Unit 2 biology revision -

unit 2, sexual reproduction and cell specialisation, stem cells and beyond, expressing the genome

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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 the number of chromosomes in a
diploid cell is characteristic for that species. If a diploid cell combines to form a new individual, the offspring would
have four sets of chromosomes, losing the characteristic specific to that particular species. If this happened then the
cells would eventually become too heavy, break down and die. To avoid this, haploid nuclei are formed with one set
of chromosomes (half number), usually within specialised cells called gametes. Sexual reproduction occurs when two
haploid nuclei fuse to form a new diploid cell called a zygote, a process called fertilisation.
Gametes are the male and female sex cells found in all organisms that reproduce sexually. They join together at
fertilisation to form a zygote, which divides and develops into a new organism. In animals, the male gametes are
sperm and the female gametes are egg cells (ova). In plants, the male gametes are contained in pollen grains and the
female gametes are contained in ovules. Normal body cells of plants and animals contain the full number of
chromosomes. Humans have two sets of 23 chromosomes ­ one set from each parent ­ giving each body cell a total
of 46 chromosomes. Gametes contain only half the number of chromosomes as body cells ­ they only contain one set
(23 in total for humans).
Meiosis ­ two nuclear divisions give rise to four haploid daughter cells, each with its own unique combination of
genetic material. These four genetically individual cells are a result of a process called crossing over. The events of
meiosis are continuous although described in separate phases.
Like mitosis, the contents of the cell are replicated, in particular DNA, whilst the cell is in Interphase.
1. The DNA replicated so there are two identical copies of each chromosome, called chromatids.
2. The DNA condenses to form double-armed chromosomes, made from two sister chromatids.
3. The chromosomes arrange themselves into homologous pairs ­ pairs matching chromosomes (one from each
set of 23, e.g. both number 1s)
4. First division ­ the homologous pairs are separated, halving the chromosome number
5. Second division ­ the pairs of sister chromatids are separated
6. Four new cells (gametes) that are genetically different from each other are produced
Meiosis produces cells that are genetically different ­ genetic variation is the difference that exists between
individuals' genetic material. The reason meiosis is so important is because it creates genetic variation.

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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 their
genetic material
4. The chromatids still contain the same genes, but they now have a different combination of alleles
5.…read more

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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 species are around 50m long.…read more

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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.
Fertilisation process:
1. Sperm swim towards the egg cell in the oviduct
2.…read more

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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 sac, the tube nucleus disintegrates and the tip of the pollen tube bursts, releasing the two
male nuclei.
5. One male nucleus fuses with the egg nucleus to make a zygote.…read more

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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 in humans are only present in the early life of an embryo ­ they differentiate into
extraembryonic cells and pluripotent stem cells. The pluripotent stem cells then differentiate into the
specialised cells in a foetus.…read more

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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 switched on and activated.
Other genes, like ones to remove the nucleus, are switched on too.…read more

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Adult stem cells aren't as flexible as embryonic stem Once the embryos are approx.…read more

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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 block colour.
The agouti alleles are A and a. AA or Aa gives an agouti coat ­ grey hairs with a yellow band, resulting in a range of
brownish colours.…read more

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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.
15% of human cancers are the result of viral infections of cells ­ i.e.…read more


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