BIO121 Exam 2

1. Meiosis I and Ploidy Reduction

1. Reduction in Ploidy during Meiosis I

   • Meiosis I involves the segregation of homologous chromosomes. During Anaphase I, homologous chromosomes (each consisting of two sister chromatids) separate and move to opposite poles of the cell, rather than sister chromatids separating as in mitosis. Each daughter cell receives one chromosome from each homologous pair.

   • This segregation reduces the chromosome number from diploid ($2n$) to haploid ($n$). Although each chromosome still consists of two sister chromatids, the genetic information per cell is effectively halved in terms of unique chromosome sets.

2. Genetic Variation in Haploid Cells

   • No two haploid cells resulting from meiosis are alike in terms of genotype due to two main processes:

     • Crossing Over: Occurs during Prophase I, where homologous chromosomes exchange segments of DNA, leading to new combinations of alleles on the chromatids.

     • Independent Assortment: Occurs during Metaphase I, where homologous pairs orient randomly at the metaphase plate. The orientation of one pair is independent of the orientation of other pairs, leading to different combinations of maternal and paternal chromosomes in the resulting daughter cells.

   • Importance for Offspring Fitness: This genetic variation is crucial for the evolutionary success and fitness of offspring. It increases the genetic diversity within a population, providing a wider range of phenotypes. This diversity allows populations to adapt to changing environments, as some individuals with beneficial new gene combinations may be better suited for survival and reproduction.

2. Chromosome Behavior in Meiosis

When observing a cell undergoing meiosis, the following events occur to the chromosomes:

• Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis) to form bivalents, and crossing over occurs between non-sister chromatids. The nuclear envelope begins to break down.

• Metaphase I: Homologous pairs (bivalents) align randomly (independent assortment) at the metaphase plate.

• Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell. Sister chromatids remain attached.

• Telophase I: Chromosomes decondense, the nuclear envelope reforms (in some species), and cytokinesis usually follows, resulting in two haploid cells, each with chromosomes still composed of two sister chromatids.

• Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II): Similar to mitosis, sister chromatids separate, resulting in four haploid cells, each containing single chromosomes.

3. Mendel's Principles and Meiosis

1. Principle of Segregation

   • Meiotic Event: The segregation of homologous chromosomes during Anaphase I explains Mendel's principle of segregation. Each homologous chromosome carries one allele for a gene. When homologous chromosomes separate, the two alleles for a given gene also separate, ensuring that each gamete receives only one allele.

   • Reasoning: If a parent has genotype Aa, the homologous chromosome carrying allele A moves to one pole, and the homologous chromosome carrying allele a moves to the opposite pole. Therefore, $50\%$ of gametes will receive A and $50\%$ will receive a.

2. Principle of Independent Assortment

   • Meiotic Event: The random orientation of homologous pairs at the metaphase plate during Metaphase I explains independent assortment. Genes located on different chromosomes assort independently because the alignment of one homologous pair is independent of the alignment of other pairs.

   • Reasoning and Genetic Variation: Consider two genes, A/a and B/b, located on different chromosomes. During Metaphase I, the homologous pairs can align in two equally probable ways:

     1. Pair A/A aligns with B/B leading to gametes AB and ab.

     2. Pair A/A aligns with b/b leading to gametes Ab and aB.

        This random alignment creates four possible combinations of alleles (AB, Ab, aB, ab) in equal proportions, generating significant genetic variation in the offspring. Each gamete receives a unique combination of alleles for genes located on non-homologous chromosomes.

4. Punnett Squares and Genetic Crosses

1. Elements of a Punnett Square

   • The alleles listed horizontally across the top represent the possible sperm genotypes (or pollen in plants).

   • The alleles listed vertically down the left side represent the possible egg genotypes (or ovules).

   • The combinations within the inner squares represent the possible offspring genotypes.

2. Determining Frequencies

   • Egg and Sperm Genotype Frequencies: These are determined by the principle of segregation. For a parent with genotype Aa, $50\%$ of gametes will be A and $50\%$ will be a. For a parent with AABB genotype, all gametes will be AB. For unlinked genes, the product rule can be used (e.g., for AaBb, $1/2 A \times 1/2 B = 1/4 AB$).

