condenses to form chromosomes.</p> Signup and view all the answers

Describe the role of centrioles in cell division.

<p>Centrioles migrate to opposite ends of the cell and organize spindle fibers.</p> Signup and view all the answers

Explain the process of cytokinesis.

<p>Cytokinesis is the division of the cytoplasm, resulting in two daughter cells.</p> Signup and view all the answers

What structure do chromosomes consist of after prophase?

<p>Each chromosome consists of two chromatids joined at the centromere.</p> Signup and view all the answers

How does the positioning of chromosomes in metaphase facilitate mitosis?

<p>Chromosomes are aligned at the nuclear equator, ensuring equal separation.</p> Signup and view all the answers

In which phase do the chromatids separate, and what are they referred to afterward?

<p>In anaphase, the chromatids separate and are referred to as chromosomes.</p> Signup and view all the answers

What marks the end of mitosis and the beginning of cytokinesis?

<p>The completion of telophase marks this transition.</p> Signup and view all the answers

Why is DNA replication crucial during interphase?

<p>DNA replication ensures that each daughter cell receives a complete set of chromosomes.</p> Signup and view all the answers

How does the structure of chromatin differ from that of chromosomes?

<p>Chromatin is a less condensed form of DNA, while chromosomes are tightly packed for cell division.</p> Signup and view all the answers

What is the significance of mitosis in tissue regeneration?

<p>Mitosis allows for the replacement of damaged or dead cells, facilitating tissue regeneration.</p> Signup and view all the answers

What is the result of homologous crossing over during prophase 1 of meiosis?

<p>It leads to an exchange of genetic material between homologous chromosomes, creating new gene combinations and genetic variation.</p> Signup and view all the answers

How does spermatogenesis differ from oogenesis in terms of gamete production?

<p>Spermatogenesis produces four functional sperm cells in each division, while oogenesis yields one functional oocyte and three polar bodies.</p> Signup and view all the answers

What are the structural differences between DNA and RNA?

<p>DNA has two strands and contains deoxyribose and thymine, whereas RNA is single-stranded, contains ribose, and has uracil instead of thymine.</p> Signup and view all the answers

During which phase of meiosis do homologous chromosomes line up next to each other?

<p>This occurs during prophase 1.</p> Signup and view all the answers

What is the role of mRNA in protein synthesis?

<p>mRNA is synthesized in the nucleus during transcription and carries the genetic code from DNA to the ribosomes for protein production.</p> Signup and view all the answers

What are the phases of mitosis and their primary purpose?

<p>The phases of mitosis are prophase, metaphase, anaphase, and telophase. Their primary purpose is to ensure the accurate division of the cell's duplicated genetic material into two daughter cells.</p> Signup and view all the answers

What role do centrioles play during mitosis?

<p>Centrioles organize the mitotic spindle, which is essential for separating chromosomes during cell division. They facilitate the proper alignment and movement of chromosomes to ensure accurate distribution to daughter cells.</p> Signup and view all the answers

Describe the process of cytokinesis and its importance in cell division.

<p>Cytokinesis is the final stage of cell division where the cytoplasm divides, forming two separate daughter cells. It is important because it ensures that each daughter cell has its own organelles and sufficient cytoplasm to function properly.</p> Signup and view all the answers

How do chromatin and chromosome structure differ during the cell cycle?

<p>Chromatin is a relaxed form of DNA, allowing for gene expression during interphase, while chromosomes are tightly coiled structures that are visible during mitosis when the DNA condenses. This compaction is necessary for accurate segregation during cell division.</p> Signup and view all the answers

Explain how mitosis contributes to tissue regeneration.

<p>Mitosis allows for the proliferation of somatic cells, which is crucial for replacing damaged or lost cells in tissues. This regenerative process is essential for growth, healing, and maintenance of multicellular organisms.</p> Signup and view all the answers

What is the significance of the diploid number of chromosomes in somatic cells?

<p>The diploid number of chromosomes in somatic cells (46) ensures that each daughter cell receives a complete set of genetic information after cell division. This is critical for maintaining genetic stability across generations.</p> Signup and view all the answers

How does the process of meiosis differ from mitosis?

<p>Meiosis results in four genetically diverse haploid gametes, while mitosis produces two genetically identical diploid daughter cells. Meiosis includes two rounds of division and promotes genetic diversity through recombination.</p> Signup and view all the answers

In what way does interphase contribute to the overall cell cycle?

