Cell Cycle, Meiosis, and Patterns of Inheritance: Study Notes

Chapter 12 – The Cell Cycle & Mitosis

Ploidy: Haploid vs. Diploid

The concept of ploidy refers to the number of sets of chromosomes in a cell. Understanding ploidy is essential for distinguishing between different cell types and their roles in reproduction and development.

• Haploid (n): Cells with one set of chromosomes (e.g., gametes in animals).

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells in animals).

• Example: Human somatic cells are diploid (2n = 46), while human gametes are haploid (n = 23).

Genetic Structure of Cells & Chromosomes

Genetic information is organized into structures called chromosomes, which are composed of DNA and proteins. The total genetic content of a cell is its genome.

• Chromosome: A single DNA molecule with associated proteins, carrying genetic information.

• Genome: The complete set of genes or genetic material in a cell or organism.

• Chromosome Number: Characteristic for each species (e.g., humans have 46 chromosomes).

The Cell Cycle: Interphase and Mitotic Phase

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (growth and DNA replication) and the mitotic phase (division of the nucleus and cytoplasm).

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

• Mitotic (M) Phase: Includes mitosis and cytokinesis.

Phases of Interphase

• G1 Phase (First Gap): Cell grows and carries out normal functions.

• S Phase (Synthesis): DNA is replicated (chromosome duplication).

• G2 Phase (Second Gap): Cell prepares for division.

Chromosome Duplication

• During the S phase, each chromosome is replicated to produce two sister chromatids joined at the centromere.

The Process of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical nuclei. (You do not need to memorize the names of the phases for this course.)

• Purpose: To produce genetically identical daughter cells for growth, repair, and asexual reproduction.

• Key Steps: Chromosome condensation, alignment, separation of sister chromatids, and division of the nucleus.

Chapter 13 – Meiosis

Homologous Chromosomes vs. Sister Chromatids

Understanding the difference between homologous chromosomes and sister chromatids is crucial for grasping the mechanics of meiosis.

• Homologous Chromosomes: Chromosome pairs (one from each parent) that are similar in size, shape, and gene content.

• Sister Chromatids: Identical copies of a single chromosome, produced during DNA replication and joined at the centromere.

The Process of Meiosis

Meiosis is a two-part cell division process that reduces the chromosome number by half, producing four haploid cells (gametes). It increases genetic diversity through recombination and independent assortment.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

• Key Difference: Meiosis I separates homologs; Meiosis II separates chromatids.

Genetic Variation in Meiosis

• Independent Assortment: Random orientation of homologous pairs during Meiosis I leads to genetic variation in gametes.

• Crossing Over: Exchange of genetic material between non-sister chromatids during prophase I, creating new allele combinations.

• Sources of Genetic Variability: Independent assortment, crossing over, and random fertilization.

Chapter 14 – Mendelian and Complex Inheritance

The DNA Basis of Alleles

Alleles are different versions of a gene, arising from variations in DNA sequence. They determine the traits expressed in an organism.

Mendel's Laws

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, leading to genetic variation.

Genetic Terminology

• Character: A heritable feature (e.g., flower color).

• Trait: A variant of a character (e.g., purple or white flowers).

• P Generation: Parental generation in a genetic cross.

• Fn Generation: Filial generations (offspring of the parental generation and subsequent generations).

Complex Patterns of Inheritance

• Multiple Alleles: More than two possible alleles exist for a gene (e.g., ABO blood types).

• Codominance: Both alleles are fully expressed in the phenotype (e.g., AB blood type).

• Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., red x white flowers produce pink offspring).

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers).

Punnett Squares

• Punnett squares are used to predict the probability of offspring genotypes and phenotypes from parental crosses.

• Example: Crossing two heterozygotes (Aa x Aa) for a single gene:

$\begin{array}{c|cc} & A & a \\ \hline A & AA & Aa \\ a & Aa & aa \\ \end{array}$

Chapter 15 – Chromosomal Inheritance

Chromosomal Basis of Inheritance

Genes are located on chromosomes, and their behavior during meiosis explains Mendel's laws. This is known as the chromosomal theory of inheritance.

Sex-Linked Inheritance

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

• Not all genes on the X chromosome are related to sex determination.

• All three genes for color vision are located on the X chromosome.

• X Inactivation: In females, one X chromosome is randomly inactivated in each cell, forming a Barr body.

Gene Linkage

• Genes located close together on the same chromosome tend to be inherited together; this is called linkage.

• Locus: The specific physical location of a gene on a chromosome.

Alterations of Chromosome Number and Structure

• Polysomy: The presence of more than two chromosomes in a set (e.g., trisomy 21 causes Down syndrome).

• Structural Changes: Deletions, insertions, inversions, and translocations can alter chromosome structure and affect gene function.

Type of Chromosomal Alteration	Description	Example
Deletion	Loss of a chromosome segment	Cri du chat syndrome
Insertion	Addition of a chromosome segment	Additional info: Can disrupt gene function
Inversion	Reversal of a chromosome segment	May affect gene regulation
Translocation	Segment moves to a non-homologous chromosome	Chronic myelogenous leukemia (CML)

Additional info: These notes provide a concise overview of key concepts in cell division and inheritance, suitable for exam preparation in introductory biology courses.