Meiosis, Genetics, and DNA: Study Guide for BIOL 1201 Exam 3

Meiosis and Sexual Reproduction

Sexual vs. Asexual Reproduction

Reproduction is fundamental to biology, and organisms can reproduce sexually or asexually. Sexual reproduction involves the fusion of gametes, resulting in offspring with genetic variation, while asexual reproduction produces genetically identical clones.

• Sexual Reproduction: Fusion of sperm and egg (gametes) forms a zygote; offspring inherit a unique combination of genes.

• Asexual Reproduction: Single parent produces clones; variation arises only from mutations.

• Example: Humans reproduce sexually; bacteria reproduce asexually by binary fission.

Chromosomes and the Human Genome

Chromosomes are structures that house genetic information. Humans have 46 chromosomes (23 pairs), including autosomes and sex chromosomes.

• Homologous Chromosomes: Pairs with same length and gene loci, but may carry different alleles.

• Karyotype: Visual display of chromosomes arranged by size.

• Diploid (2n): Two sets of chromosomes; somatic cells.

• Haploid (n): One set; gametes.

• Autosomes: 22 pairs; Sex Chromosomes: 1 pair (XX or XY).

Life Cycles and Chromosome Number

Organisms maintain chromosome number through fertilization, meiosis, and mitosis. Life cycles differ among animals, plants, fungi, and protists.

• Animal Life Cycle: Only gametes are haploid; fertilization restores diploid state.

• Plant/Algae Life Cycle: Alternation of generations; both multicellular haploid (gametophyte) and diploid (sporophyte) stages.

• Fungi/Protist Life Cycle: Multicellular haploid stage; diploid zygote undergoes meiosis.

Meiosis Overview

Meiosis is a two-part cell division process that produces four genetically unique haploid cells. It consists of Meiosis I (separates homologs) and Meiosis II (separates sister chromatids).

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

• Result: Four haploid cells.

Stages of Meiosis

Each stage of meiosis contributes to genetic variation and chromosome reduction.

• Prophase I: Synapsis and crossing over occur.

• Metaphase I: Homologous pairs align; independent assortment.

• Anaphase I: Homologs separate; sister chromatids remain attached.

• Telophase I & Cytokinesis: Two haploid cells form.

• Meiosis II: Similar to mitosis; chromatids separate.

Genetic Variation in Meiosis

Meiosis generates genetic diversity through three mechanisms:

1. Independent Assortment: Random orientation of homologs during Metaphase I. Formula: $2^n$ possible combinations (n = haploid number).

2. Crossing Over: Exchange of genetic material between homologs in Prophase I.

3. Random Fertilization: Any sperm can fertilize any egg, multiplying possible combinations.

Comparing Mitosis vs. Meiosis

Mitosis and meiosis differ in purpose, division number, genetic outcome, and chromosome behavior.

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Gamete formation
Divisions	One	Two
Resulting Cells	Two diploid (2n)	Four haploid (n)
Genetic Identity	Identical	Unique
Chromosome Number	Conserved	Reduced by half
Unique Events	No synapsis/crossing over	Synapsis, crossing over
Homologs	Do not pair	Pair in Prophase I

Genetics & Inheritance

Mendelian Genetics

Gregor Mendel's experiments with pea plants established the laws of inheritance. Traits are inherited as discrete units (genes), not blended.

• Law of Segregation: Alleles separate during gamete formation.

• Law of Independent Assortment: Alleles for different traits assort independently.

• Alleles: Alternate versions of a gene.

• Dominance: Dominant allele masks recessive allele.

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

Punnett Squares and Probability

Punnett squares visualize genetic crosses. Probability rules help predict inheritance outcomes.

• Multiplication Rule: Probability of independent events occurring together.

• Addition Rule: Probability of mutually exclusive events.

Pedigrees and Human Genetics

Pedigrees track inheritance of traits across generations. Symbols indicate affected, unaffected, and carrier individuals.

Genetic Disorders

• Recessive Disorders: Two copies of recessive allele required (e.g., albinism).

• Dominant Disorders: One copy of dominant allele required (e.g., Huntington's disease).

• Multifactorial Disorders: Influenced by genetics and environment (e.g., heart disease).

Chromosomal Theory of Inheritance

Genes and Chromosomes

Genes are located on chromosomes. Chromosome movement during meiosis explains Mendel's laws.