<p>Interphase is the longest phase of the cell cycle, during which the cell grows, replicates its DNA, and prepares for mitosis. It is critical for ensuring the cell is fully ready for division.</p> Signup and view all the answers

What are the consequences of errors occurring during mitosis?

<p>Errors during mitosis can lead to aneuploidy, where daughter cells have an abnormal number of chromosomes, potentially causing diseases such as cancer. Genetic instability can arise from such errors, influencing cell function and proliferation.</p> Signup and view all the answers

How can post-translational modifications affect protein function after translation?

<p>Post-translational modifications, such as phosphorylation or glycosylation, can change a protein's activity, localization, or stability, thereby regulating its function and involvement in cellular processes. These modifications are critical for activating or inactivating proteins as needed.</p> Signup and view all the answers

What is the primary outcome of meiosis in terms of genetic composition of the gametes produced?

<p>Meiosis produces four non-identical gamete cells, each having a haploid number of chromosomes, resulting in genetic variation.</p> Signup and view all the answers

During which stage of meiosis does homologous crossing over occur, and what is its significance?

<p>Homologous crossing over occurs during Prophase I, and it creates new gene combinations which enhance genetic diversity.</p> Signup and view all the answers

How does spermatogenesis differ from oogenesis in terms of functional gamete production?

<p>Spermatogenesis produces four functional sperm cells per division, while oogenesis results in one functional oocyte and three polar bodies.</p> Signup and view all the answers

What are the key structural differences between DNA and RNA?

<p>DNA consists of two strands forming a double helix and contains deoxyribose and thymine, while RNA is a single strand made of ribose and uracil.</p> Signup and view all the answers

What is the role of mRNA in the process of protein synthesis?

<p>mRNA carries the genetic code from DNA in the nucleus to ribosomes for protein synthesis.</p> Signup and view all the answers

What are the primary functions of globular proteins in biological processes?

<p>Globular proteins play critical roles in nearly all biological processes and are more functional due to their compact, water-soluble structures.</p> Signup and view all the answers

How does the proteome of a muscle cell differ from that of a skin cell?

<p>The proteome differs due to the expression of different genes, which leads to the production of distinct proteins specific to each cell type.</p> Signup and view all the answers

What are the two main steps of gene expression and where do they occur?

<p>The two main steps of gene expression are transcription, which occurs in the nucleus, and translation, which takes place in the cytoplasm.</p> Signup and view all the answers

Describe the role of transcription factors in the process of transcription.

<p>Transcription factors recruit the enzyme RNA polymerase to initiate the polymerization of messenger RNA from the DNA template.</p> Signup and view all the answers

What is the significance of the central dogma in molecular biology?

<p>The central dogma describes the flow of genetic information from DNA to RNA to protein, highlighting how genes direct the synthesis of proteins.</p> Signup and view all the answers

In protein synthesis, what determines the properties and functions of a protein?

<p>The specific arrangement of amino acids in a polypeptide chain determines the protein's shape, properties, and functions.</p> Signup and view all the answers

What is the role of mRNA during translation?

<p>mRNA serves as the template that carries the genetic code from DNA to ribosomes, where it is translated into a protein.</p> Signup and view all the answers

What are some examples of globular proteins mentioned in the content?

<p>Examples of globular proteins include haemoglobin, myoglobin, insulin, and most enzymes.</p> Signup and view all the answers

How does the cellular DNA contribute to protein synthesis?

<p>Cellular DNA contains the genetic information needed to produce proteins by defining the sequences that determine amino acid arrangement.</p> Signup and view all the answers

What is the difference between the coding strand and the template strand during transcription?

<p>The coding strand contains the gene sequence and is identical to the mRNA, while the template strand is used to produce the complementary RNA.</p> Signup and view all the answers

What is the primary function of keratin in biological structures?

<p>Keratin provides strength to skin, hair, and nails.</p> Signup and view all the answers

How do proteins compare to carbohydrates in terms of energy yield?

<p>Proteins yield as much energy as carbohydrates per unit of weight.</p> Signup and view all the answers

What are the two main types of secondary protein structures?

<p>The two main types are alpha helices and beta pleated sheets.</p> Signup and view all the answers

What determines the tertiary structure of a protein?

<p>The tertiary structure is determined by the 3D folding of the secondary structures.</p> Signup and view all the answers

What distinguishes fibrous proteins from globular proteins?

<p>Fibrous proteins are elongated and provide structural support, while globular proteins are more compact and functional.</p> Signup and view all the answers

Describe the quaternary structure of a protein with an example.

