lecture 11 Flashcards
: Meiosis is
the type of cell division that gives rise to the sex cells, or gametes.
One of the key features of Meiosis
is that the gametes are haploid, each containing just a single copy of the genome. Meiosis results in haploid cells because it is, in effect, two successive cell divisions, with no DNA replication occurring during either of them.
During meiosis I,
the two members of a pair of homologous chromosomes line up alongside one another to form a bivalent.
• Meiosis requires two successive cell divisions
• During meiosis I, bivalents are formed between homologous chromosomes
• Formation of bivalents ensures that siblings are not identical to one another
• Recombination occurs between homologous chromosomes within a bivalent
During S phase
of the cell cycle, each chromosome is replicated, the daughters remaining attached to one another at their centromeres (Tetraploid).
Meiosis I: the homologus chomomosmes
are by no means independent; each chromosome finds its homolog and forms a bivalent.
Mitosis:
Homologous chromosomes remain separated from one another.
After formation of the bivalents, meiosis I
continues in a similar fashion to a mitotic cell division. Microtubule radiates out from the centrosomes, attach to the kinetochores, and begin to pull in opposite directions. The tension exerted on a bivalent breaks it apart, and the plate set of chromosomes are pulled in opposite directions. A complete set of chromosomes is therefore assembled at each of the poles of the mitotic spindle. Meiosis I is then completed by cytokinesis.
Meiosis II:
The events are similar to those occurring during meiosis I, except that each starting cell has only one member of each pair of homologous chromosomes, so no bivalents are formed
Formation of bivalents
ensures that siblings are not identical to one another
Allele:
the variation of a biological characteristic. The homologous chromosomes in a bivalent are not identical.
Random segregation of homologous chromosomes during anaphase I
means that the gametes resulting from meiosis are not all identical.
The random segregation of chromosomes during anaphase I, when the bivalent separate
, gives rise to a vast range of possible combinations in the resulting gametes.
causes of variabilitu of gametes
A crossover between two chromatids in a bivalent Random segregation of chromosomes during anaphase . The bivalent contributes to the resulting variability in a second, even more important way: crossing over or recombination within the bivalent, the chromosomes arms – the chromatids – can exchange segments of DNA. This exchange is called crossing over or recombination. The variability of the gametes is increased even further by crossing over between homologous chromosomes within a bivalen
crossing over or recombination
within the bivalent, the chromosomes arms – the chromatids – can exchange segments of DNA. This exchange is called crossing over or recombination.
Recombination:
The outcome of crossing over between pairs of homologous chromosomes and the generation of new allele combinations during meiosis. It involves the breakage and subsequent rejoining of DNA molecules.
THE MOLECULAR BASIS OF RECOMBINATION
- Homologous recombination begins with formation of a DNA heteroduplex
- Cleavage of the Holliday structures results in recombination
- The biochemical pathways for homologous recombination have been studied in E. coli
- The biochemical basis of recombination in eukaryotes is less well understood
The initial steps in homologous recombination,
resulting in formation of a heteroduplex.
- Homologous recombination begins when the two double stranded molecules line up adjacent to one another.
- A double stranded cut is made in one of the molecules, breaking this one into two pieces.
-One strand in each half of this molecule is then shorten by removal of a few nucleotides, giving each end a 3’
overhang.
The partnership between the chromosomes is set up when one of the 3’ overhangs invades the uncut DNA molecules, displacing one of its strands and forming D –loop.
After strand extension, the free polynucleotide ends are joined together. This gives a structure called a heteroduplex, in which the two double – stranded. Molecules are linked together by a pair of Holliday structures.
Each Holliday structure is dynamic and can move along the heteroduplex. This branch migration results in the exchange of longer segments of DNA.
a heteroduplex
After strand extension, the free polynucleotide ends are joined together. This gives a structure called , in which the two double – stranded. Molecules are linked together by a pair of Holliday structures. Each Holliday structure is dynamic and can move along the heteroduplex. This branch migration results in the exchange of longer segments of DNA.
The two possible ways of resolving a Holliday structure.
Separation, or resolution, of the heteroduplex back into individual double stranded molecules occurs by cleavage of the Holliday structure
picture of homologus
type of resou;tuion