   • Offspring Genotype and Phenotype Frequencies: Count the occurrences of each genotype within the Punnett square and divide by the total number of squares (e.g., $1/4 AA$). Phenotype frequencies are determined by summing the frequencies of genotypes that express the same phenotype (e.g., for dominant A, genotypes AA and Aa both show dominant phenotype).

3. Completing Genetic Crosses

   • Steps: Construct a Punnett square with parental gametes. Fill in squares to determine offspring genotypes. Translate genotypes to phenotypes based on allele information (dominant/recessive, codominant). Calculate frequencies by counting.

   • Types of Crosses: Punnett squares can accommodate:

     • Autosomal: Genes on non-sex chromosomes.

     • X-linked: Genes on the X chromosome (e.g., recessive X-linked traits are more common in males).

     • Linked: Genes on the same chromosome that are close together and tend to be inherited together. Punnett squares can be adjusted to reflect recombinant frequencies.

     • Unlinked: Genes on different chromosomes or far apart on the same chromosome, assort independently.

     • Alleles: Dominant, recessive, codominant (both alleles expressed, e.g., $R<em>1R</em>2$ produces unique phenotype).

5. Determining Allele Characteristics from Phenotypes

1. Dominant, Recessive, or Codominant

   • Dominant/Recessive: If one trait masks another in heterozygotes (e.g., $F_1$ generation of purebred cross all show one phenotype), the expressed trait is dominant.

   • Codominant: If heterozygotes display both parental phenotypes simultaneously, or an intermediate phenotype that is distinct from either parent.

2. Autosomal or X-linked

   • X-linked traits often show different inheritance patterns in males and females. Males (XY) are hemizygous for X-linked genes, meaning they express whatever allele is on their single X chromosome. Affected fathers cannot pass X-linked traits to their sons. X-linked recessive traits typically appear more often in males and can skip generations.

   • Autosomal traits affect males and females roughly equally and do not show these sex-specific patterns.

3. Linked or Unlinked

   • Unlinked genes assort independently, producing gametes in frequencies that follow Mendel's laws (e.g., for a dihybrid cross of AaBb x aabb, expected offspring phenotypes are $1:1:1:1$).

   • Linked genes on the same chromosome will tend to be inherited together. Deviations from expected Mendelian ratios (e.g., fewer recombinants than expected for unlinked genes) suggest linkage. The frequency of recombinant offspring indicates the distance between linked genes on a chromosome.

6. Mutation Impact Ranking

Ranking of mutations from greatest to least impact on gene structure and function:

1. Frameshift (greatest impact): Insertion or deletion of nucleotides not in multiples of three. This changes the reading frame downstream, altering every subsequent codon and typically leading to a completely different, non-functional protein or a premature stop codon.

2. Nonsense: Changes an amino acid codon into a premature stop codon. This results in a truncated protein, which is usually non-functional because it is incomplete.

3. Missense: Changes one nucleotide, resulting in a codon that codes for a different amino acid. The impact varies; it can be mild if the new amino acid is chemically similar or maintains protein function, or severe if it alters a critical region or changes protein folding.

4. Silent (least impact): Changes a nucleotide but does not change the amino acid coded due to the degeneracy of the genetic code. This typically has no impact on the protein product or its function.

Reasoning: The impact largely depends on how drastically the protein’s primary structure is altered. Frameshift and nonsense mutations fundamentally change or prematurely truncate the protein, almost always leading to loss of function. Missense mutations can have variable effects, while silent mutations usually have none.

7. Mutation as the Ultimate Source of Genetic Variation

1. Mutation as the Ultimate Source: Mutations are random, heritable changes in the DNA sequence. They introduce new alleles into a population's gene pool that did not exist before. Other processes like recombination (crossing over and independent assortment) rearrange existing genetic variation, but they do not create new variants.

2. Randomness of Mutation: Mutations are random with respect to their impact on an individual's fitness. The occurrence of a mutation is independent of whether it will be beneficial, neutral, or detrimental in a given environment. Environmental pressures do not direct mutations to occur in a way that would be adaptive. Instead, mutations arise spontaneously, and then natural selection acts upon any advantageous mutations that happen to occur.

8. Consequences of Altering DNA Coding Strand / Transcription / Translation

1. DNA Coding Strand Change (e.g., point mutation in coding region):

   • Silent Mutation: Change in a nucleotide that does not alter the amino acid sequence due to the degeneracy of the genetic code. Consequence: No change in the protein product.