• Diploid organisms: Homologous pairs; alleles segregate during meiosis.

• Thomas Hunt Morgan: Fruit fly experiments proved genes are on chromosomes.

Sex-Linked Inheritance

Traits linked to sex chromosomes show different inheritance patterns in males and females.

• XY System: Males (XY), females (XX).

• ZW System: Females (ZW), males (ZZ).

• Haplo-diploid System: Females diploid, males haploid.

X-Linked Genes and Disorders

• Hemizygous males: Express recessive X-linked traits (e.g., color blindness).

• X-Chromosome Inactivation: One X in females is inactivated (Barr body).

Linked Genes and Genetic Mapping

Genes close together on a chromosome are inherited together (linkage). Recombination frequency is used to map gene distances.

Chromosomal Alterations

• Nondisjunction: Failure to separate chromosomes; leads to aneuploidy.

• Structural Changes: Deletion, duplication, inversion, translocation.

DNA as the Genetic Material

Discovery of DNA as Genetic Material

Experiments by Griffith, Avery-MacLeod-McCarty, and Hershey-Chase established DNA as the molecule of heredity.

DNA Structure

• Double Helix: Two antiparallel strands; sugar-phosphate backbone; base pairs (A-T, G-C).

• Chargaff's Rules: A=T, G=C.

• Watson and Crick: Double helix model.

DNA Replication

DNA replication is semi-conservative, producing two molecules each with one old and one new strand. Replication involves several enzymes and occurs in the 5' to 3' direction.

• Helicase: Unwinds DNA.

• Single-strand binding proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves strain.

• Primase: Synthesizes RNA primer.

• DNA Polymerase III: Adds nucleotides.

• DNA Polymerase I: Removes primer, fills gaps.

• DNA Ligase: Seals fragments.

Leading vs. Lagging Strands

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized in Okazaki fragments.

DNA Damage and Mutations

• Mutations: Changes in DNA sequence; source of genetic variation.

• DNA Repair: Mechanisms fix errors (e.g., nucleotide excision repair).

Telomeres

• Telomeres: Repetitive sequences at chromosome ends; protect genes.

• Telomerase: Enzyme that lengthens telomeres.

DNA Packaging

• Prokaryotes: Circular chromosome; supercoiled.

• Eukaryotes: Linear DNA; wrapped around histones (nucleosomes).

• Euchromatin: Loosely packed; active.

• Heterochromatin: Tightly packed; inactive.

Gene Expression: Transcription and Translation

The Central Dogma

Genetic information flows from DNA to RNA to protein. Transcription copies DNA into mRNA; translation synthesizes protein from mRNA.

• Transcription: DNA to mRNA.

• Translation: mRNA to protein.

Transcription and Translation Overview

• Template Strand: Used to synthesize mRNA.

• Coding Strand: Sequence matches mRNA (except T replaced by U).

• Triplet Code: mRNA read in codons (three bases).

Stages of Transcription

• Initiation: RNA polymerase binds promoter.

• Elongation: RNA polymerase synthesizes RNA.

• Termination: RNA polymerase releases RNA at terminator.

Eukaryotic RNA Processing

• 5' Cap: Modified guanine added.

• Poly-A Tail: Adenine nucleotides added.

• RNA Splicing: Introns removed, exons joined; alternative splicing allows multiple proteins from one gene.

Stages of Translation

• Initiation: Ribosome binds mRNA; initiator tRNA finds start codon.

• Elongation: Amino acids added; tRNA and ribosome facilitate codon recognition and peptide bond formation.

• Termination: Stop codon reached; polypeptide released.

Post-Translational Modifications and Protein Targeting

• Modifications: Glycosylation, methylation, etc.

• Protein Targeting: Signal peptides direct proteins to specific cellular locations.

Mutations and Their Effects

• Point Mutations: Single base changes; can be silent, missense, or nonsense.

• Frameshift Mutations: Insertions/deletions alter reading frame.

• Mutagens: Physical/chemical agents causing mutations.

Summary Table: Types of Mutations

Type	Effect	Example
Silent	No change in amino acid	Redundancy in code
Missense	Change in amino acid	Sickle cell anemia
Nonsense	Premature stop codon	Truncated protein
Frameshift	Altered reading frame	Disastrous protein