<p>The quaternary structure is the combined 3D structure of two or more polypeptide chains, such as haemoglobin.</p> Signup and view all the answers

How does the protein intake recommendation relate to total calorie consumption?

<p>The recommended protein intake is 10-35% of total calorie consumption.</p> Signup and view all the answers

Explain the role of protein receptors in cell communication.

<p>Protein receptors facilitate cell-to-cell communication by transmitting signals across the cell membrane.</p> Signup and view all the answers

What are the basic units that make up proteins?

<p>Proteins are made up of amino acids linked by peptide bonds.</p> Signup and view all the answers

What impact do side chains have on protein structure?

<p>Side chains affect how a protein folds and its final shape.</p> Signup and view all the answers

What occurs during prophase in terms of chromatin and centrioles?

<p>During prophase, chromatin condenses into chromosomes and centrioles migrate to opposite ends of the cell.</p> Signup and view all the answers

Describe the alignment of chromosomes during metaphase.

<p>In metaphase, chromosomes align at the nuclear equator, attaching to the spindle fibres.</p> Signup and view all the answers

What is the outcome of anaphase regarding chromatid separation?

<p>During anaphase, spindle fibres separate the chromatids, resulting in two identical sets of chromosomes moving towards opposite ends.</p> Signup and view all the answers

Explain what happens during telophase and its significance in cell division.

<p>In telophase, the nuclear envelope reforms around each set of chromosomes as they decondense back into chromatin.</p> Signup and view all the answers

What role do spindle fibres play during mitosis?

<p>Spindle fibres attach to the centromeres of chromosomes and help in their alignment and separation during cell division.</p> Signup and view all the answers

How do centrioles influence the formation of spindle fibres?

<p>Centrioles organize the formation of spindle fibres and move to opposite ends of the cell before cell division.</p> Signup and view all the answers

Describe the main event occurring during cytokinesis.

<p>Cytokinesis involves the division of the cytoplasm, resulting in two separate daughter cells.</p> Signup and view all the answers

What is the significance of the 'S' phase in interphase?

<p>The 'S' phase is crucial for DNA replication, ensuring that each daughter cell receives a complete set of chromosomes.</p> Signup and view all the answers

How does chromatin change through the stages of mitosis?

<p>Chromatin condenses into visible chromosomes during prophase and decondenses back into chromatin during telophase.</p> Signup and view all the answers

What mnemonic can help remember the stages of mitosis and what does it represent?

<p>The mnemonic 'I Pay My Annual Taxes' helps remember the stages: Interphase, Prophase, Metaphase, Anaphase, Telophase.</p> Signup and view all the answers

What are the main differences between DNA and RNA in terms of structure and function?

<p>DNA is double-stranded with deoxyribose sugar, whereas RNA is single-stranded with ribose sugar. DNA stores genetic information, while RNA is involved in protein synthesis.</p> Signup and view all the answers

What are the components of a nucleotide and how do they contribute to the formation of nucleic acids?

<p>A nucleotide consists of a pentose sugar, a phosphate group, and a nitrogenous base. These components link together to form polynucleotide chains, which make up DNA and RNA.</p> Signup and view all the answers

Explain the significance of homologous chromosomes and provide examples of their functions.

<p>Homologous chromosomes are pairs of chromosomes, one from each parent, that contain genes for the same traits. They are essential for genetic diversity and proper segregation during meiosis.</p> Signup and view all the answers

What is a karyotype and what information can it provide about an organism?

<p>A karyotype is a map of the chromosomes in a dividing cell, displaying their number and structure. It can reveal chromosomal abnormalities or genetic disorders.</p> Signup and view all the answers

Describe the roles of mRNA, tRNA, and rRNA in the process of protein synthesis.

<p>mRNA carries genetic information from DNA to the ribosome, tRNA brings amino acids to the ribosome, and rRNA is a structural component of ribosomes. Together, they facilitate translation.</p> Signup and view all the answers

What is the significance of non-coding DNA, and how does it differ from coding DNA?

<p>Non-coding DNA comprises 98.5% of DNA and includes regulatory sequences and introns, which do not code for proteins. It plays crucial roles in gene regulation and genome structure.</p> Signup and view all the answers

In terms of protein structure, what distinguishes essential, non-essential, and conditional amino acids?

<p>Essential amino acids must be consumed through diet, non-essential amino acids can be synthesized in the body, and conditional amino acids are usually not essential but needed during illness or stress.</p> Signup and view all the answers

How do dominant and recessive alleles influence phenotype expression and what is an example?