   • Missense Mutation: Change in a nucleotide that results in a codon specifying a different amino acid. Consequence: Protein may have altered function, reduced function, or no change depending on the new amino acid's properties and location.

   • Nonsense Mutation: Change in a nucleotide that results in a premature stop codon. Consequence: Truncated, likely non-functional protein.

   • Frameshift Mutation (insertion/deletion not in multiples of 3): Shifts the reading frame. Consequence: Completely altered downstream amino acid sequence, usually leading to non-functional protein or premature termination.

2. Error in Transcription (e.g., incorrect mRNA nucleotide):

   • An incorrect nucleotide incorporated into mRNA during transcription. Consequence: If the mRNA is then translated, it can lead to a codon change, potentially resulting in a missense, nonsense, or silent mutation in the protein product. The original DNA template, however, remains unchanged, so subsequent transcripts may be correct.

3. Error in Translation (e.g., tRNA carrying wrong amino acid):

   • A tRNA molecule binds to a codon but carries an incorrect amino acid. Consequence: The protein will incorporate the wrong amino acid at that position. This could alter protein structure and function, similar to a missense mutation, but it is an error in protein synthesis, not in the genetic code itself.

9. Eukaryotic Cell Cycle Stages

The eukaryotic cell cycle is divided into four main stages:

1. M (Mitotic) Phase: Occurs after interphase.

   • Major Events: Mitosis (nuclear division) and cytokinesis (cytoplasmic division). Sister chromatids separate (Anaphase) and the cell divides into two daughter cells.

2. G1 (First Gap) Phase: Occurs after M phase and before S phase.

   • Major Events: Cell growth, synthesis of proteins and organelles, and preparation for DNA replication. The cell monitors its internal and external environment to decide if it should divide.

3. S (Synthesis) Phase: Occurs after G1 phase.

   • Major Events: DNA replication. Each chromosome is duplicated, resulting in two identical sister chromatids.

4. G2 (Second Gap) Phase: Occurs after S phase and before M phase.

   • Major Events: Cell continues to grow and synthesizes proteins necessary for mitosis, such as microtubules. The cell checks for DNA damage and ensures all chromosomes have been replicated correctly.

10. Cancer and Cell Cycle Regulation

1. Cancer Associated with Mutations Regulating the Cell Cycle:

   • Cancer is fundamentally a disease of uncontrolled cell division. The cell cycle is tightly regulated by a series of checkpoints involving proteins (e.g., cyclins and cyclin-dependent kinases, tumor suppressor genes like p53, and proto-oncogenes).

   • Mutations in proto-oncogenes can turn them into oncogenes, which promote cell division without proper signals (e.g., mutations in RAS leading to constant cell proliferation).

   • Mutations in tumor suppressor genes (e.g., p53 or Rb) can remove the 'brakes' on cell division, allowing damaged cells to proliferate or cells to bypass checkpoints (e.g., loss of p53 function allows cells with damaged DNA to continue dividing).

   • Accumulation of such mutations disrupts the normal regulatory mechanisms, leading to unchecked cell growth and division, forming tumors.

2. Cancer More Common in Older than Younger People:

   • Cancer is largely a multi-step process, requiring the accumulation of several mutations in cell cycle regulatory genes. Each mutation moves the cell closer to becoming cancerous.

   • Over a person's lifetime, cells are exposed to various mutagens (e.g., UV radiation, chemicals, errors during DNA replication). The longer an individual lives, the more opportunities there are for these mutations to occur and accumulate in somatic cells.

   • Therefore, older individuals have had more time for the critical number of mutations to occur in a single cell lineage, leading to the development of cancer.

11. Consequences of Altering Cell Cycle Stages

1. Altering M (Mitotic) Phase: If M phase is disrupted (e.g., by drugs preventing spindle formation like colchicine), chromosomes may not segregate properly, leading to aneuploidy (cells with an abnormal number of chromosomes) or arrest at metaphase. Consequence: Cell death or formation of polyploid cells; ineffective cell division.

