Biology Exam 4 Review: Meiosis, Mendelian Genetics, Chromosomes & Inheritance, DNA Structure & Replication

CHAPTER 10 — MEIOSIS

Terminology

Understanding key terms is essential for mastering meiosis and inheritance.

• Heredity: The transmission of traits from parents to offspring.

• Variation: Differences among individuals in a population.

• Genetics: The scientific study of heredity and variation.

• Genes: Units of heredity made up of DNA; encode traits.

• Gametes: Reproductive cells (sperm and egg) with half the chromosome number.

• Somatic cells: All body cells except gametes; contain a full set of chromosomes.

• Homologous chromosomes: Chromosome pairs, one from each parent, with similar structure and gene content.

Sexual vs. Asexual Reproduction

Organisms reproduce in two main ways, affecting genetic diversity.

• Sexual reproduction: Involves fusion of gametes; offspring are genetically unique.

• Asexual reproduction: Offspring arise from a single parent; genetically identical to parent.

• Example: Bacteria reproduce asexually by binary fission; humans reproduce sexually.

Diploid (2n) vs. Haploid (n) Cells

Chromosome number distinguishes somatic cells from gametes.

• Diploid (2n): Cells with two sets of chromosomes (e.g., human somatic cells, 2n = 46).

• Haploid (n): Cells with one set of chromosomes (e.g., human gametes, n = 23).

Human Sexual Life Cycle and Role of Meiosis

Meiosis is central to maintaining chromosome number across generations.

• Meiosis reduces chromosome number by half, producing haploid gametes.

• Fertilization restores diploid number.

• Alternation between diploid and haploid stages is key to the life cycle.

Events of Meiosis I and II

Meiosis consists of two consecutive divisions, producing four haploid cells.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

• Stages: Prophase, Metaphase, Anaphase, Telophase (I and II).

• Example: In humans, meiosis produces sperm or eggs with 23 chromosomes.

Three Factors Contributing to Genetic Diversity

Meiosis and sexual reproduction generate variation.

• Independent assortment: Random distribution of homologous chromosomes.

• Crossing over: Exchange of genetic material between homologous chromosomes during Prophase I.

• Random fertilization: Any sperm can fertilize any egg.

CHAPTER 11 — MENDELIAN GENETICS

Why Mendel Used Pea Plants

Gregor Mendel chose pea plants for their distinct traits, ease of cultivation, and controlled mating.

• Pea plants have many observable traits.

• Self-pollination and cross-pollination are possible.

Terminology

• Dominant: Trait expressed when at least one dominant allele is present.

• Recessive: Trait expressed only when both alleles are recessive.

• Alleles: Different versions of a gene.

• Genotype: Genetic makeup (e.g., AA, Aa, aa).

• Phenotype: Observable traits (e.g., purple or white flowers).

Mendel’s P, F1, F2 Generations

Mendel tracked inheritance across generations.

• P (parental): True-breeding parents.

• F1 (first filial): Offspring of P generation; all showed dominant trait.

• F2 (second filial): Offspring of F1; showed both dominant and recessive traits.

Monohybrid Crosses and Predicting Disorders

Monohybrid crosses examine one trait; used to predict inheritance of genetic disorders.

• Cross between two heterozygotes (Aa x Aa) yields a 3:1 phenotypic ratio.

• Example: Predicting cystic fibrosis inheritance.

Dihybrid Cross Phenotypic Ratio

Dihybrid crosses involve two traits.

• Cross between two heterozygotes (AaBb x AaBb) yields a 9:3:3:1 phenotypic ratio.

Mendel’s Four Concepts

• Genes exist in pairs (alleles).

• Alleles segregate during gamete formation.

• Dominance relationships determine phenotype.

• Law of segregation: Each gamete receives one allele.

Law of Segregation

Alleles for a trait separate during gamete formation.

• Explains why offspring inherit one allele from each parent.

CHAPTER 12 — CHROMOSOMES & INHERITANCE

Terminology

• Wild-type: Most common phenotype in a population.

• Mutant: Phenotype resulting from a genetic mutation.

• Sex-linked: Genes located on sex chromosomes (X or Y).

• Linked vs. unlinked: Linked genes are inherited together; unlinked genes assort independently.

• Hemizygous: Having only one allele for a gene (e.g., males for X-linked genes).

X-linked Punnett Squares

Punnett squares are used to predict inheritance of X-linked traits.

• Females (XX) can be homozygous or heterozygous; males (XY) are hemizygous.

• Example: Color blindness inheritance.

Why X-linked Traits Affect Males

Males are more likely to express X-linked recessive traits.

• Males have only one X chromosome; any recessive allele is expressed.

• Females require two copies of the recessive allele.

Nondisjunction, Aneuploidy, Polyploidy

Errors in chromosome separation can lead to abnormal chromosome numbers.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis.

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy 21).

• Polyploidy: More than two complete sets of chromosomes (common in plants).

Chromosome Structure Alterations

Structural changes can affect gene function.

• Deletions, duplications, inversions, translocations.

• Example: Cri du chat syndrome (deletion on chromosome 5).

CHAPTER 13 — DNA STRUCTURE & REPLICATION

DNA Structure and Function

DNA is the hereditary material in cells, encoding genetic information.

• Double helix structure discovered by Watson and Crick.

• Stores, replicates, and transmits genetic information.

Nucleotide Components, Base Pairing, Purines vs. Pyrimidines

Nucleotides are the building blocks of DNA.

• Nucleotide: Consists of a phosphate group, deoxyribose sugar, and nitrogenous base.

• Base pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Purines: A and G; double-ring structure.

• Pyrimidines: C and T; single-ring structure.

DNA vs. RNA; 5' vs. 3' Ends

DNA and RNA differ in structure and function.

• DNA: Double-stranded, deoxyribose sugar, bases A, T, G, C.

• RNA: Single-stranded, ribose sugar, bases A, U, G, C.

• 5' end: Phosphate group attached to the fifth carbon of sugar.

• 3' end: Hydroxyl group attached to the third carbon of sugar.

DNA Replication Enzymes; Leading vs. Lagging Strands

DNA replication is a complex process involving several enzymes.

• Helicase: Unwinds DNA.

• Primase: Synthesizes RNA primer.

• DNA polymerase: Adds nucleotides to growing DNA strand.

• Ligase: Joins Okazaki fragments on lagging strand.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized in short fragments (Okazaki fragments).

Semiconservative Replication

Each new DNA molecule contains one old and one new strand.

• Ensures accurate transmission of genetic information.

• Equation:

$\text{Parent DNA} \rightarrow \text{Daughter DNA:} \text{one old strand} + \text{one new strand}$

Practice DNA Replication Example

Given a DNA template, predict the complementary strand.

• Example: Template: 5'-ATGC-3' → Complement: 3'-TACG-5'