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Cell Biology > Meiosis and mammalian SC > Flashcards

Meiosis and mammalian SC Flashcards

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1
Q

What is the synaptonemal complex (SC)?

A

A conserved structure in all meiotic organisms that holds 2 homologous chromosomes together in prophase 1 and facilitates exchange of DNA (crossing over).

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2
Q

What is meiosis?

A

A process to halve the chromosome number of a cell to produce gametes with only one of each chromosome.

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3
Q

What is the purpose of crossing over?

A

To hold homologous chromosomes together in metaphase 1 so meiosis is regulated. No crossovers results in aneuploid daughter cells (abnormal chromosome number).
Also to generate genetic diversity.

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4
Q

What happens in the crossing over process?

A

An enzyme generates a DSB in the DNA, which is then repaired by inter-chromosome recombination (usually inter-sister chromatid recombination, but this is blocked in meiosis). The chromosome has to ‘search’ for the matching sequence elsewhere (its homologous chromosome).

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5
Q

What is the meiotic telomere complex?

A

A complex that tethers both telomeric ends of meiotic chromosomes to the inner nuclear membrane.

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6
Q

What is the LINC complex?

A

A complex that spans the nuclear envelope and joins the meiotic telomere complex (ie chromosomes) to MTs in the cytoplasm.

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7
Q

What is the function of the LINC complex?

A

Allows cytoplasmic MT forces to act on chromosomes within the nucleus. This allows the broken chromosome to move rapidly and search for its homologous chromosome in prophase.

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8
Q

What is a recombination intermediate?

A

A structure that forms in prophase 1 where homologous chromosomes are tethered to each other at the homologous sites of the DSBs (no genetic exchange though).

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9
Q

What is synapsis?

A

Formation of the SC. The homologous chromosomes are ‘zipped up’.

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10
Q

What is DSB resolution?

A

All DSBs are repaired using the homologous chromosome, apart from 1 per arm (in the middle third) which is where the crossover is. DSB resolution is regulated by the SC - transmits signal to prevent further crossing over once one has formed. Then meiosis can progress.

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11
Q

What is the structure of a synapsed pair of chromosomes?

A

SC joins 2 dense chromosome axes, and less dense chromatin loops extrude from the axes.

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12
Q

What is the SC structure?

A

Tripartite - 2 outer lateral elements, transverse filaments, central element. All 3 systems self assemble.

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13
Q

What is the width of the SC?

A

100nm

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14
Q

What is the length of the SC?

A

The length of the chromosome - up to 24 µm.

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15
Q

What is the transverse filament protein?

A

SYCP1.

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16
Q

What orientation is SYCP1?

A

N terminus is in the central element, C terminus is in the lateral element.

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17
Q

What are the lateral element / chromosome axis proteins?

A

SYCP2 and SYCP3.

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18
Q

What are the central element proteins?

A

SYCE1-3, SIX6OS1 and TEX12.

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19
Q

What happens in mouse KOs of any SC proteins?

A
  • Infertility
  • Failure of SC assembly
  • Failure of DSB repair
  • Failure of crossover formation
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20
Q

When have we know the approximate SC structure since?

A

The 1970s.

21
Q

What is current SC research looking at?

A

Characterising the structures and functions of specific SC proteins and using this to predict mutant phenotypes.

22
Q

What are the roles of SYCP3?

A
  • Organising the chromosome axis (binds DNA at N and C termini)
  • Compacting chromosomes
23
Q

What is the structure of SYCP3?

A

Core is a tetramer of alpha helical coiled coils (20nm). They are antiparallel (2 are C -> N and 2 are N -> C). Termini are flexible regions that allow many copies to assemble into a massive fibre (3nm). The N and C termini interact with each other and DNA is threaded through this interacting region.

24
Q

What is SEC-MALS used for?

A
  • Outputs a molecular weight which allows us to define the oligomeric state of a protein (e.g. dimer, tetramer).
  • Validates the isolated core has the same structure are the core within the full length protein.
25
Q

Is SYCP3 stable?

A

Yes - it has a high Tm at 65˚C. This is due to the stable core.

26
Q

How is chromatin arranged by SYCP3?

A

The DNA winds through the SYCP3 fibre; bound by N terminus. The DNA in the centre of the SC is tightly looped and the DNA at the edge has long loops (lecture 12, slide 15).
The loops made by cohesin are condensed by the SYCP3 fibre. SYCP3 KO makes the chromosome axis twice as long.

27
Q

Which SYCP3 heterozygous mutants have a more deleterious phenotype?

