Biology IB HL - 1.6 Cell Cycle Part 2

The genetically identical chromosomes move to the opposite poles of the cell

Once the two chromosome sets arrive at the poles, spindle fibres dissolve

Chromosomes decondense (no longer visible under light microscope)

Nuclear membranes reform around each chromosome set

Cytokinesis occurs concurrently, splitting the cell into two

NO

cytokinesis in animal cells

After anaphase, microtubule filaments form a concentric ring around the centre of the cell

cytokinesis in animal cells

The microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the centre

When the furrow meets in the centre, the cell becomes completely pinched off and two cells are formed

cytokinesis in animal cells

Because this separation occurs from the outside and moves towards the centre, it is described as centripetal

cytokinesis in plant cells

After anaphase, carbohydrate-rich vesicles form in a row at the centre of the cell (equatorial plane)

cytokinesis in plant cells

The vesicles fuse together and an early cell plate begins to form within the middle of the cell

cytokinesis in plant cells

The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells

cytokinesis in plant cells

Because this separation originates in the centre and moves laterally, it is described as centrifugal

The mitotic index is a measure of the proliferation status of a cell population (i.e. the proportion of dividing cells)

The mitotic index may be elevated during processes that promote division, such as normal growth or cellular repair

It also functions as an important prognostic tool for predicting the response of cancer cells to chemotherapy

Cells undergoing mitosis will lack a clearly defined nucleus and possess visibly condensed chromosomes

number of cells in mitosis / total number of cells

Cyclins are a family of regulatory proteins

control the progression of the cell cycle

Cyclins activate cyclin dependent kinases (CDKs)

control cell cycle processes through phosphorylation

When a cyclin and CDK form a complex, the complex will bind to a target protein and modify it via phosphorylation

The phosphorylated target protein will trigger some specific event within the cell cycle (e.g. centrosome duplication, etc.)

After the event has occurred, the cyclin is degraded and the CDK is rendered inactive again

Cyclin concentrations need to be tightly regulated in order to ensure the cell cycle progresses in a proper sequence

YES

| Different cyclins specifically bind to, and activate, different classes of cyclin-dependent kinases

Cyclin levels will peak when their target protein is required for function and remain at lower levels at all other times

Tumours are abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ

Diseases caused by the growth of tumours are collectively known as cancers

A mutagen is an agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery)

physical, chemical and biological

Sources of radiation including X-rays (ionising), ultraviolet (UV) light and radioactive decay

DNA interacting substances including reactive oxygen species (ROS) and metals (e.g. arsenic)

Viruses, certain bacteria and mobile genetic elements (transposons)

Mutagens that lead to the formation of cancer are further classified as carcinogens

An oncogene is a gene that has the potential to cause cancer

Most cancers are caused by mutations to two basic classes of genes – proto-oncogenes and tumour suppressor genes

code for proteins that stimulate the cell cycle and promote cell growth and proliferation

code for proteins that repress cell cycle progression and promote apoptosis

When a proto-oncogene is mutated or subjected to increased expression it becomes a cancer-causing oncogene

Tumour suppressor genes are sometimes referred to as anti-oncogenes, as their normal function prevents cancer

Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour

when tumour cells remain in their original location

when tumour cells spread and invade neighbouring tissue (malignant)

Secondary tumours are made up of the same type of cell as the primary tumour – this affects the type of treatment required

E.g. If breast cancer spread to the liver, the patient has secondary breast cancer of the liver (treat with breast cancer drugs)