Cell reproduction

cell cycle

the events that take place from one cell division to the next

• cells reproduce so that organs can grow larger

• cells that are damaged, worn out or diseased must be replaced

• some cells have a short lifespan and some have much longer

  • cells in the stomach lining → 2 days

  • nerve cells in the brain → lifelong

G1 + G2 phase

• growth phases (separated by synthesis)

• cell produces new proteins, grows and carries out normal tasks

• phase ends when the cell starts duplicating DNA

s phase

• synthesis phae

  • DNA molecules in the cell nucleus form exact duplicates of themselves

m phase

• mitotic phase

• after division cells may continue the cycle and re-enter G1 → some cells may stop dividing (G0 phase)

mitosis

the process by which a single parent cell divides to produce two identical daughter cells, each containing the same number of chromosomes as the original cell

• it is used for growth, repair and replacements of cells in multicellular organisms

• mitosis ensures genetic consistency, meaning the DNA in each new cell is identical as the original cell

chromosome structure

• if it was possible to see chromosomes in a non-dividing cell (its not as they are in their chromatin form) they would look like the adjacent figure, each chromosome consisting of one chromatid

• just before cell division occurs the DNA duplicates, when the DNA condenses into chromosomes during prophase they consist of two chromatids (the original and the copy) joined by a centromere

interphase - not a stage of mitosis

• cell goes through G1, S and G2 phases

• in the S phase: DNA molecules duplicate themselves → the quantity of DNA is doubled due to DNA replication

• some cells will be in the G0 phase which is when cells are not preparing to divide and are performing normal cellular functions

• during interphase the centrioles replicate, DNA replication occurs (DNA is still in the form of chromatin) and the nuclear membrane is clearly visible

prophase

• 2 pairs of centrioles become visible and move to opposite ends (poles) of the cell

• microtubules begin to radiate from them

• nucleolus disappears and the nuclear membrane begins to break down

• chromatin becomes tightly coiled (condenses) and can be seen as chromosomes

• each chromosome is comprised of a pair of chromatids due to the prior DNA replication

• By the end of prophase:

  • centrioles have reached opposite poles and microtubules radiate from them to form a spindle

  • nuclear membrane has disappeared completely

  • spindles begin to attach to the centromere of the chromosomes

metaphase

• spindle fibres move chromosomes towards the centre of the cell

• centromere is attached to a spindle fibre

• the chromosomes (two chromatids) line up at the equator of the cell (metaphase plate)

• the centrioles are at opposite ends and the spindle fibres are attached to the centromere

anaphase

• the spindle fibres contract causing the centromeres to break, dividing the sister chromatids

• the centrioles ‘pull’ on the spindle fibres

• the chromosomes are pulled to opposite poles

• each chromosome goes from having 2 sister chromatids to being two separate chromosomes

telophase

• two sets of chromosomes form tight groups at each pole of the cell

• nuclear membrane forms around each group

• nucleolus appears in each new nucleus

• spindle fibres disappear

• chromosomes gradually uncoil to become chromatin threads again

cytokinesis

• involves the division of the cell contents (cytoplasm + organelles)

• occurs concurrently with telophase

• a furrow develops in the cytoplasm between the two nuclei. The furrow deepens until it cuts the cytoplasm into two parts resulting in two identical daughter cells, each with a full set of chromosomes / DNA

mitosis

• mitosis and cytoplasmic division have resulted in the formation of 2 daughter cells

• each chromosome was duplicated

• each daughter cell has identical number and type of chromosomes as the parent cell

• the genetic information is therefore passed from parent cell to daughter cells and without change

cancer

when normal differentiation of cells goes wrong

• this results in a tumour → an abnormal mass of tissue from uncontrolled division of cells

how does cancer form

• cells failing to follow normal cell division and multiply excessively into a mass of proliferating cells

• normal cells die when they lose contact with surrounding matrix

• carcinogens cause mutations where DNA is altered changing the expression of certain genes

• certain genes produce proteins that are essential for cell division, growth, cellular adhesion and other things

• exposing these genes to carcinogens lead to the failure of producing these genes leading to the formation of cancer

malignant tumours

• cells are able to spread to other parts of the body (metastasis)

  • secondary tumour can develop well away from the original tumour

  • cancerous type of tumour

benign tumours

• cells are not able to spread to other parts of the body

  • they grow and press on surrounding tissues

  • normally have a capsule surrounding them making them easier to remove

  • non cancerous type of tumour

causes of cancer

• certain environmental factors (carcinogens) can trigger malignant tumours:

  • UV radiation

  • X rays

  • ionising radiation (radium, uranium) → single exposure to high dose may result in leukaemia

  • viruses (e.g. HPV)

