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Chapter 3.2 Sexual reproduction and cell
I can explain the role of meiosis in the production of gametes and genetic variation through recombination of alleles
and genes including independent assortment and crossing over:
A cell containing two full sets of chromosomes (23 pairs) is called a diploid cell and…

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Crossing over of chromatids ­
1. Before the first division of meiosis, homologous pairs of chromosomes come together and pair up
2. Two of the chromatids in each homologous pair twist round each other
3. The twisted bits break off their original chromatid and re-join onto the other chromatid, recombining…

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3. When those gametes are produced, different combinations of those maternal and paternal chromosomes go
into each cell
4. This is called independent assortment (separation) of the chromosomes.

I can explain how mammalian gametes are specialised for their functions:
Spermatozoa: many, mini, motile
The male gametes (sperm) of most mammalian…

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In mammals, fertilisation occurs in the oviduct
Sperm is deposited high up in the female vagina, close to the cervix. Once there, they have to make their way up
through the cervix and uterus, and into one of the oviducts. Once the sperm are in the oviduct fertilisation may occur.…

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behind it. The tube nucleus makes enzymes that digest surrounding cells, making a way through for the
pollen tubes.
3. When the tube reaches the ovary, it grows through the micropyle (a tiny hole in the ovule wall) and into the
embryo sac within the ovule.
4. In the embryo…

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Totipotency ­ ability to produce all cell types, including all the specialised cells in an organism and
extraembryonic cells (cells of the placenta and umbilical cord)
Pluripotency ­ the ability of a stem cell to produce all the specialised cells in an organism (not
extraembryonic cells)
6. Totipotent stem cells…

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Example ­ Red Blood Cells:
Red blood cells = produced from a type of stem cell in the bone marrow. They contain lots of haemoglobin
and have no nucleus to make space for the oxygen carrier
The stem cell produces a new cell in which the genes for haemoglobin are…

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bone (usually the hip) and a small quantity of bone
marrow is removed
Adult stem cells aren't as flexible as embryonic stem Once the embryos are approx. 4-5 days old, stem cells
cells ­ they can only develop into a limited range of cells are removed from them and the…

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An example of this is coat colour in a domestic cat. Cats have a number of genes which affect the expression of coat
colour, two code for the coat pattern. The agouti gene causes the `tabby' appearance in some cats ­ without the
agouti gene, the cat would be a…

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severe spiral of decline if the cancer spreads in this way. This splitting is called metastasis. The tumour that invades
surrounding tissues is known as a malignant tumour. Cancer cells divide more and live longer; the enlarging tumour
leads to disruption of normal tissues and can kill the organism.


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