5.1 Cell Division

Gene, Deoxyribonucleic Acid (DNA) and Chromosome

• Genetics: the study of genes, inheritance and variation in organisms.
• Nucleus: cell structure that controls all cell activities; it contains chromosomes.
• Chromosome: a long, coiled, fine thread-like structure made of nucleic acid and protein, located in the nucleus.
  • Each chromosome contains one long DNA molecule.
  • A chromosome that has replicated consists of two sister chromatids joined at a centromere.
• Deoxyribonucleic acid (DNA):
  • consists of basic units called nucleotides;
  • each nucleotide contains deoxyribose sugar, a nitrogenous base and a phosphate group;
  • two antiparallel polynucleotide chains twist around each other to form a double helix.
• Gene: the basic unit of inheritance that determines an individual’s characteristics.
  • Controls inherited characteristics such as the ability to roll the tongue in humans, pea-seed shape and cat-fur colour.
  • Arranged as a segment of DNA along a chromosome.
  • Thousands of genes occur in one DNA molecule; each chromosome may carry up to 200–300 types of genes.

Structural relationship:

Cell → nucleus → chromosome → DNA → gene (a segment of DNA)

Human Chromosomes and Karyotype

• Every species has a fixed number of chromosomes.
• A normal human somatic cell contains 46 chromosomes.
• Chromosomes occur in pairs of similar shape and size. Each pair is called a pair of homologous chromosomes.
• A karyotype is an arrangement of homologous chromosomes according to their shape and size.
• Human chromosomes comprise:

Type	Function and number
Autosomes	Carry genes controlling characteristics such as eye colour, ability to roll the tongue and hair type; 44 autosomes (22 pairs) in a somatic cell
Sex chromosomes	Carry genes that determine sex; XX in females and XY in males

• Human somatic cells: 46 chromosomes.
• Human gametes—sperm and ovum: 23 chromosomes.

Mitosis and Meiosis

• Interphase occurs before mitosis or meiosis:
  • DNA replicates;
  • the cell grows and prepares for cell division.

Mitosis

• Mitosis is cell division that produces two identical daughter cells.
• Each daughter cell has the same chromosome number and genetic content as the parent cell.
• Occurs in somatic cells of humans and animals and meristematic tissues at plant root tips and shoot tips.

1. Prophase: chromosomes shorten, thicken and become visible; each consists of two sister chromatids joined at the centromere; spindle fibres form; nuclear membrane and nucleolus disintegrate.
2. Metaphase: chromosomes line up at the equatorial plane; spindle fibres attach to centromeres.
3. Anaphase: centromeres divide; sister chromatids separate and move to opposite poles.
4. Telophase: chromatids reach opposite poles; nuclear membrane and nucleolus reform; cytoplasm divides; two daughter cells with the same chromosome number and genetic information as the parent cell form.

Meiosis

• Meiosis is cell division that produces four non-identical daughter cells.
• Each daughter cell contains half the chromosome number of the parent cell.
• Occurs in reproductive organs to produce gametes: human testes and ovaries; plant anthers and ovaries.

Meiosis I

1. Prophase I: chromosomes shorten, thicken and become visible; homologous chromosomes pair; crossing over exchanges genetic information between homologous chromosomes.
2. Metaphase I: homologous chromosomes line up at the equatorial plane; spindle fibres attach to centromeres.
3. Anaphase I: homologous chromosomes separate and move to opposite poles.
4. Telophase I: cytoplasm divides; the first meiotic division ends.

Meiosis II

1. Prophase II: spindle fibres begin to form.
2. Metaphase II: chromosomes line up at the equatorial plane; spindle fibres attach to centromeres.
3. Anaphase II: centromeres divide; sister chromatids separate and move to opposite poles.
4. Telophase II: nuclear membranes and nucleoli reform; cytoplasm divides; four non-identical daughter cells (gametes), each with half the parent-cell chromosome number, form.

Comparison of Mitosis and Meiosis

Mitosis	Meiosis
DNA replicates before cell division	DNA replicates before cell division
Produces 2 daughter cells	Produces 4 daughter cells
No crossing over	Crossing over occurs
No genetic variation	Genetic variation occurs
Daughter cells are genetically identical	Daughter cells are genetically different
Daughter-cell chromosome number equals that of the parent cell	Daughter-cell chromosome number is half that of the parent cell
Occurs in somatic cells	Occurs in reproductive cells

Importance of Mitosis and Meiosis

Mitosis is important for:

• growth of organisms, including growth at plant root tips;
• replacement of damaged or dead cells, such as formation of new skin cells to heal a wound;
• asexual reproduction in certain organisms such as Amoeba sp.; the new cells are identical to the parent cell.

Meiosis is important for:

• production of gametes for sexual reproduction.