<p>Dominant alleles express their traits even in a single dose, while recessive alleles require two doses for expression. For instance, in pea plants, a dominant allele for tallness (T) masks a recessive allele for shortness (t).</p> Signup and view all the answers

Study Notes

Cell Division

  • Cytokinesis begins during anaphase and continues as a cleavage furrow forms

  • In telophase, chromosomes migrate to opposite ends of the cell, unravel into chromatin, the nuclear envelope reforms around each set of chromosomes and cytokinesis continues to form two daughter cells

  • Mitosis is complete once a new interphase begins. Chromosomes have unraveled to form chromatin, cell division has produced two daughter cells each with DNA which is identical to the parent cell’s DNA

Mitosis Examples

  • Skin: keratinized stratified squamous epithelium is constantly regenerating from the basal layer

  • High abrasion areas: Skin, gastrointestinal tract (oral cavity to anus), female reproductive system (cervix and vagina)

  • Normal growth: in utero (pregnancy), puberty, wound healing, red blood cells, mitosis inhibitors used in cancer treatment -- stop tumor growth

  • Brains, heart, skeletal muscle: slow growth

Meiosis

  • Germ cells divide and produce gametes (sex cells)

  • Specialized for sexual reproduction

  • DNA replication followed by two cell divisions

  • Produces four genetically different daughter cells:

    • Gametes are haploid (n)
    • Only one homolog from each homologous pair
  • Resulting gametes (egg/sperm) unite to form a zygote -- a new genetically unique human being

Meiosis Steps

    1. Early prophase I: The duplicated chromosomes become visible chromatids
    1. Middle prophase I: Homologous chromosomes synapse to form tetrads. Crossing over may occur at this stage. (Homologous pairs of chromosomes come together and form tetrads, the process is called synapsis, and the chromosomes swap a bit of DNA)
    1. Metaphase I: Homologous chromosomes align at the center of the cell, random assortment (which chromosome ends up on which side is random) of homologous chromosomes occurs.
    1. Anaphase I: Homologous chromosomes move apart to opposite sides of the cell
    1. Telophase I: New nuclei form and the cell divides.
    1. Prophase II: There is no DNA replication before the start of meiosis II; it starts with two non-identical cells both with 23 chromosomes and the DNA is replicated.
    1. Metaphase II: Chromosomes align along the center of the cell
    1. Anaphase II: Chromatids separate and each is now called a chromosome.
    1. Telophase II: New nuclei form around the chromosomes.

Interphase

  • Ongoing normal cell activities:
    • E.g., makes hormones, transmits action potentials, contracts, replicates DNA, and prepares for division
  • It has three phases:
    • First phase: Regular metabolism
    • Second phase: DNA replication
    • Third phase: Preparation for division

Mitosis

  • A series of events that leads to the production of two somatic cells by division of one mother cell into two daughter cells. These cells are genetically identical.

  • Four stages of mitosis:

    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
  • Cytokinesis: Division of cell cytoplasm

Chromosomes and Chromatin

  • Chromatin: DNA complexed with proteins (histones)

  • During cell division, chromatin condenses into pairs of chromatids called chromosomes. Each pair of chromatids is joined by a centromere

Chromosomes

  • Humans:

    • 23 pairs of chromosomes
    • 46 diploid number
    • 22 autonomic pairs
    • One sex determining pair:
      • XX – Female
      • XY – Male
  • Karyotype: A map of chromosomes

    • To obtain a karyotype, a cell must be stained and photographed during metaphase
  • Homologous: Pairs of chromosomes, one from the father and one from the mother (gametes)

  • Locus: The location of a gene on a chromosome

  • Allele: Different forms of the same gene. The same locus will be found on the other homologous chromosome

DNA Replication

  • Interphase: DNA replication occurs. Each chromosome becomes doubled, consisting of two identical strands of DNA.
    • During interphase, the number of chromosomes is not increased, but the amount of DNA is.
    • The cell is getting ready to divide.

Structure of a Meiotic Chromosome

  • The DNA of a chromosome is dispersed as chromatin

  • The DNA molecule unwinds, and each strand of the molecule is replicated

  • During mitosis, the chromatin from each replicated DNA strand condenses to form a chromatid. The chromatids are joined at the centromere to form a single chromosome

  • The chromatids separate to form two new, identical chromosomes. The chromosomes will unwind to form chromatin in the nuclei of the two daughter cells.

