Can someone explain what happens during synapsis and what the difference between it & crossing over

[mrh125]

I know they both happen during prophase 1 in meiosis, but what is the difference between them? I think synapsis involves the pairing of non sister chromatids of homologous chromosomes as well as recombination. Crossing over also involves recombination though? Please help?

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[The Helpful Aye Aye]

Synapsis is when the homologous chromosomes associate, which allows recombination. Recombination is the molecular mechanism of crossing over.

 

[inasensegone]

I know they both happen during prophase 1 in meiosis, but what is the difference between them? I think synapsis involves the pairing of non sister chromatids of homologous chromosomes as well as recombination. Crossing over also involves recombination though? Please help?

notajock2day was spot on, but here is a bit more detail if you are interested:

Let's take a quick look at Prophase I of Meiosis:

Prophase 1 of meiosis can actually be broken up into 5 individual stages (Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis)

Leptotene: Condensed chromosomes are visible. (46 chromosomes = 23 pairs of homologous chromosomes = 92 chromatids)

Zygotene: Homologous chromosomes are paired up [remember that homologues don't pair up in mITosis] because they are about to perform genetic recombination (crossing-over). This pairing up of the Homologous chromosomes is what is referred to as synapsis. Now that you have 2 chromosomes paired up you have 4 chromatids in each of these pairs. This is where the term "Tetrad" comes from.

Pachytene: Crossing over actually occurs here. This is where sections of genetic material are exchanged among the chromatids usually amongst "linked genes", so called because they code for similar proteins.

Diplotene: Crossing over generally is complete by this stage and the homologous chromosomes separate just a little. They are still attached at the points where crossing over occurred (again, during Pachytene) and these attachment points are called Chiasmata. The most important aspect of this Diplotene stage, aside from the end of crossing over, may be that ALL of the females eggs are stuck in this stage until they reach puberty.

Diakinesis: Nuclear envelope starts to break down. Homologous chromosomes separate a bit more but still connected at their ends until they are separated later in Anaphase 1.

AFTER DIAKINESIS, Prophase 1 is over and METAPHASE 1 begins.

Some pics to help illustrate:

 

[The Helpful Aye Aye]

notajock2day was spot on, but here is a bit more detail if you are interested:

Let's take a quick look at Prophase I of Meiosis:

Prophase 1 of meiosis can actually be broken up into 5 individual stages (Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis)

Leptotene: Condensed chromosomes are visible. (46 chromosomes = 23 pairs of homologous chromosomes = 92 chromatids)

Zygotene: Homologous chromosomes are paired up [remember that homologues don't pair up in mITosis] because they are about to perform genetic recombination (crossing-over). This pairing up of the Homologous chromosomes is what is referred to as synapsis. Now that you have 2 chromosomes paired up you have 4 chromatids in each of these pairs. This is where the term "Tetrad" comes from.

Pachytene: Crossing over actually occurs here. This is where sections of genetic material are exchanged among the chromatids usually amongst "linked genes", so called because they code for similar proteins.

Diplotene: Crossing over generally is complete by this stage and the homologous chromosomes separate just a little. They are still attached at the points where crossing over occurred (again, during Pachytene) and these attachment points are called Chiasmata. The most important aspect of this Diplotene stage, aside from the end of crossing over, may be that ALL of the females eggs are stuck in this stage until they reach puberty.

Diakinesis: Nuclear envelope starts to break down. Homologous chromosomes separate a bit more but still connected at their ends until they are separated later in Anaphase 1.

AFTER DIAKINESIS, Prophase 1 is over and METAPHASE 1 begins.

Some pics to help illustrate:
View attachment 178446
View attachment 178449

In my expert "I took the MCAT twice because I'm an idiot" opinion, that level of detail is beyond scope. The 5 prophase I stages don't appear in Kaplan/TPR meaning that their test trolls haven't come across it (nor have I). Understanding the differences between mitosis and meiosis I is much more high yield. You're obviously familiar with basic terminology and that will get you where you need to be. Consider the above post a detailed guide to what you should understand conceptually.