Intracellularly - Meiosis Flashcards
Meiosis
This is cell division where a diploid cell splits into four haploid cells in sexual reporduction in eukaryotes.
Synapsis
Fusion of chromosome pairs at the start of meiosis
What is the process of meiosis brief?
Four strand structure forms where the homologous chromosomes attach
Chromosomes seperate to poles, with cleavage(haploid)
Chromatids seperated into two seperate daughter cells, haploid
Meiocytes
A cell differnetiating into a gamete through meisosi
What are the two meiocytes?
Oocytes and Spermatocytes
What are the four phases of mieosis?
Prophase, metaphase, anaphase, telophase
What are the five stages of prophase?
Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis
What happens in leptotene?
Chromosomes become visible and threadlike by condensation, with synapsis
What carries out synapsis?
Synaptonemal complex
Synaptonemal Complex
This holds the two homologous chromosomes together
What happens in Pachytene
Crossing over occurs
Homologous Recombination
This is the exchange of segments of chromosomes by cross-wise joining across holliday junctions
First step in HR?
Introduction of DSBs by Spo11 protein
Why is DSB break important in HR?
Allow linkage of the DNA segments and spatial coalignment of the homologs
Second step in HR…
Spo11 remains bound after DS cleavage, Exo1 exonuclease resects DNA ends exposed, exposing 3’s tails
Third step in HR…
Tails bound by RecA, which catalyze strand invasion into homologous duplex DNA
Holliday junction
Homologous recombination intermediates, being key sites of genetic recombination.
Roles of recombination..
Increase genetic diversity, pairs/segregates the homologs
What happens in Diplotene
Synaptonemal Complex breaks down, and chiasmata begin to seperate
Chiasmata
Chromatin structures that link homologs together until anaphase
What happens in Diakinesis?
Chromosomes fully condensed and the homologs repel
Metaphase
Where the two bivalents, by their homologous chromosomal centromeres, via MT action, orient to opposite poles
Where do chromosomes allign in metaphase?
Metaphase plate.
How is bivalent split in progeny random?
Orientation in metaphase plate, which is random, determines which bivalent is found.
What are the gametes two heterozygous homologs could form?
A B and a b or A b and a B with 1;1;1;1 ratio
What is the physical basis of independent assortment?
The randomness of allel seperate in metaphase
Anaphase
Poleward segregation occurs and allele seperate
What is the physical basis for Principle of Segregation?
Anaphase poleward segregation and allele separation.
Srtucture of Synap Complex…
Zipper like structure, in prophase 1 the sister chromatids develop axial element, connceted by transever filaments
Axial Element
An SC structure organising chromatids into DNA loops, with bases anchroed to it.
Functional importance of chiasmata?
Attachment of the two homologous chromosomes to opposite spindle poles, for segregation in meiosis 1
Importance of chiasmata?
Efficient homolog disjunction
Process of crossover…
Spo11 DSB, DNA resection producing a DNA strand with a 3’ end subsequently bound by Rad51
Crossover with invasion of 3’ tail into OTHER homolog, forming the holliday junction.
Crossover Interference
Describes the non-random placement of crossovers with respect to each other in meiosis.
Why does crossover interference occur?
Formation of crossover is tension dependent, however crossovers also relieve tension, so crossovers can form only locally.
Heterochromatin
A chromosome material of increased density than normal, supressing/modifying gene activity.
Structure of the holliday junction in crossover….
D loop formation with strand invasion, where DNAP extends 3’ end, forming holliday junction by annealing of 3’ end to the D loop