Mitosis

  • Produces two identical daughter cells

  • Happens all the time, during wound healing, skin renewal, etc.

  • Prophase: Chromatin condenses to form chromosomes, centrioles migrate to either end of the cell, spindle fibres attach to centromeres

  • Metaphase: Chromosomes are aligned at the nuclear equator

  • Anaphase: Spindle fibres separate the chromatids, two identical sets of chromosomes are moved to separate ends of the cell, cytokinesis begins

  • Telophase: The nuclear envelope reforms around each set of chromosomes, chromosomes decondense into chromatin, cytokinesis continues

  • Cytokinesis: Cytoplasmic division

Centrioles and Spindle Fibres

  • Two centrioles located in the centrosome

  • Center of microtubule (spindle fibre) formation

  • Before cell division, centrioles divide and move to either end of the cell and organise spindle fibres

Mitosis – IPMAT

  • Can be remembered by "I Pay My Annual Taxes"

  • Interphase: The time between cell divisions. DNA is present as thin threads of chromatin in the nucleus. DNA replication occurs during the 'S' phase of interphase, and organelles other than the nucleus and centrioles duplicate during interphase

  • Prophase: The chromatin condenses into chromosomes. Each chromosome consists of two chromatids joined at the centromere. The centrioles move to opposite ends of the cell, and the nucleus and the nuclear envelope disappear. Microtubules form near the centrioles and project in all directions, some of the microtubules end blindly and are called astral fibres. Others, known as spindle fibres, project towards an invisible line known as the equator and overlap with fibres from the opposite side

  • Metaphase: The chromosomes align in the center of the cell in association with the spindle fibres. Some spindle fibres are attached to kinetochores in the centromere of each chromosome

  • Anaphase: The chromatids separate, and each chromatid is then referred to as a chromosome. Thus, when the centromeres divide, the chromosome number is double, and there are two identical sets of chromosomes. The chromosomes assisted by the spindle fibres move towards the centrioles at each end of the cell. Separation of the chromatids signals the beginning of anaphase, and by the time anaphase has ended, the chromatids have reached the poles of the cell

Leucine

  • DNA code is 'degenerate' meaning several code words have the same meaning

  • Three codons do not code for an amino acid but the termination of the peptide chain:

    • Stop codons: UAG, UAA, UGA

Translation Summary

  • mRNA carries genetic information from the nucleus to the ribosomes

  • The sequence is read by the translational machinery in the ribosomes, in lots of 3 nucleotides (nucleotide triplets = codon)

  • Translation starts at the start codon (AUG) of each gene in the mRNA

  • Each codon codes for a specific amino acid

  • As each codon is read, a tRNA with a specific complimentary sequence (anticodon) binds to the triplet

  • The tRNA carries the amino acid specified by the codon

  • Amino acids are joined together by peptide bonds, in the sequence specified by the mRNA, to make a peptide/protein

Post-translational Modification

  • The chemical modification of a protein following translation

  • It is one of the last steps in protein synthesis

  • After translation, proteins can be modified by attaching other functional groups which can change or extend its functions:

    • E.g., lipids (lipoproteins), carbohydrates (glycoproteins)
  • Amino acids may be cleaved off the end of the protein, or a polypeptide can be cut in half, e.g. insulin

  • Other modifications such as phosphorylation are a common way of controlling the behaviour of a protein, for instance, activating or inactivating an enzyme

    • Proteins cannot be produced in active form because if they are produced in active form, they will start digesting the cell in which they are produced
    • Proteins are activated when they reach their destination

Cells

  • Somatic cell:

    • A biological cell forming the body of a multicellular organism
    • Most cells
    • 46 chromosomes (diploid number)
    • Mitosis
    • E.g., epithelial cells, muscle cells, and neurones
  • Germ cell:

    • Cells that give rise to gametes
    • Located in the gonads (ovaries and testes)
    • Diploid (46 chromosomes)
    • Meiosis
  • Gamete cell:

    • Cells that fuse during sexual reproduction
    • Sperm or egg (spermatocyte or oocyte)
    • 23 chromosomes (haploid number)

Cell Life Cycle

  • Cells spend the majority of their life in interphase

  • Interphase: The phase between cell divisions. Cells divide to form four daughter gamete cells with a haploid number of chromosomes (n).

Homologous Crossing Over

  • Prophase I: Homologous chromosomes line up next to each other. DNA is exchanged between the adjacent homologous chromatids. Sister chromatid strands of each chromosome are no longer identical resulting in an exchange of genetic material between mother and father chromosomes.

