Cellular Replication and Variation

Question 1

Cell Replication

Answer 1

Form of cell division in which a parent cell divides to produce two genetically identical daughter cells.

Question 2

Binary Fission

Answer 2

Cell replication in prokaryotes, very fast process.

Question 3

Cell Cycle Phases

Answer 3

1. Interphase
   - The cell prepares for division by growing and duplicating its DNA
2. DNA Replication (Mitosis)
   - The cell divides its nucleus and distributes the duplicated chromosomes into two daughter nuclei.
3. Cytokinesis
   - Cytoplasm divides, resulting in two genetically identical daughter cells.

Question 4

Mitosis Result

Answer 4

Two daughter cells that have identical genetic material as the parent cell. A diploid (2n) parent cell that undergoes mitosis will produce two diploid (2n) daughter cells.

Question 5

Mitosis Phases

Answer 5

1. Prophase
   - Chromosomes begin to condense, begin to see two chromatids held together at the centromere
   - Centrioles replicated during interphase, move to opposite ends of the cell to form the poles.
   - Nuclear membrane breaks down. The centrioles begin to form a network of fibres, called spindles.
2. Metaphase
   - Centromeres continue to be drawn by the spindle fibres so that the chromosomes are aligned in the middle of the cell.
3. Anaphase
   - Spindle fibres contract, pulling the centromere in two directions.
   - Centromere splits, separating the two chromatids.
   - Separated chromatids are pulled to opposite poles, results in two daughter cells each receiving the same genetic information: one copy of every chromosome that was in the original nucleus at interphase.
4. Telophase
   - Nuclear membrane forms around the chromosomes at each pole. The spindle is dismantled and disappears.
   - The chromosomes become longer and thinner.

Question 6

Cytokinesis Steps

Answer 6

• Cytoplasm divides and the two new nuclei separate
• Division marks creation of new cells

Question 7

Meiosis (I and II) Stages

Answer 7

1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I
5. Prophase II
6. Metaphase II
7. Anaphase II
8. Telophase II

Question 8

Meiosis I Phases

Answer 8

1. Prophase I
   - Chromosomes condense from fine threads and spindle forms. Each chromosome composed of two chromatids, connected at the centromere.
   - Crossing over of homologous chromosomes occurs in late prophase.
2. Metaphase I
   - Homologous chromosomes pair up and align on the equatorial plate
   - Chromatids (two per chromosome) become apparent, and the nuclear membrane breaks down.
   - The homologous pairs are independently assorted, each side will have a random number of maternal and paternal chromosomes for each homologous pair.
3. Anaphase I
   - The spindle draws members of each chromosome pair to opposite poles of the cell.
   - Cell membrane begins to pinch in.
4. Telophase I + Cytokinesis
   - The cytoplasm divides and nuclear membranes form.
   - Two haploid daughter nuclei are created, but each chromosome is still in the replicated state.

Question 9

Meiosis II Phases

Answer 9

1. Prophase II
   - Nuclear envelope breaks down and the meiotic spindle is re-created.
2. Metaphase II
   - Chromosomes at each end of the cell move to a central position (begins the second division).
3. Anaphase II
   - Centromeres split, separating the sister chromatids
   - Single-strand chromosomes move to opposite poles of the cell (results in haploid cells).
4. Telophase II + Cytokinesis
   - The cytoplasm divides and nuclear membranes re-form.
   - Four haploid daughter cells are created with different genetic combinations of the original set of homologous chromosomes.

Question 10

Sister Chromatids

Answer 10

Identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common centromere.

Question 11

Crossing Over and How It Creates Genetic Variation

Answer 11

• Produces chromosomes that have brand new combinations of genetic information
• Genetic material being swapped between the chromosomes.
• DNA strands from the chromatids of two homologous chromosomes are cut at an equivalent point.
• A segment is exchanged and the strands are reconnected.
• Paternal segments of DNA end up on the maternal chromosome, and maternal DNA segments end up on the paternal chromosome.

Question 12

Spermatogenesis

Answer 12

• Happens after meiosis in males
• Four haploid sperm are formed from the original diploid parent cell. Each sperm cell contains 23 chromosomes.
• Spermatogonia, divide mitotically to produce spermatocytes.
• Spermatocytes are diploid and divide meiotically to produce haploid spermatozoa.

Question 13

Oogenesis

Answer 13

• Happens after meiosis in females
• Produces only one haploid ovum: the other three haploid cells are much smaller and eventually degenerate.
• Only completed after fertilisation