5.2.1 Processes of Cell Replication

Question 1

Cell Division of Unicellular Organisms

Answer 1

The cell division of unicellular organism occurs through binary fission.

It is important that the genetic code remains identical when passing to the offspring cell for the genetic stability of the species.

Question 2

Cell Cycle Phases

Answer 2

G1 Phase - cell growth and enlargement constantly occur, as well as metabolic changes to prepare for division.

S Phase - DNA is copied and replicated for cell division.

• Chromatids and centromeres become a full chromosome with two sister chromatids, each containing an identical DNA molecule.

G2 Phase - enzymes check and repair the duplicated chromosomes for any errors, and the cytoplasmic material accumulates for division.

M Phase - the nucleus divides before cytokinesis divides the cytoplasm.

• Prophase - nuclear membrane begins to break down as chromosomes condense (shorten and thicken) and become visible as the centrioles move to opposite sides of the nucleus to form poles and spindle fibres.
• Metaphase - centromeres (highly condensed chromosomes) attach to spindles and align in the equatorial plain (between the two poles).
• Anaphase - spindle fibres contract to split the centromeres, separate sister chromatids, and pull them to opposite poles.
• Telophase - nuclear membrane reforms around the sets of chromosomes to form two distinct nuclei, which also causes the spindles to disappear chromosomes to elongate.

Cytokinesis - division of the cytoplasm to separate into two daughter cells

• In animal cells, the cytoplasm constricts the centre of the cell and pinches off.
• In plant cells, a cell plate forms at the equator during telophase, made up of pectin compounds and eventually cellulose.

Cells spend most of their time in interphase, where the cell grows by producing proteins and additional organelles. It includes the G1, S and G2 phase.

Question 3

Mitosis

Answer 3

Somatic cells, which are all cells other than gametes, undergo cell division by mitosis. This allows cells to differentiate and specialise over the lifetime of an adult, forming tissues and some losing the ability to reproduce once they have reached a particular stage and location.

Mitosis produces two identical daughter cells.

Mitosis allows for the growth of multicellular organisms followed by cell assimilation, enlargement and differentiation. It also allows for the repair of damaged tissue and replacement of worn-out cells. It is a process ofasexual reproduction and allows for genetic stability – daughter cells resulting in same number of chromosomes and genetic information.

Question 4

Stem Cells

Mitosis

Answer 4

Stem cells are a type of somatic cell that have the ability to specialise into any type of tissue. **Embryonic stem cells **a pluripotent, meaning they have to potential to develop into any type of tissue. Adult stem cells are multipotent and pre-specialised to become a specific type of cell when necessary, but stem cells in bone marrow are multipotent in that they can develop into all kinds of blood cells. This means that adult stem cells are less applicable to study as embryonic.

Question 5

Meristem Cells

Mitosis - Plants

Answer 5

Meristem cells occur at sites of plants that undergo growth, such as the tip of the stem and root. The tissue is able to divide and form any other tissue necessary.

Question 6

Meiosis

Answer 6

Gametes undergo cell division by meiosis, which produced genetically different daughter cells.

Meiosis is a type of reduction division in gametes of both plants and animals, that also ensures that the chromosome number of each species is maintained (not doubled) during sexual reproduction. It does this by forming gametes (sperm and egg cells), that are haploids with only one set of chromosomes (23). This means that when they fuse to form a zygote, the embryo will have a full set of 46 chromosomes.

Meiosis produces four haploid daughter cells by dividing twice, meaning the final daughter cells end up with half the original number of chromosomes.

Question 7

Stages of Meiosis

Answer 7

Meiosis 1 (Reductional Cell Division)

1 diploid → 2 haploids

• Prophase 1 - chromosomes condense, nuclear membrane breaks down and crossing over
• Metaphase 1 - pairs of homologous chromosomes move to the equator
• Anaphase 1 - homologous chromosomes move to opposite poles but chromatids do NOT split
• Telophase 1 - chromosomes gather at the poles and nuclei form
• Cytokinesis - cytoplasm divides

2. Meiosis 2 (Equational Division)

2 haploids → 4 haploids

• Prophase 2 - spindles form around the chromosomes
• Metaphase 2 - chromosomes line up at the equator
• Anaphase 2 - centromeres divide and chromatids move to opposite poles
• Telophase 2 - chromosomes gather at the poles and nuclei form
• Cytokinesis - cytoplasm divides