  • This causes new gene combinations and genetic variation.

Spermatogenesis

  • Meiosis occurring in the gonads

  • 4 functional sperm cells per division

  • Non-identical with 23 chromosomes

  • Lifelong process in the testes

Oogenesis

  • The ovaries make gametes (oocytes) via meiosis

  • At birth, the ovaries contain all the oocytes they will ever have - stalled in prophase I

  • 1 functional oocyte per division

  • 3 polar bodies produced

  • Non-identical with 23 chromosomes

Nucleic Acids

  • First discovered in the nuclei of cells, found in all cells
  • Organic macromolecules (C,H,O,N,P)
  • Main information-carrying molecules that direct protein synthesis and determine hereditary traits
  • Polynucleotides/polynucleotide chains are chains of repeating monomers called nucleotides
  • Nucleotides contain:
    • Pentose sugars (deoxyribose, ribose)
    • A phosphate group
    • A nitrogenous base (adenine, guanine, cytosine, thymine, uracil)
  • Nucleoside = pentose sugar + nitrogenous base
  • Nucleotide = nucleoside + phosphate group
  • Sequence of nitrogenous bases carries the information
  • Two major classes: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)

DNA

  • DNA = deoxyribonucleic acid
  • Mainly found in the nucleus, also in mitochondria
  • Contributes to the blueprint coding for protein synthesis
  • Approximately 20,000 to 25,000 genes in the human genome
  • Only 1.5% of DNA is genes
  • 98.5% of DNA is non-coding (regulatory sequences, introns, and non-coding DNA such as repeat elements)

Structure of DNA

  • Double helix molecule (Watson and Crick 1953)
  • Double-stranded polymer: two nucleotide chains are antiparallel
  • Alternating sugar-phosphate backbone
  • Pentose sugar: deoxyribose
  • Complementary nitrogenous bases form rungs of the ladder:
    • Adenine – thymine
    • Guanine – cytosine
  • Nitrogenous bases held together by weak hydrogen bonds

Organisation of DNA

  • Double strands of DNA in a twisted ladder
  • DNA is wrapped around proteins called histones
  • DNA and histones bundled together is called chromatin
  • Chromatin twists and condenses to form chromosomes
  • Each chromosome contains hundreds to thousands of genes

Quantity of DNA

  • Each somatic human cell nucleus contains 2 copies of each chromosome (one from the mother and one from the father)
  • Somatic cells with 46 chromosomes (23 pairs) have the full amount of DNA, called diploid
  • Maternal and paternal chromosomes in a pair are called homologous chromosomes (make a homologous pair)
  • Gametes (sperm and egg) have one chromosome from each pair (23 chromosomes), have half the normal amount of DNA, called haploid
  • Humans have 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes
  • Women have 2 X chromosomes, men have 1 X and 1 Y chromosome
  • When cells divide, chromatin condenses to form chromosomes, which are easier to see and can be arranged next to their pair. This map is called a karyotype.

Karyotype

  • A map of chromosomes in a dividing cell
  • Male karyotype (22 autosomes + XY)
  • Each chromosome has segments represented by colours. These segments represent a gene, which controls the production of a protein
  • Many genes coding for different proteins on each chromosome
  • When comparing a homologous pair, both will have genes for the same function, but could code differently. The dominant gene will be displayed.
  • The two different genes are referred to as alleles

RNA

  • RNA = ribonucleic acid
  • Single-stranded polymer, self-complimentary sequences that form folds, bulges, and helices
  • Supports DNA during protein synthesis
  • Found in both the nucleus and the cytoplasm
  • Alternating sugar phosphate backbone
  • Pentose sugar: ribose
  • Nitrogenous bases:
    • Adenine – uracil
    • Guanine – cytosine
  • 3 types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)
  • Different relative sizes, shapes, and roles in protein synthesis:
    • mRNA: carries information from DNA to the ribosome through transcription
    • tRNA: brings amino acids to the ribosome for protein production in translation
    • rRNA: integral part of the ribosomes

Terminology

  • Genetics: the study of heredity, looking at DNA, chromosomes, and gene expression
  • Gene: segment of DNA that codes for a protein
  • Allele: alternative form of a gene
  • Genotype: the actual gene (AA, Aa, aa)
  • Phenotype: the person's appearance (blue eyes, brown hair)
  • Dominant and recessive alleles: an allele is dominant if it can express itself in a single dose, recessive if it requires 2 doses of that allele for expression
  • Sex-linked traits: traits affected by genes on the sex chromosomes

