Biology Forums - Study Force

I would greatly appreciate it if somebody can answer this question, but manly that can explain to me how to truly solve it. I would like to know how to approach such question if I get something similar on the exam. Thank you!!!!!!!



Two homologous human chromosomes have the following structure:


 –– –– A –– –– B ––.–– C ––  –– D ––  –– E ––  –– F ––  –– G ––  –– H ––  –– I ––  –– J ––  ––
 –– –– A –– –– D –– –– C ––. –– B ––  –– E ––  –– F ––  –– G ––  –– J ––  –– I ––  –– H––  ––


    where the letters represent genetic markers and the black dot represents the centromere.

        Diagram the two chromosomes as they would be when they synapse during meiosis.
        
        Show the resolution products of a reciprocal recombination event between loci C and D.
        
        Show the resolution products of a reciprocal recombination event between loci E and F.
        
        Show the resolution products of a reciprocal recombination event between loci H and I.

A) Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired.

This is what prophaseI stage would look like with pairing synapsis: See attachment.

SO... for this.. make sure the centromeres are lined up.

--A--B-.-C--D--E--F--G--H--I--J--
--A--D--C-.-B--E--F--G--J--I--H--

They are most likely to synasis towards the sides rather than closer to the centromere. The closer the genes are to the centromere, the less likely they will experience synapsis. These are the two portions I'm going to cross (known as pairing synapsis) as a form of genetic recombination:

--A--B-.-C--D--E--F--G--[H--I--J--]
--A--D--C-.-B--E--F--G--[J--I--H--]

Pairing synapsis might look like this:

--A--B-.-C--D--E--F--G--[J--I--H--]
--A--D--C-.-B--E--F--G--[H--I--J--]

Now just remove the brackets:

--A--B-.-C--D--E--F--G--J--I--H--
--A--D--C-.-B--E--F--G--H--I--J--

In genetics, alocus(pluralloci) is the specific location of a gene or DNA sequence on a chromosome.
Recombination (aka genetic recombination) is basically that they switch positions on the loci.

---------------------

B. Show the resolution products of a reciprocal recombination event between loci C and D.?

Again make sure the centromeres are lined up.

--A--B-.-C--D--E--F--G--H--I--J--
--A--D--C-.-B--E--F--G--J--I--H--

Then switch loci C and D (this kind of recombination is very rare in real life), like so:

--A--B-.-D--C--E--F--G--H--I--J--
--A--C--D-.-B--E--F--G--J--I--H--

---------------------

c.Show the resolution products of a reciprocal recombination event between loci E and F.?

Again make sure the centromeres are lined up.

--A--B-.-C--D--E--F--G--H--I--J--
--A--D--C-.-B--E--F--G--J--I--H--

Then switch the loci (in this example, E and F get switched but still technically don't make a genotypic change to the genome since the loci remained constant in this example)

--A--B-.-C--D--E--F--G--H--I--J--
--A--D--C-.-B--E--F--G--J--I--H--

---------------------

d.Show the resolution products of a reciprocal recombination event between loci H and I.

Again make sure the centromeres are lined up.

--A--B-.-C--D--E--F--G--H--I--J--
--A--D--C-.-B--E--F--G--J--I--H--

Then switch H and I: (this type of recomination is more common in nature, because the loci are towards the end. Also this one caused a genotypic change since the loci recombination caused a change in sequence.

--A--B-.-C--D--E--F--G--I--H--J--
--A--D--C-.-B--E--F--G--J--H--I--

zoeksyrianos  thank you!!! I understand so much better than before, but...

I do have a question in regards to "making sure the centromeres are lined up". Do I have to write it out as
    --A--B-.-C--D--E--F--G--H--I--J--
--A--D--C-.-B--E--F--G--J--I--H--       where the black dot is literary lined up with each other? Also, when i do line it up and the loci are not lined up...can the recombination occur?  I hope I make sense and sorry if my questions seem too silly :( :s