Questions and Answers
What are the three stages of interphase?
G1, S, G2
Proto-oncogenes act as brakes in the cell cycle.
False
What proteins do tumor suppressor genes encode?
During the G0 stage, cells continue to perform their normal functions, but no longer prepare for cell ___________.
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What is a trait?
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What did Gregor Mendel formulate regarding heredity? Select the correct option.
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Mendel observed meiosis directly to formulate his laws of inheritance.
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A cell has two pairs of ________ chromosomes recognized by length, not color.
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What percentage of tumors are found to have an altered Ras protein?
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Nearly half of all human cancers have a mutation in the p53 gene.
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Which genes are mentioned as examples of tumor suppressor genes in the text?
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A human cell typically contains at least 2 m of DNA, packed into a nucleus that is about 5 μm in diameter. The histones are responsible for packaging the DNA so that it can fit into such a small space. When a eukaryotic cell is not undergoing division, the ______ is dispersed or extended.
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Match the following mitosis phases with their descriptions:
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What is a dihybrid cross?
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What are the possible gametes when two dihybrids (FfSs) reproduce?
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In a dihybrid cross, the expected phenotypic ratio is __.
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Explain the Law of Segregation.
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Dominant alleles are always represented by lowercase letters.
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What is a mutation?
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Which types of RNA are involved in translation at the ribosome?
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Mutations are always harmful.
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Point mutations refer to alterations in a very small segment of the DNA, such as a single ____.
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Match the following types of mutations with their descriptions:
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Study Notes
The Cell Cycle
- The cell cycle is an orderly set of stages that take place between cell division and the next cell division
- Interphase is divided into three stages: G1, S, and G2
- G1 stage: cell doubles its organelles and accumulates materials needed for DNA synthesis
- S stage: DNA replication occurs, resulting in duplicated chromosomes
- G2 stage: cell synthesizes proteins needed for cell division
- Some cells, such as nerve and muscle cells, can enter a G0 phase, where they do not prepare for cell division
The Mitotic Stage
- Mitosis is nuclear division, where the chromosome number stays constant
- Overview of mitosis: 2n → 2n
- Before mitosis, DNA replication occurs, duplicating the chromosomes
- Each replicated chromosome is composed of two sister chromatids held together at a centromere
- At the completion of mitosis, each chromosome in the daughter cells consists of a single chromatid
Control of the Cell Cycle
- Eukaryotic cells have evolved a complex system for regulation of the cell cycle
- The cell cycle is controlled by internal and external signals
- Internal signals ensure that the stages follow one another in the correct sequence
- External signals tell the cell whether or not to divide
- Checkpoints in the cell cycle: G1, G2, and M stages
- Cyclins increase and decrease as the cell cycle continues, allowing the cell to proceed to the next stage
Proto-oncogenes and Tumor Suppressor Genes
- Proto-oncogenes encode proteins that promote the cell cycle and prevent apoptosis
- Tumor suppressor genes encode proteins that stop the cell cycle and promote apoptosis
- Mutations in proto-oncogenes or tumor suppressor genes can lead to cancer
- Examples of tumor suppressor genes: p53 and BRCA1### Mitosis
- Chromosomes attached to centromeric spindle fibers line up at the metaphase plate during metaphase
- Polar spindle fibers reach beyond the metaphase plate and overlap
- At the beginning of anaphase, centromeres uniting sister chromatids divide, and sister chromatids separate, becoming daughter chromosomes that move toward the opposite poles of the spindle
- Daughter chromosomes have a centromere and a single chromatid
- Movement of daughter chromosomes is due to:
- Kinetochore spindle fibers shortening, pulling daughter chromosomes toward poles
- Polar spindle fibers pushing poles apart as they lengthen and slide past each other
- Telophase: spindle disappears, and nuclear envelope components reassemble around daughter chromosomes
- Each daughter nucleus contains the same number and kinds of chromosomes as the original parental cell
- Cytokinesis occurs, and soon there will be two individual daughter cells, each with a nucleus that contains the diploid number of chromosomes
Mitosis in Plant Cells
- Mitosis in plant cells permits growth and repair
- Meristematic tissue, found at root tip and shoot tip of stems, retains the ability to divide throughout the life of a plant
- Lateral meristematic tissue accounts for the ability of trees to increase their girth each growing season
- Phases of mitosis in plant cells are the same as in animal cells
- Plant cells have a centrosome and spindle, but no centrioles or asters during cell division
Cytokinesis in Animal and Plant Cells
- Cytokinesis usually accompanies mitosis, but they are separate processes
- Division of cytoplasm begins in anaphase and continues in telophase
- By the time of next interphase, newly forming cells have received a share of cytoplasmic organelles that duplicated during previous interphase
- In animal cells, a cleavage furrow forms, and a band of actin filaments, called the contractile ring, slowly forms a constriction between the two daughter cells
- The action of the contractile ring can be likened to pulling a drawstring ever tighter about the middle of a balloon, causing the balloon to constrict in the middle
Meiosis
- Meiosis I:
- Crossing over occurs in prophase I
- End result of meiosis I: two haploid cells, each with a unique combination of chromosomes
- Meiosis II:
- Each haploid cell from meiosis I divides again to produce two haploid cells
- End result of meiosis II: four haploid cells, each with a unique combination of chromosomes
Laws of Heredity
- Law of Segregation:
- Each organism carries two alleles for each trait
- Alleles segregate or separate during gamete formation
- Each gamete carries only one allele for a particular trait
- Law of Independent Assortment:
- Alleles for different traits segregate independently of one another during gamete formation
- The inheritance of one trait does not influence the inheritance of another trait
- Dominant Allele:
- Expressed when present in the genotype, even if only one copy is inherited
- Represented by uppercase letters
- Recessive Allele:
- Only expressed when two copies are inherited
- Represented by lowercase letters
- Homozygous:
- When an organism carries two identical alleles for a particular trait
- Heterozygous:
- When an organism carries two different alleles for a particular trait
- Genotype:
- The genetic makeup of an organism, represented by the combination of alleles it carries
- Phenotype:
- The observable traits of an organism, resulting from the interaction between its genotype and the environment### Central Dogma of Life
- The Central Dogma of life is crucial for the functioning of every cell in our body.
