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Biology - Topic 6 - Inheritance, (DNA structure/meiosis/types of…
Biology - Topic 6 - Inheritance, (DNA structure/meiosis/types of reproduction/mutation)
DNA
DNA stands for doxyribonucleic acid. It's the chemical that all of the genetic material in a cell is made up from.
DNA is found in the nucleus of animal and plant cells, in really long structures called chromosomes (normally come in pairs).
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Genes
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Each gene tells the cells to make a particular sequence of amino acids which are put together to make a specific protein.
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DNA determines what proteins the cell produces and that, in turn, determines what type of cell it is (e.g. red blood cell, skin cell etc)
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Reproduction
Asexual reproduction
Bacteria, some plants and some animals reproduce asexually.
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In asexual reproduction, there's only one parent. There's no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring.
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Fungi
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These species release spores, which can become new fungi when they land in a suitable place.
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Malarial Parasites
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When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human.
The parasite reproduces sexually when it's in the mosquito and asexually when it's in the human host.
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Sexual Reproduction
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Sexual reproduction is where genetic information from two organisms is combined to produce offspring which are genetically different to either parent.
Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents' genes.
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Meiosis
Gametes
To make gametes which only have half the original number of chromosomes, cells divide by meiosis.
This process involves two cell divisions. In humans, it only happens in the reproductive organs (in females it's the ovaries and in males, the testes.
Gametes only have one copy of each chromosome so that when gamete fusion takes place, you get the right amount of chromosomes again.
The Process
- In the second division, the chromosomes line up in the centre of the cell and the arms are pulled apart.
- The cell duplicates its genetic information, forming two armed chromosomes(called chromatids). After replication, the chromosomes arrange themselves into pairs.
- In the first division, the chromosome pairs line up in the centre of the cell. At this point crossing over can occur - this is where sections of chromosomes can swap as they are positions alongside each other - this increase the mixing of genes and variety
- The pairs are pulled apart so each new cell only has one copy of each chromosomes (some of the father's and some of the mother's go into each new cell).
You get four gametes, each with only a single set of chromosomes in it. Each of the gametes is genetically different from the others because the chromosomes get all shuffled up during meiosis and each gamete only gets half of them, at random.
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