Mitosis and meiosis
Mitosis and meiosis share the same starting machinery — an S phase that replicates the DNA and a spindle that distributes the chromosomes — but they solve different biological problems: mitosis maintains the chromosome complement so that the organism can grow and renew its tissues; meiosis halves it to make sexual reproduction possible and to generate genetic variability.
Mitosis: a division that maintains the chromosome complement
When you look at cells at the tip of an onion root under the light microscope in your school lab, you can see nuclei in interphase and others in which the chromatin is condensed into little rods that line up in the centre and then separate into two symmetrical groups. By what mechanical process does the cell make sure that its two daughter cells inherit exactly the same genetic information?
Explain the biological function of mitosis in multicellular organisms and give two examples of processes in which it plays an essential role.
Explain the biological function of mitosis in multicellular organisms and give two examples of processes in which it plays an essential role.
Meiosis: two divisions to produce gametes
If human gametes were formed by mitosis and kept their 46 chromosomes, the zygote resulting from fertilisation would have 92 and its descendants 184, doubling the genome in every generation. Which type of cell division halves the chromosome complement in order to keep the number constant for the species?
Crossing over
In prophase I (the first stage of meiosis I, after the S phase), homologous chromosomes pair up to form bivalents. The first diagram shows the crossing over in the starting cell only to summarise it: it actually happens during prophase I, before the homologues separate. Non-sister chromatids of the two homologues can break at matching points and swap equivalent segments. The points of exchange are called chiasmata. The result is chromatids carrying a new combination of alleles, so even two gametes that receive the same homologue differ from each other. (Crossing over is not required by the syllabus, but you need it to explain variation.)
How many daughter cells are obtained after mitosis and how many after complete meiosis? State also the chromosome complement of these daughter cells compared to the…
How many daughter cells are obtained after mitosis and how many after complete meiosis? State also the chromosome complement of these daughter cells compared to the parent cell in each process.
What is fertilisation? Reason why meiosis is essential for keeping the chromosome number constant from one generation to the next in sexually reproducing species.
What is fertilisation? Reason why meiosis is essential for keeping the chromosome number constant from one generation to the next in sexually reproducing species.
During prophase I of meiosis, crossing over occurs. Explain what this phenomenon consists of and its consequence for the genetic variation of the gametes.
During prophase I of meiosis, crossing over occurs. Explain what this phenomenon consists of and its consequence for the genetic variation of the gametes.
Mitotic metaphase, metaphase I and metaphase II: what lines up and what separates
When the distribution fails: non-disjunction
The karyotype of a person with Down syndrome (as screened by the HSE) reveals 47 chromosomes instead of 46, due to the presence of three copies of chromosome 21 in all of their somatic cells. At what point in meiotic division does the distribution fail so that a gamete receives an extra copy?