A

Those with C terminal deletion. The mutant proteins still bind DNA and form a complex with the WT protein, but the compaction function is lost.

28
Q

Which SYCP3 heterozygous mutants have a less deleterious phenotype?

A

Those with complete core deletion. The mutant proteins don’t exist so the WT copies interact with each other normally and there is partial function.

29
Q

What is the trend in mutation severity in oligomeric proteins?

A

In heterozygotes, mild mutations have the most deleterious impacts due to their interference with WT function.

30
Q

What is standard protein mutation notation?

A
  • Starts with p (protein)
  • Original amino acid
  • Position of change
  • New amino acid or del (deletion)
  • fs* x = frame shift and a stop codon after x amino acids.
31
Q

Which part of an alpha helical core is critical for self assembly of larger structures?

A

The Ctip.

32
Q

What is the structure of the SYCE2-TEX12 complex.

A

Core is a 2:2 complex of alpha helical coiled coils (20nm). TEX12 is on top and SYCE2 is on the bottom. They are antiparallel (1 of each is C -> N and 1 of each is N -> C). 2 of these form a 4:4 complex. The C terminus of each coil sticks out the end of the subunit to facilitate fibre assembly. Only a few subunits are needed for the whole SC. The fibres are long, thin and amorphous (lacking defined structure) and resemble the central element (same width - 4nm).

33
Q

What is the function of the SYCE2-TEX12 complex?

A

Provides a ‘backbone’ for the formation of the SC. Gives it longitudinal strength. In KOs, only very short sections of the SC form.

34
Q

How can we study subunit structure for these large assemblies?

A

Deletion of the Ctip, which inhibits assembly.

35
Q

Which form of the SYCE2-TEX12 complex is thought to be important in fibre assembly?

A

The 4:4 complex - the 2:2 complex assembles underneath the 2:2 structure in an inverted form (SYCE2 on top this time). The 4:4 complexes can then assemble end to end.

36
Q

Is SYCE-TEX12 stable?

A

Yes - it has a high Tm at 70˚C. This is due to the stable core.

37
Q

What is the complete structure of a SYCE2-TEX12 fibre?

A

2 end-to-end 4:4 complexes bundle into a 10nm fibre, which assembles into a rope like 40nm bundle, similar to keratin.

38
Q

What mutation have we observed in the SYCE2-TEX12 complex?

A

One homozygous deletion of the TEX12 core. This knocks out TEX12 function, prevents backbone assembly and causes infertility.

39
Q

Why have we only observed one TEX12 mutant?

A

The mutations cause infertility so are therefore not passed on to offspring. We would only see a mutation if it was de novo, which is rare.

40
Q

What is the typical infertility phenotype that SC mutations cause?

A

Males - total infertility.
Females - early and recurrent miscarriages.

41
Q

Why is it easier to identify homozygous mutations that heterozygous but dominant mutations?

A

There is a lot of noise caused by SNPs; which polymorphism is the cause of infertility? Whereas with homozygotes there is more certainty, so genome sequence searches are usually filtered for homozygotes only (biased).

42
Q

What is the structure of SYCP1?

A

Core is a dimer of alpha helical coiled coils. They are parallel (both same direction). 2 dimers associate to form a tetramer. The N terminal tip mediates self-assembly - SYCP1 molecules associate at the central element via their Ntips (head-to-head assembly). SYCP1 is hypothesised to associate with the lateral element (chromosome axis) via their Ctips. The dimers in one tetramer form head-to-head associations with dimers in different tetramers (Lecture 12, slide 27).

43
Q

What prevents assembly of a large SYCP1 structure?

A

Deletion of the Ntip. It is only able to form a tetramer (small).

44
Q

What does SYCE3 do to SYCP1?

A

Binds and disrupts its core tetramer. This disrupts the lattice. However once more SYCE3s self assemble (bind themselves) the lattice structure can be rescued.

45
Q

What happens when SYCE3 is knocked out?

A

SYCP1 form tetramers but not a lattice.

46
Q

What happens when SYCE3 is mutated?

A

SYCP1 cannot form any kind of structure. The organism is infertile.

47
Q

Why is SYCE3 addition beneficial to the SC?

A

SYCE3 binds central and lateral element proteins. It holds the whole SC together!

48
Q

What happens in SYCE1-SIX6OS1 mutations?

A

Infertility if they are homozygous, relatively little if heterozygous. Haven’t characterised the proteins yet though.

49
Q

What type of mutations does mutating the proteins talked about in this lecture result in?

A

Dominant negative. Heterozygotes experience loss of function (infertility) and are not passed on. Therefore not many mutations observed.

Cell Biology (7 decks)

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