- It involves the synthesis of Proteins, which depends on the code present on DNA.
Transcription
- Transcription produces an RNA copy of the information in DNA.
- The process is DNA-directed synthesis of RNA by the enzyme RNA polymerase.
- Only one of the two DNA strands needs to be copied, known as the template strand.
- The RNA transcript's sequence is complementary to the template strand.
- The strand of DNA not used as a template is called the coding strand, which has the same sequence as the RNA transcript except that U (uracil) in RNA is T (thymine) in DNA.
Translation
- Translation uses information in RNA to synthesize proteins.
- The process is more complex than transcription, requiring the participation of multiple kinds of RNA and many proteins.
- RNA has multiple roles in gene expression, including:
- Messenger RNA (mRNA): carries the DNA message to the ribosome for processing.
- Ribosomal RNA (rRNA): found in ribosomes and critical to their function.
- Transfer RNA (tRNA): the intermediary adapter molecule between mRNA and amino acids.
- Small nuclear RNA (snRNA): part of the machinery involved in nuclear processing of eukaryotic "pre-mRNA".
- SRP RNA: mediates the process of protein synthesis on the rough endoplasmic reticulum (RER) in eukaryotes.
- Small RNAs: includes micro-RNA (miRNA) and small interfering RNA (siRNA).
DNA Characteristics
- DNA is able to store information that pertains to the development, structure, and metabolic activities of the cell or organism.
- DNA is stable, allowing it to be replicated with high accuracy during cell division and transmitted from one generation to the next.
The Nature of the Genetic Material
- Frederick Griffith discovered that genetic material can be transferred between organisms, leading to the transformation of one bacterium into another.
- Alfred Hershey and Martha Chase used radioactively labeled DNA to demonstrate that DNA is the genetic material.
DNA Structure
- James Watson and Francis Crick determined the structure of DNA in the early 1950s.
- DNA is a chain of nucleotides, each consisting of phosphoric acid, a pentose sugar, and a nitrogen-containing base.
- There are four possible bases: adenine (A), guanine (G), thymine (T), and cytosine (C).
DNA Replication
- DNA replication is the process of copying one DNA double helix into two identical double helices.
- The process involves several enzymes and proteins, including:
- DNA helicase: unwinds and separates the double-stranded DNA.
- DNA polymerase: synthesizes new DNA strands by adding nucleotides to the template strands.
Gene Expression
- Gene expression is the process of using the information within a gene to synthesize a protein.
- It relies on the participation of several different forms of RNA molecules.
- Gene expression requires two main processes:
- Transcription: produces an RNA copy of the information in DNA.
- Translation: uses information in RNA to synthesize proteins.
Mutations
- A mutation is a permanent change in the nucleotide sequence of DNA.
- Mutations can arise during replication and/or recombination.
- Types of mutations include:
- Point mutations: involve changes in a single nucleotide pair.
- Gross mutations: involve changes in multiple nucleotide pairs or entire genes.
- Chromosomal mutations: involve changes in the structure or number of chromosomes.
Gene Mutations
- Gene mutations are heritable changes in the genetic coding instructions of DNA.
- They can be classified into different types based on factors such as:
- The type of cell involved (somatic or germline).
- The size and quality of the mutation (point or gross).
- The direction of the mutation (loss or gain of function).
- The type of chromosome involved.
Chromosomal Mutations
- Chromosomal mutations involve changes in the structure or number of chromosomes.
- They can be classified into different types, including:
- Structural changes: involve changes in the arrangement of genes on a chromosome.
- Changes in number: involve changes in the number of chromosomes.
Repair of DNA
- There are several mechanisms for repairing DNA damage, including:
- Base excision repair: eliminates modified bases.
- Nucleotide excision repair: repairs large-scale defects in DNA.
- Mismatch repair: corrects single mismatch base pairs.
- Double-strand break repair: repairs breaks in both strands of DNA.
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Description
Explore the cell cycle, cell growth, and cell reproduction, with a focus on cancer and uncontrolled cell growth. Learn about cell cycle checkpoints and control.
