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

sources of variation

1. crossing over - recombination of chromosomal sections in prophase 1

2. independent and random assortment of chromosomes into gametes

3. random fusion of gametes and fertilisation

genetic variation

• the advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring

• the three main sources of genetic variation arising from sexual reproduction are:

  • crossing over

  • random assort of chromosome

  • random fusion of gametes from different parents

crossing over

• during meiosis 1, homologous chromosomes (1 from each parent) pair along their length

• the chromosomes may cross over at point called chiasma

• at each chiasma, the chromosomes break and rejoin, trading some of their genes

• crossing over can result in a new combination of alleles along the chromosome, called recombination

• therefore, crossing over creates a new combination of genes so that the chromosomes passed on to the offspring are not exactly the same as those inherited from the parents

crossing over - recombination

• crossing over is an exchange of segments of chromosome between homologous chromatids during meiosis 1 (prophase 1)

• it may occur at one or more places along the chromosome

• allele closer together are less likely to be separated

independent (random) assortment

• describes how pairs of alleles separate independently from one another during gamete formation → the inheritance of genes / traits is independent to the inheritance of any other gene / trait

• this is due to the random orientation of pairs of homologous chromosomes in meiosis 1 → the orientation of each homologous pair is random and is not affected by the orientation of any other

• this means an allele on one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (the inheritance is independent)

• this random, independent assortment takes place for each of the 23 pairs of human chromosomes → any human egg receives one of two possible chromosomes 23 times (possible combinations are 2^23)

random fertilisation

• the fusion of two haploid gametes results in the formation of a diploid zygote

• this zygote can then divided by mitosis and differentiate to form a developing embryo

• as meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes

non disjunction (error in replication)

• refers to the chromosomes failing to separate correctly, resulting in gametes with one extra or one missing chromosome (aneuploidy)

• failure of chromosomes to separate may occur via:

  • failure of homologues to separate in anaphase 1 (resulting in four affected daughter cells)

  • failure of sister chromatids to separate in anaphase 2 (resulting in only two daughter cells being affected

aneuploidy - trisomy

• trisomy is a condition in which an individual inherits an extra copy of a chromosome - 3 copies instead of the normal 2

• one such chromosome defect is down syndrome, or trisomy 21

aneuploidy - monosomy

• monosomy is where an individual is missing a chromosome - they have only one copy instead of the normal two

• partial monosomy and partial trisomy can also occur

  • in partial monosomy, part of a chromosome is missing - part of the chromosome has two copies, but part only has one copy

  • partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes

mutation

• changes to the DNA code

• can happen spontaneously through mutagens (e.g. radiation / chemicals) or errors in replication

• if this occurs in gametes, it will be passed onto the next generation

mutation as the source of new variation

• many cellular processes exist to repair mutations in DNA because:

  • harmful mutations can stop a protein, and therefore a cell, from functioning properly

  • harmful mutations may impair the process of apoptosis, leading to cancer

• if a cell cannot repair a mutation it will try to undergo apoptosis

effects of mutation on survival

• neutral: does not change the amino acid or changes it to one with a similar shape and charge. Protein essentially unchanged

• deleterious: deletes, impairs or enhances the proteins activity in such a way that the organism is adversely affected. Can lead to premature death of the organism

• beneficial: changes the proteins activity in such a way that the organism benefits

• mutations occur throughout the genome and non coding regions are also affected

variation

• while variation between species allows us to tell them apart, variation is a common and important observation within species

• this is infraspecific variation

• variation in phenotypes can be genetically, or epigenetically, determined

morphological phenotypic variation

• shape and structure including internal anatomy

  • size and shape of noses

biochemical phenotypic variation

• chemical structure and composition of organisms including proteins, lipids, carbohydrates, and other molecules

  • expression of enzymes creating pigments and resulting colour of hair

physiological phenotypic variation

• metabolic and other bodily processes

  • blood group, haemophilia

behavioural phenotypic variation

• the ways individuals perceive, think, and react. This includes congnition and behaviour

  • agression, inquisitiveness, mate selection

mitosis vs. meiosis