Genetics Lecture 2: Cell Division- Mitosis & Meiosis

Prokaryote vs. Eukaryote

Cellular Organization

Cells are classified as either prokaryotic or eukaryotic based on their structural features, particularly the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotes are organisms without a cell nucleus or other membrane-bound organelles. Most prokaryotes are unicellular, though some may have multicellular stages.

• Eukaryotes are organisms with complex cells in which the genetic material is organized into a membrane-bound nucleus. Eukaryotic cells also contain other membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

Examples: Bacteria and Archaea are prokaryotes; animals, plants, fungi, and protists are eukaryotes.

Chromosomes: Structure and Properties

Physical Properties of Chromosomes

Chromosomes are thread-like structures composed of DNA and proteins, carrying genetic information. Their physical properties are important for genetic analysis and classification.

• Number: Each species has a characteristic number of chromosomes.

• Size: Chromosomes vary in length and DNA content.

• Structure: Chromosomes can be metacentric, submetacentric, acrocentric, or telocentric, depending on the position of the centromere.

• Centromere Position: The centromere is a constricted region important for chromosome movement during cell division.

• Heterochromatin Patterns: Regions of tightly packed DNA, often genetically inactive.

• Banding Patterns: Staining techniques reveal characteristic bands useful for identification.

• Nucleolar Organizers: Chromosomal regions associated with nucleolus formation.

• Chromomere Patterns: Bead-like structures visible during certain stages of cell division.

Chromosome Numbers in Organisms

Different organisms have different chromosome numbers. The diploid number (2n) refers to the total number of chromosomes in a somatic cell, while the haploid number (n) is found in gametes.

Organism	Body Cell (2n)	Gamete (n)
Human	46	23
Fruit fly (Drosophila)	8	4
Adder's tongue fern	1260	630
Additional info: Other organisms may be included in the original table, such as mouse, chimpanzee, etc.

Homologous Chromosomes and Alleles

• Homologous chromosomes are pairs of chromosomes in a diploid organism that are similar in size, shape, and banding pattern, carrying the same genes at corresponding loci.

• Allele: An allele is one of several alternative forms of a gene found at a specific locus on a chromosome.

Example: In humans, each somatic cell contains 23 pairs of homologous chromosomes (46 total).

Chromosome Structure

Duplicated Chromosomes and Chromatids

• A chromatid is one of two identical copies of DNA making up a duplicated chromosome, joined at the centromere.

• Sister chromatids are the two identical chromatids formed by DNA replication of a chromosome, held together by cohesin proteins at the centromere.

Centromere and Chromosome Arms

• The centromere is a specialized DNA sequence that links sister chromatids and is essential for proper chromosome segregation during cell division.

• Chromosomes have two arms: the p arm (short arm, from French 'petite') and the q arm (long arm).

The Cell Cycle

Phases of the Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• G1 phase: Cell grows and synthesizes proteins.

• S phase: DNA replication occurs, resulting in duplicated chromosomes.

• G2 phase: Further growth and preparation for mitosis.

• M phase (Mitosis): Division of the nucleus and cytoplasm.

• G0 phase: A resting or quiescent state where cells exit the cycle and do not divide (e.g., neurons, heart cells).

Regulation of the Cell Cycle

• The cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs).

• One key complex is MPF (M-phase promoting factor), composed of cyclin B and CDK.

Equation:

$\text{MPF} = \text{Cyclin B} + \text{CDK}$

Mitosis and Meiosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two daughter cells.

• Mitosis has five stages: prophase, prometaphase, metaphase, anaphase, and telophase.

• Distinct cytological characteristics are observed at each stage.

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing four genetically distinct gametes.

• Homologous chromosomes pair, undergo crossing over, and segregate.

• Sister chromatids remain together until meiosis II.

• Genetic recombination creates new allele combinations.

Molecular Mechanisms in the Cell Cycle

Spindle Apparatus and Microtubules

• The spindle apparatus is composed of microtubules that segregate chromosomes during cell division.

• Kinetochore microtubules attach to the kinetochore of chromosomes.

• Polar microtubules extend from opposite poles and overlap at the cell's equator.

• Astral microtubules radiate outward from the spindle poles.

Microtubule Organization

• Microtubules are dynamic polymers of tubulin, with a plus end (growing) and a minus end (anchored).

• Microtubule nucleation occurs at microtubule organizing centers (MTOCs), primarily the centrosome in animal cells.

• The centrosome contains a pair of centrioles and pericentriolar material (PCM).

Kinetochore

• The kinetochore is a protein complex assembled on the centromere, serving as the attachment site for spindle microtubules.

• It is essential for accurate chromosome segregation during mitosis and meiosis.

Separation of Sister Chromatids

• Cohesin proteins hold sister chromatids together after DNA replication.

• During anaphase, separase cleaves cohesin, allowing sister chromatids to separate.

• In meiosis I, shugoshin protects cohesin at centromeres until anaphase II.

Genetic Variation in Meiosis

Independent Assortment

• Random distribution of maternal and paternal chromosomes during meiosis I leads to genetic variation.

• In humans, independent assortment alone can produce $2^{23}$ (about 8.4 million) different gamete combinations.

Crossing Over

• Crossing over between homologous chromosomes during prophase I of meiosis creates new allele combinations within chromosomes.

• This process increases genetic diversity and helps populations adapt to changing environments.

Gametogenesis

Spermatogenesis (Male Gametogenesis)

• Occurs in the testes and produces four functional sperm cells from each primary spermatocyte.

Oogenesis (Female Gametogenesis)

• Occurs in the ovaries and produces one functional ovum and three polar bodies from each primary oocyte.

Additional info: The diagrams in the original notes likely illustrate the stages of spermatogenesis and oogenesis, showing the reduction in chromosome number and the formation of gametes.