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
Meiosis results in haploid cells because it is
, in effect, two successive cell divisions, with no DNA replication
a bivalent
During meiosis I, the two members of a pair of homologous chromosomes line up alongside one another to form it
a vast range of possible combinations in the resulting gametes during anaphase 1
The random segregation of chromosomes during anaphase I, when the bivalent separate,
heteroduplex,
a structure within which two double- stranded DNA molecules are linked together by a pair of crossovers. Resolution of a heteroduplex can lead to exchange of chromosome segments.
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.
where is the recombination occure
it occurs between two double stranded DNA molecultes that have regions where the nucleotide sequences are th esame or att least very similar
Chi form
Holliday structure in its true three dimentional configuration
Mendel’s crosses revealed a
regular pattern to the inheritance of the chrachertistic
the most important aspect of his crosses was tat one of the partntal characteristic disappeared in the F1 genration and then reapperaed in the F2 generation
how one of the parental charcteristic could disappear in the f1 genration and reapperar in the F2 plants
this suggests that during sexual reproduction the alleles of each paretnt separate (segregate) ,producing intermediate structures that contain just one allele
first law of genetics
alleles segregate randomly
relatioships between paris of alleles
- dominant over the other
- incomplete dominance
- lethal alleles result in death of homozygote
- condominant
round pea has
functional gene for the strach branching enzyme SBE1 convert teh sucrose to starch
incomplete dominance ( the phenotype of th eheterozygote is inetermediate between those of the tow homozygotes
example of single functinal copy of gene in a geterozygote is not able to direct encough proetein to maintain the requried levels of th egene procuct, ANS gene of carnations
lethal alleles
homozygote cannot survive
codominant alleles
both member of pair of alleles must be functional,
codominance then occurs when bothe alleles are active in the heterozygous state
interactions between alleles of different genes
can have additive effects ot determine the nature of a phenotype
complematery gene action
interactons occur between genes controlling different steps in a biochemical pathway
-particular combination of alleles of two separate genes is neededin order to produce the phenotype
epistasis
certain alleles of one gene mask or cancel the effects of alleles of a second gene
like presence of carotenoid pigment
gene W inhibits synthesis of the green precursor
quantitive traits
interactions between multiple genes
Qunatitive traits
the productivity of most crops and farm animal is determined by quantitive traits
cross over result in
gametes wuth recombinant genotypes
the freqency of recombinats enables
the map positions of genes to be worked out
distance between genes are expressed in
Map unites ( centimorgan)
crossovers are less frequent
near the centromere of the a chromosome
-if the two genes that are close together
recombination hotspots
are more likely to be invoved in crossover than other
gene mapping
requires planned breeding experiments or pedigree analysis
gene mapping in humans is
carried out by pedigree analysis
the freqency of recombination between a pair of genrs
that lie on the same chromosome
can be used to map the relative positions of thoses two gene