Proteins

  • The most diverse biomolecules in the human body
  • Important macromolecules, at least 10,000 in our body
  • Contain carbon, hydrogen, oxygen, and nitrogen bound by covalent bonds. May also contain sulphur, phosphorus, iron, and iodine.
  • Long chain of amino acids (aa) linked by peptide bonds
  • Made from 20 amino acids. Each amino acid has specific properties due to its side chain (part of the amino acid not involved in linking to other amino acids)
  • Essential amino acids (can't be synthesized in the body): 9
  • Non-essential amino acids (can be synthesized in the body): 5
  • Conditional amino acids (not normally essential, but required in times of illness or stress): 6

Amino Acid Structure

  • Each amino acid has an amine group, a carboxyl group, a hydrogen atom, and a side chain (R)
  • Some side chains are non-polar and hydrophobic (water-fearing), while others are hydrophilic (water-loving) or positively or negatively charged

Types of Peptides

  • Dipeptide: 2 amino acids
  • Oligopeptide: 3-10 amino acids
  • Polypeptide: 10 or more amino acids
  • Protein: 50 or more amino acids

Protein Functions

Role Example
Regulation Enzymes control chemical reactions. Keratin adds strength to skin, hair, and nails.
Energy Proteins can be broken down for energy. Per unit weight, they yield as much energy as carbohydrates.
Communication Cell-to-cell communication by protein receptors on the cell membrane and neurotransmitters, which are also proteins.
  • Recommended protein intake is 10-35% of total calories

Protein Structure

  • Interactions between side groups in a long chain and the peptide bonds affect the way a protein can fold and take shape

  • Primary Structure:

    • Sequence of amino acids linked by peptide bonds
  • Secondary Structure:

    • Protein folds to form secondary structures because amino acids have different side chains
    • Two regular folding patterns: alpha helices (keratin) and beta pleated sheets (fibroin, silk)
  • Tertiary Structure:

    • 3D shape is determined by the folding of the secondary structure
    • Alpha helices and beta sheets fold to form unique structures held together by bonds between amino acids that may be far apart in the polypeptide chain.
  • Quaternary Structure:

    • Combined 3D structure of two or more polypeptide chains
    • E.g. Haemoglobin, which consists of 2 alpha and 2 beta chains

Types of Proteins

  • Fibrous:

    • Simple elongated polypeptide chains arranged in parallel fashion along a single axis.
    • Usually insoluble in water and stable.
    • Provide mechanical support and tensile strength, more structural.
    • Abundant outside the cell, making up the matrix between cells.
    • Less sensitive to changes in temperature, pH, etc.
    • E.g. Collagen, keratin, myosin, elastin, actin.
  • Globular:

    • Polypeptide chain folds up into a compact shape, like a ball with a rough surface.
    • Usually water soluble.
    • Mobile, chemically active.
    • Play critical roles in nearly all biological processes, more functional.
    • Sensitive to changes in temperature, pH, etc.
    • E.g. Haemoglobin, myoglobin, insulin, most enzymes, antibodies.
  • Membrane Proteins:

    • E.g. Histones and glucose transporters

The Proteome

  • The proteome of a cell is all the proteins that a cell makes. Proteomics is the study of the proteins in a cell.
  • Not all cells make all proteins. Comparing the proteomes of different cells can show their differences.
  • Even though all cells are derived from the same parent cell, they produce different things by switching genes on or off.
    • E.g. Muscle cell vs skin cell, melanoma vs normal melanocyte
  • Cells are protein factories that constantly synthesize many different proteins
  • These proteins are used for cell functions, or exported.
    • Intracellular use or extracellular use
  • The cell's DNA contains all the information needed to make proteins.

Protein Synthesis - Basic Concept

  • The specific arrangement of amino acids determines the shape, properties, and functions of a protein.
    • Gene: a segment of DNA that specifies the structure of a protein
    • Genetic code: specific arrangement of nucleotides in DNA and RNA that determines the amino acid sequence of a particular protein
    • Gene expression: production of proteins from the information stored in DNA
    • Central dogma: directional flow of information from DNA -> RNA -> protein

Gene Expression

  • Gene expression involves two steps:
    • Transcription:
      • Occurs in the nucleus
      • A copy of a small part of the stored information in DNA (gene) is produced
      • DNA -> mRNA
    • Translation:
      • Occurs in the cytoplasm
      • The copied information is converted into a protein
      • mRNA -> protein

