Cell Biology and Genetics

• All living organisms must reproduce to continue their species
• Offspring inherit DNA from their parents
• DNA contains information needed to develop into a functioning organism
• DNA is organized into functional units called genes
• Genes are a part of the much bigger structure, chromosomes
• Genome is the name given to all genetic material in a cell
• Genetics is the study of genes
• Knowledge in genetics effects daily life, such as genetic counseling and production of human insulin from genetically modified micro-organisms
• Human beings reproduce sexually, requiring genetic material from two parents
• It helps species evolve because every human being is genetically different from others
• The Human Genome Project completed in 2003, mapped every chromosome and identified every gene to provide the blueprint for life

Chromosomes, Genes, and DNA

• Nearly every body cell contains an identical copy of the individuals genetic material
• Red blood cells and gametes/sex cells are exceptions
• Chromatin is genetic material, see in a resting cell and hard to see under a microscope
• Chromosomes become visible when the cell prepares to divide
• Human cells have 46 chromosomes arranged as 23 pairs, one from each parent
• Diploid cells have 23 pairs of chromosomes
• Haploid cells like gametes have half the complement, 23 chromosomes
• Homologous chromosomes belong to the same pair
• The complete set of chromosomes for a cell is a karyotype
• Each pair of chromosomes is numbered, largest pair being number 1
• Autosomes are the first 22 pairs, holding the same amount of genetic material
• Sex chromosomes are pair 23, determining individual gender
• Autosomes are not necessarily the same size

Genes

• 99% of cells DNA doesn't code for protein
• Filler DNA contains signals to stop and start protein synthesis
• Genes are the DNA sequences that code for proteins interspersed with filler DNA
• Genes allow cells to make a specific protein, called a gene product
• The human genome contains about 20,500 genes
• Genes exist in pairs, matched at the equivalent site/locus

DNA

• DNA is a double-stranded molecule of 2 chains of nucleotides
• Nucleotides consist of a sugar, phosphate group, and a base
• DNA looks like a twisted ladder, with the uprights as sugar and phosphate units
• The sugar in DNA is deoxyribose
• Bases are linked to the sugars, binding to another base on the other chain to form rungs
• Chains are twisted, forming a double helix
• The double helix is wrapped highly around histones (structural proteins)
• the DNA histone material is called chromatin and is packaged into chromosomes shortly before cell divides
• DNA carries information for an organisms biological activities, passing from generation to next
• Information is kept found in the bases within DNA
• There are 4 bases: adenine (A), guanine (G), thymine (T), and cytosine (C)
• Bases are arranged to make a base code for when protein synthesis is required
• Base along one DNA strand pairs with a base on the other strand, known as complementary base pairing
• Adenine always pairs with thymine, cytosine pairs with guanine
• Bases run down in the middle of the helix and bind with hydrogen bonds

Mitochondrial DNA

• Each body cell has around 5000 mitochondria
• Mitochondria hold DNA(mitochondrial DNA) for enzymes for energy production
• this DNA passes from one generation to another via the ovum, meaning offspring's mitochondrial DNA is inherited from the mother
• Rare inherited disorders arise from faulty mitochondrial DNA and are passed through generations

Mutation

• Mutation is a heritable DNA alteration in a cell
• Most mutations occur spontaneously because of millions of DNA replications and cell divisions
• Others are from external factors such as X-rays, UV rays, or chemical exposure
• Mutagens are factors capable of mutating DNA
• Most mutations are repaired

Protein Synthesis

• DNA holds essential biological information written in the base code
• Products of this information are almost always proteins
• Proteins are essential for body function, structural elements and enzymes for all biochemical processes
• Building blocks of human proteins are 20 different amino acids
• Intermediary molecule needed to carry genetic instructions to the cytoplasm because DNA cannot leave the nucleus, this is messenger ribonucleic acid
• Messenger ribonucleic acid(mRNA) is a strand of nucleotides synthesized in the nucleus
• The mRNA is synthesized from the appropriate gene whenever the cell needs protein
• RNA differs from DNA in 3 ways, it is single-stranded
• It has sugar ribose instead of deoxyribose
• It uses the base uracil instead of thymine

Transcription

• The code is buried in the DNA, so the first step to transcription to open the helix to expose the bases
• Only the gene to be transcribed if opened
• Both base strands are exposed, but the enzyme that makes mRNA uses only one, so the mRNA molecule is single-stranded
• As enzyme moves along DNA strand and reads code it adds a complimentary base to the mRNA
• The complementary base added to the mRna is dependent on the DNA base, if the DNA base is cytosine, guanine is added; if the DNA base is thymine, adenine is added
• Uracil is added if adenine is the DNA base; mRNA does not contain thymine
• Synthesis of mRNA is terminated when the enzyme reaches a 'stop' signal and the mRNA is released
• DNA is then zipped up and the mRNA leaves the nucleus

