Genetics Study Notes: Mutations, Protein Translation, Mitosis, and Meiosis

Mutations

Structure of a Nucleotide and DNA Directionality

A nucleotide, the building block of DNA, has a specific structure and orientation that is crucial for genetic processes.

• Components of a nucleotide: phosphate group, 5-carbon sugar (deoxyribose), and a nitrogenous base.

• 5' end: The phosphate group is attached to the 5' carbon of the sugar.

• 3' end: The hydroxyl (OH) group is attached to the 3' carbon of the sugar.

• DNA synthesis and reading: DNA is always synthesized and read in the 5' to 3' direction.

• Antiparallel strands: The two DNA strands run in opposite directions (one 5'→3', the other 3'→5').

Reasons for Antiparallel DNA Strands

• Hydrogen bonding: Base pairing (A with T, C with G) requires opposite orientation for proper hydrogen bond formation.

• DNA polymerase activity: DNA polymerase can only synthesize DNA in the 5'→3' direction, necessitating antiparallel strands for replication.

• Functional necessity: Without antiparallel orientation, replication and base pairing would not occur correctly.

Effects of Single Nucleotide Changes on Protein Function

Changes in a single nucleotide can have significant consequences for protein structure and function.

• Central Dogma: DNA is transcribed to mRNA, which is translated into protein.

• Codons: Every three nucleotides (codon) code for one amino acid.

• Mutation impact: A single nucleotide change can alter a codon, potentially changing the amino acid, which may affect protein folding, shape, and function.

• Genetic disease: Many genetic diseases are caused by such mutations affecting protein function.

Types of Mutations

• Silent mutation: DNA changes, but the same amino acid is encoded; no change in protein.

• Missense mutation: DNA change results in a different amino acid; protein function may be altered.

• Nonsense mutation: DNA change converts a codon to a STOP codon, producing a truncated, usually nonfunctional protein.

• Insertion: Addition of one or more nucleotides.

• Deletion: Removal of one or more nucleotides.

• Frameshift mutation: Insertion or deletion not in multiples of three shifts the reading frame, altering all downstream codons and usually severely affecting protein function.

Frameshift Mutations: Example

• Original sequence: AUG AAA GGC UUU (Met Lys Gly Phe)

• After deletion of one base: AUG AAG GCU UU... (all codons after the deletion are changed; this is a frameshift)

• Deletion of three bases: Only one amino acid is removed; the reading frame is preserved.

Protein Translation

Start Codon and Directionality

• Start codon: AUG (codes for Methionine, Met); translation always begins at AUG.

• Reading direction: Ribosome reads mRNA from 5' to 3'.

Steps of Translation

1. Initiation: Ribosome binds to mRNA, recognizes the start codon (AUG), Met-tRNA binds, followed by the second tRNA.

2. Elongation: Peptide bonds form (catalyzed by rRNA), ribosome moves forward one codon, empty tRNA exits, and the process repeats.

3. Termination: Stop codon (UAA, UAG, UGA) is reached, and the completed protein is released.

Mitosis

Stages of the Cell Cycle

• G1 phase: Cell growth and normal function.

• S phase: DNA replication; sister chromatids are formed.

• G2 phase: Further growth and preparation for division.

• M phase: Mitosis and cytokinesis.

• Interphase: Includes G1, S, and G2 phases.

Cell Cycle Checkpoints

• Definition: Control points that monitor and regulate the cell cycle.

• Functions: Check for DNA damage, complete replication, and proper chromosome alignment.

• Importance: Errors at checkpoints can lead to uncontrolled cell division and cancer.

Homologous Chromosomes vs. Sister Chromatids

• Homologous chromosomes: One from each parent, same genes but possibly different alleles; present in diploid cells.

• Sister chromatids: Identical copies of a chromosome, formed during S phase, attached at the centromere.

Centromere

• Definition: Region where sister chromatids are joined.

• Function: Site of spindle fiber attachment; essential for proper chromosome segregation.

Order of Mitosis (PMAT + Cytokinesis)

1. Prophase: Chromosomes condense, nuclear envelope breaks down.

2. Prometaphase: Spindle fibers attach, chromosomes move to the center.

3. Metaphase: Chromosomes align at the metaphase plate.

4. Anaphase: Sister chromatids separate and move to opposite poles.

5. Telophase: Nuclear envelope reforms, chromosomes decondense.

6. Cytokinesis: Cytoplasm divides, forming two identical diploid cells.

Meiosis

Overview of Meiosis

Meiosis is a two-division process that reduces chromosome number by half, producing genetically diverse gametes.

Meiosis I: Separation of Homologous Chromosomes

1. Prophase I: Homologous chromosomes pair (synapsis), crossing over occurs, tetrads form.

2. Metaphase I: Tetrads align at the center.

3. Anaphase I: Homologous chromosomes separate.

4. Telophase I: Two haploid cells form.

Meiosis II: Separation of Sister Chromatids

1. Prophase II

2. Metaphase II

3. Anaphase II: Sister chromatids separate.

4. Telophase II: Four haploid cells are produced.

End Results of Meiosis

• Four haploid (1n) cells are produced.

• Cells are genetically different due to crossing over and independent assortment.

• Homologous chromosomes separate in Anaphase I; sister chromatids separate in Anaphase II.

Haploid vs. Diploid

• Haploid (1n): One set of chromosomes; characteristic of gametes.

• Diploid (2n): Two sets of chromosomes; one from each parent; characteristic of somatic cells.

Asexual vs. Sexual Reproduction

• Asexual reproduction: One parent, mitosis, offspring are genetically identical.

• Sexual reproduction: Two parents, meiosis produces gametes (1n), fertilization restores diploid state (1n + 1n = 2n), increases genetic variation.

Crossing Over

• Occurs during Prophase I of meiosis.

• Exchange of DNA between homologous chromosomes.

• Produces recombinant chromosomes, increasing genetic diversity.

Key Differences Between Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of cells produced	2	4
Genetic identity	Identical diploid cells	Non-identical haploid cells
Crossing over	No	Yes (Prophase I)