Genetic Basis: Chromosomes and Cell Division

Genetic Basis: Chromosomes and Cell Division

1. Chromosomes: The Building Blocks of Heredity

• Definition: Chromosomes are structures that are readily stained in the nucleus of a cell, and play an important role in storing and transmitting genetic information.

• Chromosome set: The total number of chromosomes present in a cell of an organism.

• Homologous chromosomes: A pair of chromosomes that are identical in structure, morphology, and size. One chromosome in the pair is inherited from the father, and the other from the mother (except for the XY and XO pairs, which are not homologous).

• Diploid chromosome set (2n): Present in somatic cells or germ cells in the reproductive region, with chromosomes existing in pairs.

• Haploid chromosome set (n): Present in eggs or sperm cells (gametes), with chromosomes existing as single entities.

2. Chromosome Structure and Shape:

• Shape: Chromosomes can be rod-shaped, V-shaped, or dot-shaped.

• Size: Length ranges from 0.2 – 50 µm, diameter from 0.2 – 2 µm.

• Microscopic structure:

• Chromatid: A double-stranded helix structure of a chromosome, formed by DNA replication.

• Centromere (primary constriction): The point where a chromosome attaches to the spindle fibers (mitotic spindle), helping the chromosome move towards the poles of the cell during cell division.

• Secondary constriction: The location where rRNA is synthesized.

3. Nucleosome Structure:

• A DNA strand (146 base pairs, wrapped 1 3/4 turns) binds to 8 histone protein molecules.

4. Chromosomes in Organisms:

• Prokaryotes (bacteria, cyanobacteria): Chromosomes are naked DNA, in a circular form.

• Acellular organisms (viruses): Chromosomes are DNA or RNA, with only half a strand.

5. Mechanism of Mitosis:

• Stages: Interphase, prophase, metaphase, anaphase, telophase.

• Interphase:

• 2n = 4 single chromosomes.

• 0 chromatids.

• 4 centromeres.

• DNA replication occurs, forming 2n = 4 replicated chromosomes.

• Prophase:

• 2n = 4 replicated chromosomes.

• 8 chromatids.

• 4 centromeres.

• Chromosomes condense, the nuclear membrane and nucleolus disappear.

• Metaphase:

• 2n = 4 replicated chromosomes.

• 8 chromatids.

• 4 centromeres.

• Chromosomes align in a single line along the equator of the mitotic spindle.

• Anaphase:

• 4n = 8 single chromosomes.

• 0 chromatids.

• 8 centromeres.

• Chromatids separate from each other and move towards the two poles of the cell.

• Telophase:

• 2n = 4 single chromosomes.

• 0 chromatids.

• 4 centromeres.

• The nuclear membrane and nucleolus reappear, chromosomes decondense, and the cell divides into two daughter cells identical to the parent cell.

6. Result of Mitosis:

• One parent cell produces two daughter cells that are identical to the parent, due to one replication (interphase) and one segregation (anaphase).

7. Mechanism of Meiosis:

• Stages: Interphase, prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II.

• Result: One parent cell produces four daughter cells with half the number of chromosomes as the parent cell, due to one replication (interphase) and two segregations (anaphase I, anaphase II).

8. Number of Chromosomes Supplied by the Environment:

• Mitosis: 2n.2^x – 2^x (where x is the number of mitotic divisions)

• Meiosis:

• Germ cell: 4.2^x.n – 2n (where x is the number of mitotic divisions)

• Gamete: 4.2^x.n – 2n.2^x (where x is the number of mitotic divisions)

9. Fertilization:

• Definition: The fusion of an egg cell with a sperm cell to form a zygote.

10. Fertilization Efficiency:

• Sperm: H% = Number of sperm cells that fertilize / Total number of sperm cells.

• Egg: H% = Number of eggs that fertilize / Total number of eggs.

11. Number of Zygotes:

• Equal to the number of eggs that fertilize or the number of sperm cells that fertilize.