D2.1 Cell and Nuclear Division

Cytokinesis

Where the cytoplasm of the parent cell divides into two daughter cells

Cytokinesis in animal cells

A cleavage furrow forms when actin and myosin proteins form a contractile ring at the equator. As the proteins contract, they pull the plasma membrane towards the centre

Cytokinesis in plant cells

Vesicles fuse together to create the two plasma membranes, forming the cell plate. Once the cell plate reaches the cell walls of the parent cell, new cell walls are produced, separating the new daughter cells

Is cytokinesis equal? What must each cell receive?

Cytokinesis is usually equal, producing daughter cells that are of similar size. Each daughter cell receives at least one mitochondrion. In plant cells, each cell must receive at least one chloroplast

Examples of unequal cytokinesis

Oogenesis in humans, budding in yeast

Why is nuclear division needed before cell division?

It avoids the production of anucleate cells

What is the role of mitosis?

Mitosis produces genetically identical cells, maintaining the chromosome number and genome of cells. The cells produced are diploid

What is the role of meiosis?

Meiosis produces genetically different cells called gametes (sex cells). The cells produced are haploid

Diploid

Nuclei possessing pairs of homologous chromosomes (2n)

Haploid

Nuclei possessing only one set of chromosomes (n)

What is the structure of a chromosome?

A chromosome is made up of 2 sister chromatids that are joined together by the centromere

How does chromatin become chromosomes?

Before mitosis, the DNA molecules are loosely coiled around histones in eukaryotic cells to form chromatin. During prophase, the chromatin gets condensed by supercoiling to form chromosomes

What is the role of microtubules and microtubule motors in cell division?

They are responsible for the movement of chromosomes. Microtubules are able to lengthen and shorten to enable movement. Microtubule motors carry the chromosomes along the microtubules

What is mitosis needed for?

Growth, tissue repair, asexual reproduction

4 phases of mitosis

Prophase, metaphase, anaphase, telophase

Steps of mitosis

Prophase - Chromosomes condense and become visible. The 2 centrosomes move towards opposite poles, and form spindle fibres. The nuclear membrane breaks down.

Metaphase - Spindle fibres from both centrosomes connect to the centromere of each chromosome. This causes the chromosomes to align along the equator.

Anaphase - The sister chromatids separate at the centromere. They are pulled to opposite poles by the spindle fibres.

Telophase - Chromosomes arrive at opposite poles and begin to decondense. Nuclear membranes reform around each set of chromosomes and the spindle fibres break down. Cytokinesis occurs

Steps of meiosis I

Prophase I - Chromosomes condense and become visible. The 2 centrosomes move towards opposite poles, and form spindle fibres. Homologous chromosomes pair up to form a bivalent. Crossing over occurs, which is the exchange of genetic material between non-sister chromatids. The nuclear membrane breaks down. Recombination of genetic material between non sister chromatids which results in genetically identical haploid daughter cells (which are separated into different cells), increases genetic variation

Metaphase I - Spindle fibres from both centrosomes connect to the centromeres of each bivalent. This causes the chromosomes to align along the equator.

Anaphase I - The homologous chromosomes are separated and pulled to opposite poles by spindle fibres.

Telophase I - The nuclear membrane reforms around each daughter nucleus. Cytokinesis occurs.

Steps of meiosis II

Prophase II - Spindle fibres begin to emerge again and the nuclear membrane breaks down

Metaphase II - In each cell, the spindle fibres from both centrosomes connect to the centromere of each chromosome. This causes the chromosomes to align along the equator

Anaphase II - The sister chromatids separate at the centromere. They are pulled to opposite poles by the spindle fibres.

Telophase II - The nuclear membrane reforms around the four sets of daughter chromosomes. Cytokinesis occurs

Why is meiosis needed in a sexual life cycle?

Allows for the fusion of gametes. It is essential that the chromosome number is halved, otherwise the chromosome number would keep doubling every generation

Non-disjunction

Occurs when chromosomes fail to separate correctly during meiosis. This can occur in either anaphase I or anaphase II

Example of non-disjunction

Down syndrome (Trisomy 21), individuals have three copies of chromosome 21

How does meiosis allow for variation?

Crossing over allows for the exchange of genes, producing chromatids with new combinations of alleles. The random arrangement of bivalents in metaphase I also contributes

How to calculate the number of possible chromosomal combinations

2n, where n is the number of homologous chromosome pairs

Proliferation

A rapid increase in the number of cells as a result of cell growth and cell division

Examples of proliferation for growth

Plant meristems, early-stage animal embryos

Proliferation for cell replacement and tissue repair

Skin cells that form part of the epidermis are lost daily and will be replaced by the proliferation of the stem cells found in the basal layer of the epidermis

What are the stages of cell cycle in order?

G1, S, G2, mitosis, cytokinesis

What happens to the cell during interphase?

Increases in mass and size, carries out cellular functions in the nucleus and cytoplasm, increases the number of mitochondria, increases the number of chloroplasts (plant cell)

What happens during G1?

Cells make RNA, enzymes and other proteins required for growth

What happens during S?

DNA replication, resulting in each chromosome consisting of 2 sister chromatids

What happens during G2?

The cell continues to grow and newly synthesised DNA is checked for errors

What are the 3 checkpoints in the cell cycle?

G1, G2, mitosis

How is the cell cycle controlled?

By cyclins and kinases. There are 4 different cyclins (D, E, A, B) whose concentrations rise and fall. A threshold level of a specific cyclin is required to pass each checkpoint in the cycle

What happens when a threshold level of a specific cyclin is reached?

Cyclin will bind with cyclin-dependent kinases to form an activated complex. This complex phosphorylates a target protein which activates it, causing it to trigger the next stage of the cell cycle

How do mutations in proto-oncogenes lead to uncontrolled cell division?

Proto-oncogenes code for proteins that stimulate normal cell division. When mutated, they become oncogenes, resulting in an increase in the protein product produced or proteins which are permanently activated

How do mutations in tumour-suppressor genes lead to uncontrolled cell division?

Tumour-suppressor genes code for proteins which inhibit cell division or promote controlled cell death if the nucleus contains damaged DNA. When mutated, there will be a reduced number of proteins produced or proteins that are permanently deactivated

Malignant tumours

Tumours that cause cancer by growing rapidly, invading and destroying surrounding tissues. They can undergo metastasis

Primary tumours

The original or first tumour in the body

Secondary tumours

Tumours that develop from cells that broke off the primary tumour

Benign tumours

Non cancerous. They grow slowly and do not invade other tissues. However, they can cause blockages by exerting pressure

Mitotic index

The proportion of cells that are undergoing mitosis

Mitotic index =

Number of cells with visible chromosomes / Total number of cells