Translation

• Translation is synthesis of the final protein using information carried on mRNA and takes place on free ribosomes in the cytoplasm
• First, the mRNA attaches to the ribosome, which reads this base sequence
• Proteins are built from 20 different amino acids
• The base code in RNA is read in triplets, giving 64 base combinations, called codons
• The first codon is the start codon that initiates protein synthesis
• The ribosome slides, reads codons, and adds amino acids to form the protein molecule
• When the process arrives at a stop codon, it terminates synthesis and releases the new protein, then some proteins are used

Cell Division

• Most body cells divide, producing two identical diploid daughter cells through mitosis to help with growth and repair
• Production of gametes differs because daughter cells have half the chromosome number, 23/haploid
• Gametes are produced from meiosis, DNA replication occurs before
• DNA is the only molecule capable of self-replication
• Mistakes in copying cells yields non-functioning cells or cells that can't respond to normal cell controls, leading to tumors
• In DNA replication, the double helix unfolds and unzips to expose bases
• Enzyme moves for DNA replication along the base sequence to each strand
• It reads the genetic code and adds the complimentary base to the new chain
• This makes each strand of opened bases a double strand
• The result is two identical DNA molecules
• Enzymes cause twisting and coiling back into normal form

Meiosis

• Meiosis is gamete production, after fertilization, the gamete (sperm cell) and ovum unite, with the resulting zygote being fully diploid
• Meiosis involves two cell divisions, producing four distinct daughter cells that vary from each other
• First, DNA replication occurs forming four chromatids that cluster tightly together to exchange over genes, known as crossing over
• Crossing over causes chromatids to acquire random gene combinations, these gene combinations separate randomly to prep for cell division
• Each pair of chromosomes separates and travels, producing two unique diploid daughter cells

Genetic Basis of Inheritance

• Mixing parents' genes during meiosis leads to human genetic variety
• Each pair of homologous chromosomes contains genes for the same traits
• Alleles are paired genes that may or may not be identical
• Homozygous alleles are two identical forms
• Heterozygous alleles are two different forms
• Alleles contain genes for the same trait but are not necessarily identical

Sex-linked inheritance

• Sex-linked traits are coded for on the section of the X chromosome that has no equivalent material on the Y
• Normal colour vision is one example, carried on the X chromosomes and is the dominant form of the gene
• If the faulty gene is abnormal, this male will be colour blind because there is only one X chromosome

Ageing and DNA

• Cumulative exposure to mutagens and diminishing repairing DNA causes cells genome to gradually collect mutations
• This can lead to diminished function and increased risk of disease
• Mitochondrial DNA ages and has wear and tear damage, causing function impairment

Cell Senescence/Aging

• Occurs when a cell can divide between 50 to 60 times
• Thought to relate to the effects of aging on telomerase function
• Telomerase is an enzyme repairing the telomeres (chromosomes tips)
• Telomeres are on chromosomes and gradually decline, reducing DNA replication, so the chromosomes cannot function correctly and divide.
• If these telomeres are damaged, telomerase cannot act effectively restricting cells from further division until completely diminishing.
• Eventually, each cell shortens further after each replication, thus causing a restricted cell lifespan because of its damage and cell function is inhibited.

Genetic Basis of Disease: Cancer

• Caused by mutation of cellular DNA causing disorganised growth
• Cells collect more mutations as they age, explaining why the incidence of cancer rises as people age
• The more mutations a cell has, the likely it is to acquire a tumor cell tumor characteristics, including failing to respond to normal growth controls and its immortality capabilities
• Mutations can be inherited leading to greater risk of cancers in certain families
• However, the majority of cancer is due to the mutations caused by aging and other risk factors

Inherited Disease

• Faulty gene is passed directly through parents
• Many genes can be found and traced through human mapping
• Many diseases have inherited qualities but the single faulty gene cannot be identified so inheritance cannot be predicated
• Phenylketonuria is a metabolic disorder, causing the phenylalanine hydroxylase enzyme gene to be faulty producing an absent enzyme
• The liver fails to convert phenylalanine to tyrosine, leading to toxic levels that cause brain damage if not treated

Abnormalities of Sex Chromosomes

• Abnormal meiosis will create daughter cells to have the wrong number of chromosomes
• Individual will may not progress through sexually and will have learning disabilties

Turner's Syndrome

• Usually has one sex chromosome, an X as well 22 normal autosomes
• Karyotype is XO and affects females
• They are infertile without treatment, and can experience coarctation of the aorta with usual level intellegence

Down's Syndrome

• There are three copies of chromosome 21, an additional chromosome is included preventing the pair being diploid, leading to an additional copy
• This is caused by the chromosomes not normally separating during meiosis
• Individuals possess short stature, face abnormalities and learning disabilities ranging in their severity

Klinefelter's Syndrome

• Has a XXY Karyotype, there are extra chromosomes present and affects males
• Condition is more common than Turner's syndrome with high average for height and mild learning disabilities
• They have under developed testies which inhibits fertility
• Tend to possess more feminine characteristics, such as enlarged breasts and rounded hips without testosterone treatment

Test your knowledge of cell biology and genetics. This quiz covers chromosomes, cell types, genetic disorders, and related key concepts in biology. Perfect for students studying introductory genetics or cell biology.