  • chemical carcinogens (e.g. alcohol, asbestos, soot, tar, organic solvents in glue and paint, tobacco tar)

cancer prevention methods

• education: advertising and educational programs to limit exposue to carcinogens (e.g. Slip, Slop, Slap to limit UV exposure)

• Legislation: laws to control exposure to carcinogens

  • smoking being banned in many public places

  • tobacco advertising is not permitted

  • cigarettes must be sold in plain packages / images

  • standards for manufacture and operation of X ray machines

  • banning products containing asbestos

• reducing UV exposure: sunscreen, sunglasses, long sleeved clothing, shade and hats, stay out of direct sunlight between 10am and 3pm

• diet: adequate fibre and low fat, not overweight / obese, limit alcohol

• protective clothing when handling chemicals

• avoid smoking

cervical cancer

• caused by HPV → some people who have HPV may have cervical cells change and later become cancerous

  • pap test: cells collected from cervix smeared on microscope slide and examined. This detects early changes in cervical cells

breast cancer

mammogram: X-ray of breasts → tumours as small as 1cm in diameter can be detected

bowel cancer

• bowel cancer: most bowel cancers develop from polyps, it not all polyps become cancerous

  • Faecal Occult Blood Test (FOBT): at home tests for blood in faeces, mail to lab for analysis → can detect small amounts of blood not visible to the naked eye

  • if the test is positive, referred for a colonoscopy (visual examination of the intestine)

prostate cancer

• Digital rectal examination (DRE): insert finger into anus to feel surface of prostate → swelling, hardening or irregularities of surface may indicate cancer (some irregularities may be beyond reach)

• prostate specific antigen (PSA): blood test for presence of protein produced by prostate, if PSA rises it may indicate presence of cancer

• Biopsy: several small samples of prostate tissue checked for cancer (used once the other 2 methods have indicated positive)

meiosis interphase

• similar to mitosis interphase

• chromosomes replicate (in chromatin form) in the s phase

• each duplicated chromosome consists of two identical sister chromatids attached to their centromeres

meiosis prophase 1

• spindle fibres form

• centrioles move to the poles

• nuclear envelope dissolves

• chromatin condenses into replicated chromosomes (2 sister chromatids)

• homologous chromosomes pair up

• in prophase 1 ‘crossing over’ occurs:

  • during crossing over segments of chromosomes break off and reattach to the paired homologous chromosome → this leads to greater genetic diversity

meiosis metaphase 1

• the shortest phase

• spindle fibres attach to the centromere of each homologous chromosome

• pairs of homologous chromosomes line up at the equator of the cell

meiosis anaphase 1

• homologous chromosomes separate and move towards the poles

• sister chromatids remain attached at their centromeres

• there is no separating of chromatids

meiosis telophase 1

• chromosomes uncoil into chromatin and spindle fibres break down

• nuclear envelopes form around the DNA at each pole creating 2 nuclei

• each pole now has one of the 2 homologous chromosomes consisting of 2 sister chromatids

• cytokinesis occurs and 2 haploid daughter cells are formed

meiosis prophase 2

• the same of prophase in mitosis

• nucleolus and nuclear membrane disintegrate

• centrioles migrate to opposite poles, which are at right angles to the previous devision

• chromatin condenses to form chromosomes and become visible

• spindle fibres develop and attach to centromeres

meiosis metaphase 2

• same as metaphase in mitosis

• chromosomes are arranged at the equator of the cell in a single file line

• each chromosome is attached to a spindle fibre at the centromere with the centrioles at opposite ends

meiosis anaphase 2

• same as anaphase in mitosis

• spindle fibres constrict ‘breaking’ chromosomes to separate sister chromatids. Each chromatid is now considered a chromosomes and are pulled to opposite poles

• sister chromatids separate

meiosis telophase 2

• chromosomes uncoil into chromatin

• nuclear membrane and nucleolus reform around each set of chromosoems

• spindle fibres disappear

• cytokinesis occurs, resulting in a tetrad of haploid cells

somatic cells vs. gametes

somatic cells	gametes
normal body cells	sex cells
contain the normal number of chromosomes - one copy from each parent cell	contain half the normal number of chromosomes
called the diploid number - 2n	called the haploid number - n

meiosis

the process by which gametes are produced with half the number of chromosomes (haploid)

• during meiosis diploid cells are reduced to haploid cells

• diploid (2n) → haploid (n)

gametogenesis

• meiosis and the processes that follow result in the formation of ova and sperm, this is collectively called gametogenesis

• there are two types of gametogenesis:

  • spermatogenesis: the formation of sperm in the testes

  • oogenesis: the formation of ova in the ovary

homologous chromosomes

• pairs of chromosomes (maternal and paternal) that are similar in shape and size

• each gene is in the same position on homologues

• humans have 23 pairs of homologous chromosomes

  • 22 pairs of autosomes, 1 pair of sex chromosomes