Meiosis demystified: Part 1
All species of living organisms are able to reproduce. Reproduction may be asexual or sexual. In asexual reproduction, a single organism produces offspring that are genetically identical to itself. The cells of the new organisms are formed as a result of mitosis in eukaryotes or binary fusion in prokaryotes.
However, in sexual reproduction, the offspring that are produced are genetically different from each other and from their parent or parents. Each parent produces specialised reproductive cells, known as gametes, that fuse together in fertilisation to produce the first cell of the new organism - a zygote.
If a life cycle involves sexual reproduction, then it is necessary for the number of chromosomes to be halved at some point. This is done by a special type of cell division called meiosis. In animals such as humans, for example, meiosis occurs as gametes are formed inside the testes and ovaries (precisely, the germinal epithelium of ovary, and the germinal epithelium of seminiferous tubule). The cells from which the gametes will be produced are normal diploid (2n) cells, each containing two complete sets of chromosomes. As a result of meiosis, the gametes contain only half the normal number of chromosomes, and they are said to be haploid (n) cells. Thus, when two gametes fuse together at fertilisation. the zygote that is formed obtains two complete sets of chromosomes, returning to the diploid condition.
Meiosis does more than halve the number of chromosomes in a cell. Meiosis also introduces genetic variation into the gametes and therefore the zygotes that are produced. Genetic variation may also arise as a result of mutation, which can occur at any stage in a life cycle. Such variation is the raw material on which natural selection has worked to produce the huge range of species that live on Earth.
From Biology 2, Cambridge Advanced Sciences
However, in sexual reproduction, the offspring that are produced are genetically different from each other and from their parent or parents. Each parent produces specialised reproductive cells, known as gametes, that fuse together in fertilisation to produce the first cell of the new organism - a zygote.
If a life cycle involves sexual reproduction, then it is necessary for the number of chromosomes to be halved at some point. This is done by a special type of cell division called meiosis. In animals such as humans, for example, meiosis occurs as gametes are formed inside the testes and ovaries (precisely, the germinal epithelium of ovary, and the germinal epithelium of seminiferous tubule). The cells from which the gametes will be produced are normal diploid (2n) cells, each containing two complete sets of chromosomes. As a result of meiosis, the gametes contain only half the normal number of chromosomes, and they are said to be haploid (n) cells. Thus, when two gametes fuse together at fertilisation. the zygote that is formed obtains two complete sets of chromosomes, returning to the diploid condition.
Outline of the life cycle of an animal
Meiosis does more than halve the number of chromosomes in a cell. Meiosis also introduces genetic variation into the gametes and therefore the zygotes that are produced. Genetic variation may also arise as a result of mutation, which can occur at any stage in a life cycle. Such variation is the raw material on which natural selection has worked to produce the huge range of species that live on Earth.
From Biology 2, Cambridge Advanced Sciences
posted by Eau at 11:23 pm
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