Crossing-over occurs between a. sister chromatids of the...

Crossing-over occurs between
a. sister chromatids of the same chromosome.
b. two different kinds of bivalents.
c. two different kinds of chromosomes.
d. nonsister chromatids of a bivalent.
e. two daughter nuclei.

Key Concepts

Crossing-over

Crossing-over is a genetic process that occurs during prophase I of meiosis, where homologous chromosomes exchange segments of DNA. This exchange results in the recombination of genetic material, leading to increased genetic diversity in the resulting gametes.

Nonsister Chromatids

Nonsister chromatids refer to the individual chromatids from homologous chromosomes that are not identical copies. During meiosis, it is between these nonsister chromatids in a paired structure that crossing-over occurs, allowing the exchange of genetic segments.

Bivalent Formation

Bivalents, also known as tetrads, are pairs of homologous chromosomes that become closely associated during meiosis. This pairing creates the physical structure necessary for crossing-over to occur between nonsister chromatids, facilitating genetic recombination.

Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes. A key aspect of meiosis is the pairing of homologous chromosomes and crossing-over, which contribute to genetic diversity in sexually reproducing organisms.

Transcript

00:01 This question is asking us for some of the details about crossing over.

00:06 So to illustrate this, i'm going to just draw a simplified case where they're a hypothetical organism with just two chromosomes.

00:17 So for each chromosome, there's a maternal and a paternal copy.

00:24 So one chromosome from mom, one chromosome from dad.

00:30 And i'm going to draw each of these chromosomes in a different color.

00:37 Okay.

00:38 So if you looked at the cell, you would actually see what looks like four chromosomes, but really it's just two chromosomes, but for each chromosome, you have a distinct homologous chromosome, one maternal and one paternal cop.

00:55 Now, prior to myosis or mitosis, dna replication will occur during the s phase or dna synthesis phase, and each of these chromosomes will get copied perfectly.

01:12 So this is an identical copy of the dna that's getting made.

01:17 So i added a whole other strand to each of these chromosomes.

01:21 And these two strands are called sister chromatids.

01:34 So these are identical copies of dna.

01:38 Now prior to or rather during meiosis, these pairs of homologous chromosomes will come together and form what's called a tetrad.

01:55 So a tetrad or a bivalent chromosome is when two homologous chromosomes come together, but each chromosome is composed of two sister chromatids.

02:10 So it looks like four chromosomes all coming together.

02:15 But all together, this is called a bivalent, but another word for it is a tetrad.

02:29 Because there's four chromatids present two identical chromatids for each homologous chromosome.

02:38 Now this question is asking about what occurs during crossing over...

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