Sex, DNA and Chromosomes
Chromosomes
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Chromosomes are DNA that have condensed into discreet units. The number of chromosomes an organism has depends on its species. All species have a characteristic number of chromosomes. For instance, humans have 46 chromosomes. |
The chromosomes appear as "X's" because after the DNA has replicated, there are two identical strands connected at a central structure called the centromere.
Most sexual species actually have 2 complete sets of chromosomes, and are known as diploid. Humans have 23 unique (N) chromosomes, and because we have 2 sets, one from each parent, our total number of chromosomes is 46. This is the diploid number (2N).
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Homologues, or homologous pairs, are the pair of chromosomes from each parent; in other words, chromosome 1 from the mother and chromosome 1 from the father. These chromosomes are identical in that they contain the same genes. Genes can differ in terms of alleles, or the version of a particular gene. An example is eye color. There is one gene for eye color, but two alleles, one for brown eyes and one for blue eyes. These genes are located on the same places on the two homologues. |
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Every chromosome we receive from one parent has a homologue from the other parent. (The only exception to this is the Y chromosome received from the father in male offspring). |
Meiosis
Meiosis is the process of cell division in sexually reproducing organisms that reduces the number of chromosomes in reproductive cells from diploid to haploid, leading to the production of gametes in animals and spores in plants.
| Meiosis I | ||
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Prophase I | Chromosomes condense, nuclear membrane breaks down, spindle fibers form. Tetrads form (homologous pairs) and recombination occurs. |
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Metaphase I | Tetrads line up on metaphase plate |
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Anaphase I | Tetrads split, reducing the cells to haploid |
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Telophase I | Cell membranes starts to pinch off |
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Cytokinesis I | Cells split, leaving two haploid cells - with replicated DNA in them |
| Meiosis II | ||
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Prophase II | Chromosomes condense, nuclear membrane breaks down, spindle fibers form. NO replication or recombination. |
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Metaphase II | Chromosomes line up on metaphase plate |
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Anaphase II | The chromosomes divide at the centromeres |
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Telophase II | Chromosomes begin to unravel, nuclear envelope reforms |
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Cytokinesis II | Cells split, leaving 4 haploid gametes |
Differences Between Mitosis and Meiosis as seen in a plant cell of 2n=6
| Mitosis | Meiosis I | Meiosis II (for 1 haploid cell) | |
|---|---|---|---|
| Prophase | no synapsis occurs |
synapsis occurs forming tetrads/bivalents and crossing over
occurs |
no synapsis occurs and the cell has a haploid number |
| Metaphase | duplicated chromosomes line up single file |
tetrads line up double file |
duplicated chromosomes line up single file, cell has a
haploid number |
| Anaphase | chromatids split so the centromeres divide
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tetrads separate, centromeres do not divide
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chromatids split so the centromeres divide, cell has a
haploid number |
| Telophase | 2 diploid nuclei develop |
2 haploid nuclei develop |
2 haploid nuclei develop |
| Interphase | DNA replication does not occur |
Karyotyping
Karyotyping is used to detect genetic abnormalities. To do so, the cell containing the chromosomes is put into a hypotonic solution so that it will burst. The chromosomes can then be photographed.
The photographs are cut up and chromosomes are matched by size.
Below are two genetic anomalies dealing with the sex chromosomes.
Helpful Tutorials
Review Questions
- Illustrate and describe the 8 phases of Meiosis.
- When and why does mitosis or simple cell division take place?
- Describe the pros and cons of both sexual reproduction (meiosis) and asexual
reproduction (mitosis in single celled organisms).
- What is a karyotype?