Meiosis and recombination

Week 3

Meiosis

Cell division in gonads to make four genetically unique gametes

Independent Alignment

Orientation of homologous chromosomes in meiosis is random

Recombination

Crossing over between homologous chromosomes generates genetically unique gametes

Prophase I

Chromosomes condense, homologous chromosomes pair up forming tetrads, crossing over occurs

Synapsis

Pairing of homologous chromosomes during meiosis

Metaphase I

Tetrads align at metaphase plate, spindle fibers attach to centromere of each homolog

Anaphase I

Homologous chromosomes are pulled apart

Telophase I

Nuclear envelope reforms, chromosomes decondense

Cytokinesis in meiosis I

Cell divides into two daughter cells (2n)

Prophase II

Chromosomes condense, spindle forms

Metaphase II

Sister chromatids align at metaphase plate

Anaphase II

Sister chromatids are pulled apart

Telophase II

Sister chromatids reach poles and nuclear envelope reforms

Cytokinesis in meiosis II

Cells divide into 4 genetically unique haploid cells (n)

Synaptonemal complex

Holds homologous chromosomes together and facilitates crossing over

When does the synaptonemal complex dissolve?

Before meiosis I, leaving DNA crossover and sister chromatid cohesion to maintain homolog attachment

Recombination nodules

Protein complexes that form on the synaptonemal complex to facilitate recombination among non-sister chromatids

What maintains the physical association between homologous chromosomes during meiosis?

Homologous recombination

Spo11 endonuclease

Cleaves the phosphodiester bonds between nucleotides to induce double strand break for recombination

Exonuclease

Degrades the 5’ ends to expose 3’ single-stranded tails

Dmc1

Recombinase that promotes strand invasion and homologous pairing

Displacement loop

The invading single strand pairs with complementary strand on a homolog, displacing the original strand, displaced strand can pair with the second end of broken DNA, allowing both ends to be joined and repaired

Replication protein A

Binds to single stranded DNA and prevents its degradation

First strand invasion

Invasion of non-sister chromatid results in a heteroduplex

Heteroduplex

Double stranded DNA complex that forms when each strand comes from different chromatids during homologous recombination

Formation of a Holiday Junction

Reciprocal second strand invasion forming an X pattern

Ligase

Fix breaks in bond joining the DNA strands

Resolvase

Enzyme that cuts the holiday junctions

Branch migration

Two holiday junctions move away from each other to lengthen the heteroduplex region

Bivalent

Paired structure formed by two homologous chromosomes (Tetrad) held together by synapsis and recombination

Recombination crossovers

Physical connections between chromatids of homologous chromosomes

Chiasmata

Visible point where two homologous chromosomes connect in meiosis

Orientation of sister kinetochores in meiosis I vs II

Co-orientation (Same spindle) in meiosis I and bi-orientation (Opposite spindles) in meiosis II

Shugoshin

Blocks ability of separase to cleave cohesin near centromeres in meiosis I

How often do recombination crossovers occur per homologous pair

At least once per homologous pair

Productive vs nonproductive crossover

Productive crossovers lead to proper genetic recombination and nonproductive crossovers do not result in proper recombination

Single crossover event

Recombination between one gene and its neighbor

Double crossover event

Two independent crossovers between three linked genes on the same chromosome, much less frequent

Aneuploidy

Abnormal number of chromosomes as a result of nondisjunction in chromosome segregation during meiosis or chromatid segregation during mitosis

Nondisjunction

Failure of chromosomes to segregate properly during meiosis, resulting in aneuploidy

Heterochromatin vs euchromatin

Heterochromatin is tightly packed, AT rich, gene-poor regions

Euchromatin is loosely packed, CG rich, gene-rich regions

Trisomy

Cell containing three copies of chromosomes instead of the normal two (ex Down Syndrome)

Monosomy

Cell that has one copy of a chromosomes instead instead of the normal two (ex Turner’s syndrome)

Karyotype

Visualizing the complete set of chromosomes an individual has, and any abnormalities that could result in growth defects

Philadelphia chromosome

Translocation of chromosome 9 on 22

Chronic myeloid leukemia

Patau, Edward, and Down syndrome

Trisomy due to nondisjunction

Why is aneuploidy more common in females than males?

Females produce all oocytes during development, whereas males continue making sperm

Remain arrested in meiosis, leading to loss of cohesion of sister chromatids

What are the three classes of double crossovers?

Two-strand, three-strand, four-strand double crossover

Where does recombination typically occur?

Outside of the coding regions (Genes) of chromosomes in non-coding sequences

How do base mismatches in heterochromatin DNA affect alleles?

Mismatch repair machinery uses one strand as a template (No signal telling which strand to repair)

Gene conversion- Change in DNA sequence changing frequency of alleles