Chromosome Transmission During Cell Division and Sexual Reproduction

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Chromosome Transmission During Cell Division and Sexual Reproduction

Types of Animal Cells

Animal cells can be classified based on their role in reproduction and body function.

Somatic cells: Body cells other than gametes (e.g., blood cells).
Germ cells: Gametes, including sperm and egg cells.

Most eukaryotic species are diploid, meaning they have two sets of chromosomes. For example, humans have 46 total chromosomes (23 per set).

Eukaryotic Chromosomes and Inheritance

Chromosomes in eukaryotes are inherited in sets, with each member of a pair called a homolog. Homologous chromosomes:

Form homologous pairs
Are nearly identical in size
Have the same banding pattern and centromere location
Contain the same genes, but not necessarily the same alleles

The locus (plural: loci) is the physical location of a gene on a chromosome.

Genotype examples:
- AA: Homozygous for the dominant allele
- Bb: Heterozygous
- cc: Homozygous for the recessive allele

Cell Division

Purpose of Cell Division

Cell division serves two main purposes:

Asexual reproduction
Multicellularity (growth and development from a single fertilized egg to an adult with trillions of cells)

The Eukaryotic Cell Cycle

Eukaryotic cells destined to divide progress through the cell cycle, which consists of several stages:

G1 phase: Cell prepares to divide, reaches a restriction point, and commits to cell division.
S phase: Chromosomes are replicated, forming pairs of sister chromatids.
G2 phase: Cell accumulates materials for nuclear and cell division, then enters the M phase.
M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasm division).

Chromosomes Following DNA Replication

After replication, each chromosome consists of two sister chromatids joined at the centromere.
One chromatid is called a monad; a pair is a dyad.

Mitosis

Overview of Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.

In humans: 46 pairs of sister chromatids are separated and sorted, resulting in two daughter cells with 46 chromosomes each.

Phases of Mitosis

Prophase
Prometaphase
Metaphase
Anaphase
Telophase

Structure of the Mitotic Spindle

The mitotic spindle is composed of three types of microtubules:

Aster microtubules: Position the spindle apparatus.
Polar microtubules: Push the poles away from each other.
Kinetochore microtubules: Attach to the kinetochore at the centromere of each chromosome.

Outcome of Mitotic Cell Division

Produces two daughter cells with the same chromosome number as the mother cell.
Daughter cells are genetically identical (except for rare mutations).
Ensures genetic consistency, crucial for multicellularity.

Cell-Cycle Checkpoints and Control

Cell-cycle checkpoints ensure that all events of a particular stage are completed before the next stage begins. If a gene encoding a checkpoint molecule is defective, diseases such as cancer can result.

Progression through the cell cycle requires activated cyclin/CDK complexes.
CDK: Cyclin-dependent kinases
p53: Example of a checkpoint protein
Checkpoint proteins can halt the cycle if DNA is damaged or chromosomes are misaligned.

Meiosis

Overview of Meiosis

Meiosis is the process by which diploid cells produce haploid gametes for sexual reproduction. It involves two successive divisions (Meiosis I and II) to reduce the chromosome number by half.

Parents are diploid (2n); gametes are haploid (n).
In humans: 2n = 46 chromosomes; n = 23 chromosomes.
Gametes fuse during fertilization to restore diploidy.

Phases and Stages of Meiosis

Meiosis begins after a cell has completed interphase.
Each meiotic division includes:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
Meiosis I: Homologous chromosomes separate.
Meiosis II: Sister chromatids separate.

Nuclear Contents Through Meiosis

Stage	Number of Chromosomes	Chromatids/Equivalent
G1	46	46
S	46	92
Meiosis I	23	46
Meiosis II	23	23

Key Events of Meiosis I

Synapsis: Homologous chromosomes pair up via the synaptonemal complex.
Tetrads: Four chromatids (two homologs) align together.
Genetic recombination/crossing over: Exchange of genetic material between homologs.
Bivalents: Paired homologous chromosomes.
Chiasma: Site of crossing over.
Random alignment: Homologous pairs align randomly at the metaphase plate.
Reductional division: Chromosome number is halved.

Stages of Meiosis

Pre-Meiotic Interphase: G1, S, G2
Prophase I: Leptotene, Zygonema, Pachynema, Diplonema, Diakinesis
Metaphase I, Anaphase I, Telophase I
Interkinesis
Prophase II, Metaphase II, Anaphase II, Telophase II

Stages of Prophase I

Zygotene: Synapsis begins, homologs align.
Pachytene: Chiasma forms, crossing over occurs (physical exchange between homologs).

Types of Gametes

Isogamous species: Produce morphologically similar gametes (e.g., many fungi and algae).
Heterogamous species: Produce distinct sperm (small, mobile) and egg cells (large, nonmotile, nutrient-rich).

Gametogenesis

Spermatogenesis

The production of sperm in male animals occurs in the testes. Each primary spermatocyte undergoes meiosis to produce four haploid sperm cells.

Oogenesis

The production of egg cells in female animals occurs in the ovaries. Early in development, diploid oogonia produce diploid primary oocytes, which initiate meiosis I but are arrested in prophase I until puberty.

At puberty, primary oocytes periodically resume meiosis I.
Division is asymmetric, producing a large secondary oocyte and a small polar body.
The secondary oocyte enters meiosis II and is released during ovulation.
If fertilized, meiosis II completes, yielding a haploid egg and a second polar body.

Comparison of Mitosis, Meiosis I, and Meiosis II

Phase	Event	Mitosis	Meiosis I	Meiosis II
Prophase	Synapsis	No	Yes	No
Prophase	Crossing over	Rarely	Commonly	Rarely
Prometaphase	Attachment to the poles	A pair of sister chromatids to both poles	A pair of sister chromatids to one pole	A pair of sister chromatids to both poles
Metaphase	Alignment along the metaphase plate	Sister chromatids	Bivalents	Sister chromatids
Anaphase	Separation of	Sister chromatids	Bivalents	Sister chromatids
End result		Two diploid cells		Four haploid cells

Additional info: The above notes expand on the original slides and handwritten notes, providing definitions, context, and examples for key terms and processes in chromosome transmission, mitosis, meiosis, and gametogenesis. The tables have been reconstructed for clarity and completeness.