S-Cool Revision Summary
S-Cool Revision Summary
DNA replicates as mitosis.
Homologous chromosomes condense ( synapsis ) to form bivalents. The chromatids become coiled around each other. As the chromosomes pair up ( homologous ), the twisting produces tension, and sometimes sections of chromatid may break and exchange new partners with corresponding sections of different chromatids. These breakage points result in "cross-overs" or Chiasmata .
The bivalents move to the equator of the cell. Which pair of chromosomes orientates to which pole is completely random (called random assortment .)
The pairs of bivalents seperate into chromatid pairs, each pair of chromatid is pulled to a pole.
Note: Unlike mitosis, there is no division of the centromeres at this stage.
The pairs of chromatids reach their respective poles, the cell divides.
New spindle is formed and the centrioles have replicated. Nuclear membrane disintegrates.
The pairs of chromatids line themselves up on the equator as in mitosis, with sister chromatids orientated toward opposite poles.
The centromeres divide and the chromatids separate, migrating to opposite poles.
The cell divides. Nuclear membranes and nucleoli are reformed. The chromosomes uncoil and go into interphase. The daughter cells have half the number of chromosomes present in the original cell.
Most exam boards only require knowledge about reproduction in Angiosperms - the flowering plants.
Sexual reproduction in flowering plants centres around the flower. Within a flower, there are usually structures that produce both male gametes and female gametes.
Inside the ovary there may develop one or more ovules . Each ovule begins life as a small projection into the cavity of the ovary. As it grows and develops it begins to bend but remains attached to the ovary wall by a placenta .
At the start, the ovule is a group of similar cells called the nucellus . As it develops, the mass of cells differentiates to form an inner and an outer integument, surrounding and protecting the nucellus within, but leaving a small opening called the micropyle .
At the centre of the ovule is an embryo sac containing the haploid egg cell (the female gamete ).
Each anther contains 4 pollen sacs . Many pollen grains develop inside each pollen sac. It begins with a mass of large pollen mother cells in each pollen sac. All are diploid .
In each pollen grain the wall thickens and forms an inner layer ( the intine ) and an often highly sculptured outer layer ( the exine ). The surface pattern is different on pollen grains from different species. When the pollen grains are mature, the anther dries out and splits open (a process called dehiscence ) and the pollen is released.
Many plants favour cross-pollination , so pollen must be transferred to the stigma of another plant if sexual reproduction is to take place. Some flowers rely of the wind to carry pollen grains others rely on insects.
Self-pollination is where the pollen is transferred to the stigmas of the same flower or the stigma of another flower on the same plant. Self-pollination is obviously more reliable, particularly if the nearest plant is not very close.
If the pollen grain lands on a compatible stigma, a pollen tube will grow so that eventually the egg cell, hidden away in the embryo sac, can be fertilised . A tube emerges from the grain, its growth being controlled by the tube nucleus at the tip of the tube. It may grow downwards in response to chemicals made by the ovary (a response known as chemotropism).
When conditions are right, the seed will take up water through the micropyle by imbibition. This triggers the beginning of the growth of the seed.
The cell swells and the testa splits. With the addition of water, large molecules of carbohydrate, protein and fat can be hydrolysed (broken down) to produce substances for respiration.
Like in plants it is the male gamete that needs to be transferred to the female gamete. The female gamete is fertilised and develops inside the mother's body so the reproductive systems of both males and females are highly adapted for this.
Production of sperm is called spermatogenesis . It takes place in the gonads of the male - the testes .
Each testis is composed of numerous tiny tubes called seminiferous tubules . It is in the walls of these tubules that sperm production actually takes place.
Development begins in the outer side of the wall in a layer of cells called the germinal epithelium .
The control centres are the pituitary gland and the hypothalamus in the brain.
The hypothalamus secretes GnRH (gonadotrophin releasing hormone) . This is released into the blood and stimulates the anterior lobe of the pituitary gland.
The anterior lobe of the pituitary gland secretes ICSH (interstitial cell stimulating hormone) .
ICSH: this stimulates the leydig cells that produce testosterone.
FSH: this stimulates the seminiferous tubules, including the Sertoli cells. They produce sperm in response.
Note: Testosterone also acts on the seminiferous tubules and stimulates sperm production.
The production of eggs is called oogenesis . It takes place in the ovaries and begins before birth.
The outer layer of the ovary (the germinal epithelium ) produces primary oocytes. It also produces follicle cells that congregate around the oocytes, forming a structure called the primary follicle .