2. Altering G1 (First Gap) Phase: If G1 is prolonged or cells are permanently arrested in G1 (G0 phase), they will not divide. Conversely, if G1 checkpoints are bypassed, cells may enter S phase without proper signals or preparation. Consequence: No entry into S phase if prolonged G1/G0; uncontrolled replication if G1 checkpoint is bypassed (a hallmark of cancer).

3. Altering S (Synthesis) Phase: Incomplete or erroneous DNA replication during S phase. Consequence: DNA damage accumulates, potentially leading to cell cycle arrest in G2, apoptosis, or mutations being passed on to daughter cells, increasing cancer risk.

4. Altering G2 (Second Gap) Phase: If G2 checkpoints are bypassed, cells may enter M phase with damaged or incompletely replicated DNA. Consequence: Entry into mitosis with errors, leading to chromosome abnormalities in daughter cells or cell death. Proper G2 function ensures genomic integrity before division.

12. Flow of Information: Gene to Protein (Central Dogma)

graph LR
    DNA -- Transcription --> mRNA
    mRNA -- Translation --> Protein

• DNA $\rightarrow$ mRNA (Transcription): This arrow represents transcription, the process where the genetic information encoded in a DNA sequence is copied into a complementary mRNA molecule. RNA polymerase synthesizes mRNA from a DNA template.

• mRNA $\rightarrow$ Protein (Translation): This arrow represents translation, the process where the genetic information carried by mRNA is decoded by ribosomes to synthesize proteins. tRNA molecules bring specific amino acids to the ribosome, matching them to codons on the mRNA sequence.

13. Mutation, Heritable Variation, and Selection

1. Connection between Mutation and Heritable Variation: Mutations are changes in the DNA sequence. If these mutations occur in germline cells (sperm or egg), they are heritable, meaning they can be passed on to offspring. These heritable mutations create new alleles, which are different versions of a gene. This introduction of new alleles is the ultimate source of all heritable genetic variation within a population. Without mutation, all individuals would be genetically identical (barring rare recombination events if variation already exists), and there would be no raw material for evolution.

2. How Selection Leads to Changes in Allele Frequencies: Once heritable variation (new alleles) arises through mutation, natural selection can act upon it. Individuals with certain alleles may possess traits (phenotypes) that provide a survival or reproductive advantage in a particular environment. These individuals are more likely to survive, reproduce, and pass on their advantageous alleles to the next generation. Over many generations, the frequency of these advantageous alleles will increase in the population, while less advantageous alleles will decrease. This differential survival and reproduction based on heritable traits, driven by environmental pressures, leads to evolutionary change and adaptation.

14. Experiment Design: Evolution by Natural Selection

Scenario: Investigating the evolution of antibiotic resistance in bacteria.

• Model Organism: Escherichia coli (E. coli), a common bacterium.

• Treatment: Culture E. coli in a medium containing a sub-lethal concentration of an antibiotic (e.g., ampicillin).

• Control Conditions: Culture E. coli in an identical medium without the antibiotic.

• Outcome Variable Measured: The optical density (proxy for bacterial population size) over time, and the minimum inhibitory concentration (MIC) of the antibiotic required to inhibit growth of the bacterial population at regular intervals across generations.

Interpreting Outcomes / Predicted Outcomes Graph:

• Control (No Antibiotic):

  • E. coli population will grow robustly and maintain a high optical density. The MIC for the antibiotic will remain relatively low and constant across generations, as there is no selective pressure for resistance.

• Treatment (With Antibiotic):

  • Initially, the E. coli population may show a dip or slower growth due to the presence of the antibiotic. However, within several generations, the population will likely rebound and demonstrate robust growth. The MIC for the antibiotic will progressively increase over generations, indicating that the population is evolving increased resistance to the antibiotic.

Graphical Representation (Predicted):

• Y-axis: MIC (micrograms/mL for ampicillin).

• X-axis: Generations (or Time).

• Control Group (blue line): Will show a flat or slightly fluctuating line at a low MIC value.

• Treatment Group (red line): Will show a steadily increasing line, indicating an increase in MIC over generations, plateauing as resistance becomes widespread or reaches its maximum.

Explanation: In the antibiotic treatment, individuals with pre-existing mutations conferring even slight resistance will have a survival advantage. These resistant individuals will reproduce more successfully, passing on their resistance alleles. Over generations, these alleles will become more common, and further mutations may accumulate, leading to higher levels of resistance in the population. The