Cellular Division

Human Biology

When does Crossing over occur

Prophase 1

Crossing over

When the homologous chromosomes are paired, the chromosomes may cross, break and exchange segments.

Crossing over results in

a new combination of alleles along the chromosome, recombination

Chiasma

Point where two chromatids cross during crossing over.

Non-disjunction occurs

Anaphase 1, the first division of meiosis

Non-disjunction

Is where one or more of the chromosome pairs fail to separate when the cell divides.

Non-disjunction results in

one of the daughter cells receiving an extra chromosome and the other daughter cell lacking that chromosome.

Random/independent assortment occurs

During first meiotic division. Anaphase 1.

Random/independent assortment

When the chromosomes move apart during the first meiotic division, they do so independently. The way one pair of chromosomes separates is unaffected by the way any of the other pairs separate.

Cancer

Is the uncontrolled division of cells. This uncontrolled growth of abnormal cells produces a mass, or tumour.

Malignant

The tumour cells are able to spread to other parts of the body, known as metastasis.

benign

Are not able to invade normal tissues, blood or lymph vessels and so cannot spread to other parts of the body.

Can cancer cells differentiate

No

Carcinogens

Environmental factors that can trigger malignant tumours.

Examples of Carcinogens

UV radiation

X-rays

Ionising Radiation

Viruses

Chemical Carcinogens

Cervical Cancer

Caused by human papilloma virus (HPV), which is transferred through genital skin contact during intercourse.

Cervical Screening test

Cells are examined for abnormalities that can develop into cancer.

Mammography

Is an X-ray of the breasts.

Bowel Cancer

Is a malignant tumour in the large intestine (Colon or Rectum).

Faecal Occult Blood Test (Bowel Cancer)

Tests for small amounts of blood (unseeable) in faeces. This blood comes from polyps which are potentially cancer.

Digital Rectal Examination (DRE)

Doctor feels surface of prostate gland for swelling, hardening or irregularities.

Prostate-Specific antigen blood test (PSA)

Checks blood for presence of protein produced by prostate. Protein presence rising can indicate cancer.

Biopsy (Prostate Cancer)

Small sample of tissue checked for cancer cells

Mitosis (Interphase)

DNA replication occurs. The cell grows. New proteins are synthesised. Cell organelles are replaced.

Mitosis (Prophase)

Nuclear membrane starts to break down. Centrioles start to become visible and migrate to poles. Spindle fibres form at centrioles. Chromosomes becoming visible.

Mitosis (Metaphase)

Chromatid pairs line up on the equator of the spindle. The centromere of each pair is attached to a spindle fibre.

Mitosis (Anaphase)

Each pair of chromatids separates at the centromere, they are now called chromosomes. The new chromosomes are then pulled apart towards opposite poles of the cell by the spindle fibres. Each pole (future daughter cell) now has an identical set of genes.

Mitosis (Telophase)

The two sets of chromosomes form tight groups at each pole of the cell. A nuclear membrane forms around each group, and a nucleolus appears in each new nucleus. The spindle fibres disappear, and the chromosomes gradually uncoil to become chromatin threads once more.

Cytokinesis

Division of the cytoplasm. A furrow develops in the cytoplasm between the two nuclei. The furrow gradually deepens until it cuts the cytoplasm into two parts, each with its own nucleus. Each daughter cell has exactly the same number and type of chromosomes as the parent cell.

Differentiation

The process by which cells become specialised

Stem cells

Cells that undergo differentiation. Not specialised and are capable of repeating division by mitosis.

Totipotent

Potential to create any type of cell necessary for embryonic development

Pluripotent

Are able to give rise to most, but not all tissues of an organism.

Multipotent

Are able to give rise to cells with a specific function. e.g. blood cells to platelets, white or red.

Meiosis (Interphase)

Chromatin replicates to produce homologous pairs (not identical but code for the same gene)

Meiosis (Prophase 1 Early)

Chromosome become visible as long threads. Chromosomes gradually move so that the members of a homologous chromosomes come to lie alongside each other throughout their entire length. The chromosomes then shrink and thicken as the DNA becomes more tightly coiled.

Meiosis (Prophase 1 Late)

As each chromosome consists of two chromatids, each chromosome appears as four strands. The nuclear envelope disappears. The spindle forms stretching between the poles of the cell. Homologous pairs line up. Crossing over occurs.

Meiosis (Metaphase 1)

The paired chromosomes move towards the spindle fibres across the centre or equator of the cell. Orientation at the equator is random. This process is known as independent assortment.

Meiosis (Anaphase 1)

The pairs of homologous chromosomes move apart. One member of each pair (consisting of two chromatids) moving to one pole of the cell while the other moves to the opposite pole. The centromeres do not divide. The pairs of chromatids remain intact resulting in 23 chromosomes moving to each pole of the cell.

Meiosis (Telophase 1)

The spindle fibres disintegrate. Cytokinesis occurs. Nuclear envelope reforms. 2 cells created with half the number of chromosomes as the original cell.

Meiosis (Prophase 2)

A new spindle forms at each end of the original spindle and usually at right angles to the original. Nuclear membranes disintegrate. The chromosomes in each cell gradually move towards the equator.

Meiosis (Metaphase 2)

Chromosomes are arranged on the new spindle. The centromeres then divide so that each chromatid is now a separate chromosome.

Meiosis (Anaphase 2)

These new chromosomes migrate to opposite poles of the cell.

Meiosis (Telophase 2)

Nuclear membrane begins to form and the cytoplasm starts to divide (Cytokinesis). 4 new haploid cells (gametes) are produced.

Growth 1

Produces new proteins. Grows. Carries out normal functions.

Synthesis

DNA in nucleus forms identical copies. (DNA replication)

Growth 2

Cell further prepares for cell division (relatively short period)