Transcription

  • Transcription factors recruit the enzyme RNA polymerase, which polymerizes messenger RNA.
    • Polymerization: two molecules combine to form a larger molecule
  • Double helix structure:
    • Sugar-phosphate backbone
    • Two polynucleotide chains
    • Nitrogenous bases
    • The two strands are complementary through base pairing
  • One strand is the coding strand (contains the gene sequence) and the other is the template strand
  • The template strand is used to produce RNA

Translation

  • Ribosomes are the sites of protein synthesis
  • tRNA carry amino acids to the ribosome
  • tRNA anticodons match mRNA codons to add the appropriate amino acid to the growing polypeptide chain
  • The newly synthesized polypeptide chain folds into its correct 3D shape to become functional

Cell Cycle

  • The cell cycle is a continuous process of growth, development, and division in a cell.
  • Most of the time, cells are in interphase where they grow up, carry out their unique functions, and duplicate their chromosomes.
  • The mitotic phase is the division stage, consisting of mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

  • Mitosis is a nuclear division that produces two daughter cells with the same number and type of chromosomes as the parent cell
  • Four phases: prophase, metaphase, anaphase, telophase
  • IPMAT: Interphase, Prophase, Metaphase, Anaphase, Telophase

Interphase

  • Interphase is the time between cell divisions.
  • DNA is present as thin threads of chromatin in the nucleus.
  • DNA replication occurs during the 'S' phase of interphase.
  • Organelles other than the nucleus and centrioles duplicate during interphase.

Prophase

  • The chromatin condenses into chromosomes.
  • Each chromosome consists of two chromatids joined at the centromere.
  • The centrioles move to opposite ends of the cell, and the nucleus and the nuclear envelope disappear.
  • Microtubules form near the centrioles and project in all directions.
  • Some of the microtubules end blindly and are called astral fibres.
  • Others, known as spindle fibres, project towards an invisible line known as the equator and overlap with fibres from the opposite side.

Metaphase

  • The chromosomes align in the center of the cell in association with the spindle fibres.
  • Some spindle fibres are attached to kinetochores in the centromere of each chromosome.

Anaphase

  • The chromatids separate, and each chromatid is then referred to as a chromosome.
  • This doubles the chromosome number, resulting in two identical sets of chromosomes.
  • The chromosomes, assisted by the spindle fibres, move towards the centrioles at each end of the cell.
  • The separation of the chromatids signals the beginning of anaphase, and by the time anaphase has ended, the chromatids have reached the poles of the cell.

Telophase

  • The nuclear envelope reforms around each set of chromosomes.
  • Chromosomes decondense into chromatin.
  • Cytokinesis continues.

Cytokinesis

  • Cytokinesis is the cytoplasmic division.

Centrioles and Spindle Fibres

  • Two centrioles located in the centrosome.
  • Center of microtubule (spindle fibre) formation.
  • Before cell division, centrioles divide and move to either end of the cell and organize spindle fibres.

Meiosis

  • Meiosis is a type of cell division that produces gametes (sperm and egg cells) with half the number of chromosomes as the parent cell.
  • Two rounds of division: meiosis I and meiosis II
  • Meiosis I:
    • homologous chromosomes pair up and exchange genetic material through crossing over (prophase I)
    • homologous chromosomes separate (anaphase I)
  • Meiosis II:
    • sister chromatids separate (anaphase II)

Homologous Crossing Over

  • Prophase I: Homologous chromosomes line up next to each other. DNA is exchanged between the adjacent homologous chromatids. Sister chromatid strands of each chromosome are no longer identical, resulting in an exchange of genetic material between mother and father chromosomes.
  • This causes new gene combinations and genetic variation.

Spermatogenesis

  • Meiosis occurring in the gonads
  • Produces 4 functional sperm cells per division
  • Non-identical, with 23 chromosomes
  • Lifelong process in the testes

Oogenesis

  • The ovaries make gametes (oocytes) via meiosis.
  • At birth, the ovaries contain all the oocytes they will ever have, stalled in prophase I.
  • Produces 1 functional oocyte per division.
  • Produces 3 polar bodies.
  • Non-identical, with 23 chromosomes.

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Explore the fascinating processes of cell division, including mitosis and meiosis, through this quiz. Understand the stages of mitosis, examples of where it occurs in the body, and the roles of meiosis in gamete production. Test your knowledge on cell biology concepts and their applications in growth and healing.

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