Chromosomes, Cell Cycle, and Cell Division: Structure, Function, and Mechanisms

Chromosomes and Chromatin

Structure and Organization of Eukaryotic Chromosomes

Eukaryotic chromosomes are highly organized structures composed of DNA and proteins. They play a central role in storing and transmitting genetic information during cell division.

• Chromosomes come in pairs called homologues in diploid organisms.

• Humans have 46 chromosomes arranged in 23 pairs.

• One chromosome of each pair is inherited from the mother, the other from the father.

Types of Chromosomes

• Autosomes: Non-sex chromosomes, found in both males and females. Humans have 22 pairs of autosomes.

• Sex Chromosomes: Determine biological sex. Humans have one pair (X and Y). The X chromosome is larger and contains more genes than the Y chromosome. Females are XX, males are XY.

Chromosomes vs. Chromatin

DNA exists in two main forms within the nucleus:

Chromosomes	Chromatin
Tightly packaged DNA	Unwound DNA
Visible only during cell division	Present throughout interphase
DNA is not being used for macromolecule synthesis	DNA is being used for macromolecule synthesis

Additional info: Chromosomes are composed of DNA and proteins (mainly histones), forming a complex called chromatin. During cell division, chromatin condenses to form visible chromosomes.

Alleles and Loci

• Allele: A variant form of a gene, often differing by one or a few nucleotides.

• Locus (plural: loci): The specific location of a gene on a chromosome.

• Different alleles may or may not result in different phenotypes.

• A gene is a discrete heritable unit.

Chromatids and Centromeres

• Chromatid: One of two identical halves of a replicated chromosome, joined at the centromere.

• A chromatid is called such as long as it is attached to its sister chromatid at the centromere.

• After separation during cell division, each chromatid becomes an independent chromosome.

The Eukaryotic Cell Cycle

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase and the mitotic (M) phase.

• Interphase: The cell grows and DNA is replicated. It includes:

  • G1 phase (Gap 1): Cell growth and normal functions.

  • S phase (Synthesis): DNA replication.

  • G2 phase (Gap 2): Preparation for mitosis.

• M phase (Mitotic phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis: Mechanism of Eukaryotic Cell Division

Purpose and Overview

Mitosis ensures that each daughter cell receives an identical set of chromosomes. It is essential for growth, development, and tissue repair in multicellular organisms.

Stages of Mitosis

1. Prophase: Chromatin condenses into visible chromosomes; nucleoli disappear; mitotic spindle forms; centrosomes move apart.

2. Prometaphase: Nuclear envelope fragments; spindle microtubules attach to kinetochores on chromosomes.

3. Metaphase: Chromosomes align at the metaphase plate; spindle apparatus is fully formed.

4. Anaphase: Sister chromatids separate and move toward opposite poles; each chromatid is now considered a chromosome.

5. Telophase: Chromosomes arrive at poles; nuclear envelopes reform; chromosomes decondense; cytokinesis begins.

Mitotic Spindle and Asters

• Mitotic spindle: Structure made of microtubules that segregates chromosomes during mitosis.

• Asters: Radial arrays of microtubules that help position the spindle apparatus.

Cytokinesis

• In animal cells: Occurs by cleavage, forming a cleavage furrow that pinches the cell in two.

• In plant cells: Occurs by formation of a cell plate, which develops into a new cell wall separating the daughter cells.

Meiosis: Reduction Division for n

Reproduction

Purpose and Overview

Meiosis reduces the chromosome number by half, producing haploid gametes (sperm and egg) from diploid cells. This process introduces genetic variation.

Stages of Meiosis

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

• Meiosis II: Sister chromatids separate, similar to mitosis, resulting in four haploid cells.

Key Events in Meiosis

• Synapsis: Pairing of homologous chromosomes during prophase I.

• Crossing Over: Exchange of genetic material between non-sister chromatids, increasing genetic diversity.

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

Genetic Variation via the Sexual Cycle

Mechanisms of Variation

• Crossing Over: After crossing over and synapsis, sister chromatids are no longer identical.

• Independent Assortment: Each human can produce over 8.3 million different gametes due to random chromosome shuffling in meiosis I.

• Fertilization: The combination of gametes from two parents can produce over 64 trillion different zygotes (not accounting for crossing over).

Summary Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetic identity	Identical to parent	Genetically unique
Role	Growth, repair	Gamete production

Example: Human Sexual Cycle

• Humans alternate between diploid (2n) and haploid (n) stages.

• Meiosis produces haploid gametes (n = 23), which fuse during fertilization to restore diploid number (2n = 46).

Additional info: The processes of mitosis and meiosis are tightly regulated to ensure accurate distribution of genetic material and to maintain genetic stability across generations.