BIOL10003 Chapter Notes - Chapter ILT 2 + 4 : Synapsis, Telophase, Chromatid
ILT: Cell Biology Part 2: Cellular reactions
Enzymes
• The cell also has mechanisms to regulate the activity of the enzyme by altering the speed with
which it catalyses the reaction.
• Are aily proteis (a molecule that has a precise shape dictated by its amino acid sequence and
the prevailing physical conditions)
• Shape affects actiity. Ezye hae a specialised pocket or zoe ito hich the reactats (ko
as the substrate) fit snugly. Once the substrate, (or substrates when the reactions involves two or
more reactants) has bound to the enzyme
ILT: Meiosis
• cell division that occurs in sexually reproducing organisms in which the diploid chromosome
complement is halved, i.e. haploid. The diploid number is restored at fertilisation when two haploid
gametes fuse
• DNA is replicated prior to eiosis
Chromatid: at this stage of meiosis each homologues is
double stranded. Each strand is called a chromatid.
Chromosomes: one of the homologues comes from the
father and the other from the mother.
Sister chromatid: 2 chromatids of one homologue
Non-sister chromatid: two chromatids from different
homologues
Centromere: green bit, the position of the centromere on
homologous chromosomes is identical and the gene loci
found along the homologues are also identical.
Kinetochore: The kinetochore is the point of attachment of
the microtubules which pull the chromosomes apart.
First meiotic division – Prophase I
The homologous pair of chromosomes move together and synapse — a
process where the homologues line up so that their centromeres and gene
loci are matched exactly.
A synaptonemal complex made of protein forms between the homologous
chromosomes and holds them together.
This structure formed at synapsis is called a bivalent.
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First meiotic division — Prophase I — Crossing over
During synapsis recombination nodules (not visible under light microscopy) appear at
intervals along the bivalent. The nodules are enzyme complexes which play a role in the
exchange of pieces of chromatid. Depending on the size of the chromosome there may be
more than one cross over per bivalent.
<chiasma
First meiotic division — Metaphase I
The homologues are attached at the chiasmata (three chiasmata in this case) the centromeres repel
each other, and the maternal and paternal homologues pull away from each other. homologue are
double stranded.
First meiotic division — Early Anaphase I
The synaptonemal complex breaks down. The kinetochores of sister chromatids behave as
one and the microtubules attach to them and assist in the separation of the homologues to
opposite poles of the spindle.
The homologues remain attached at chiasmata, (singular: chiasma) which are the physical
evidence of a cross over event.
First meiotic division — Late Anaphase I
The homologous pair of chromosomes separate into different cells
First meiotic division — End of Anaphase I
At the end of anaphase I the two new cells produced contain only one chromosome of each
homologous pair. The chromosome number has been halved ie. the cell is haploid. Each
chromosome is still composed of two chromatids. Notice how there has been recombination of the
maternal and paternal genetic material.
Second meiotic division — Prophase II
At the end of the first meiotic division, nuclear envelopes reform and the chromosomes condense.
There may be a brief interphase although some cells appear to pass directly from telophase I to
prophase II.
No DNA replication occurs in this interphase.
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