Slides in this set
Why is meiosis necessary?
· Meiosis produces four daughter nuclei, each with half the number of
chromosomes as the parent cell.
· Every diploid cell of an organism has two sets of chromosomes.
· During meiosis, the chromosome pairs separate, so that only one
chromosome from each pair enters each gamete. This is known as the
haploid number of chromosomes.
· When two haploid gametes fuse, the diploid number is restored.
· If each gamete has a diploid number of chromosomes, then the cells that
they produce has double this number.
· This doubling of the number of chromosomes would continue at each
· In order to maintain a constant number of chromosomes in the adults of
a species, this number must be halved and it occurs as a result of
Process of Meiosis
Meiosis involves two nuclear divisions
· First division (meiosis 1)
- homologous chromosomes pair up
- their chromatids wrap around each other
- equivalent portions of chromatids get exchanged (crossing over)
- by the end of this stage, homologous pairs separate
- one chromosome from each homologous pair goes into one of
the two daughter cells
· Second division (meiosis 2)
- the chromatids move apart
- at the end of meiosis 2, four cells are formed
- in humans, each of these cells contain 23 chromatids…read more
Genetic Variation by Meiosis
· Meiosis also produces genetic variation among the offspring, allowing
an organism to adapt and survive in a changing world.
· Genetic variation by meiosis occurs in two ways
- independent segregation of homologous chromosomes
- recombination of homologous chromosomes by crossing over
· Refreshing some basic terms
- Gene: a section of DNA that codes for a polypeptide
- Locus: the position of a gene on a chromosome/ DNA molecule
- Allele: one of the different forms of a particular gene…read more
· During meiosis 1, each chromosome lines up alongside its homologous
· In humans, this means that there will be 23 homologous pairs lying
side by side.
· They arrange themselves in this line randomly.
· One of each pair will pass to each daughter cell.
· Which one of the pair goes into the daughter cell, and with which one
of any of the other pairs, depends on how the pairs are lined up in the
· Since the pairs are lined up at random, the combination of
chromosomes that goes into the daughter cell at meiosis 1 is also
random. This is called independent segregation of chromosomes.…read more