in the given figures
figure 1
A and B are sister chromatids
C and D are sister chromatids
can crossing over occur between
A and D chromatids
B and C chromatids
simultaneously as shown in figure 2 during pachytene stage of prophase I of meiosis I
Answers
Answer:
yes. ..it can occur during the prophase I of meiosis...
Answer:
Crossing over, or recombination, is the exchange of chromosome segments between nonsister chromatids in meiosis. Crossing over creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic diversity.
Meiosis is the process by which homologous chromosomes are separated to form gametes. Gametes contain only one member of each pair of chromosomes. Prior to meiosis, each chromosome is replicated. The replicas, called sister chromatids, remain joined together at the centromere. Thus, as a cell starts meiosis, each chromosome is composed of two chromatids and is paired with its homologue. The chromatids of two homologous chromosomes are called nonsister chromatids.
Meiosis occurs in two stages, called meiosis I and II. Meiosis I separates homologues from each other. Meiosis II separates sister chromatids from each other. Crossing over occurs in meiosis I. During crossing over, segments are exchanged between nonsister chromatids.
Mechanics of Crossing Over
The pairing of homologues at the beginning of meiosis I ensures that each gamete receives one member of each pair. Homologues contact each other along much of their length and are held together by a special protein structure called the synaptonemal complex. This association of the homologues may persist from hours to days. The association of the two chromosomes is called a bivalent, and because there are four chromatids involved it is also called a tetrad. The points of attachment are called chiasmata (singular, chiasma).
The pairing of homologues brings together the near-identical sequences found on each chromosome, and this sets the stage for crossing over. The exact mechanism by which crossing over occurs is not known. Crossing over is controlled by a very large protein complex called a recombination nodule. Some of the proteins involved also play roles in DNA replication and repair, which is not surprising, considering that all three processes require breaking and reforming the DNA double helix.
The Consequences of Crossing Over
A chiasma occurs at least once per chromosome pair. Thus, following crossing over, at least two of the four chromatids become unique, unlike those of the parent. (Crossing over can also occur between sister chromatids; however, such events do not lead to genetic variation because the DNA sequences are identical between the chromatids.) Crossing over helps to preserve genetic variability within a species by allowing for virtually limitless combinations of genes in the transmission from parent to off-spring.