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The meaning of REARRANGE is to arrange (something or someone) again in a different way. How to use rearrange in a sentence.
- Overview
- Introduction
- Phases of meiosis
- Meiosis I
- Meiosis II
- How meiosis "mixes and matches" genes
How meiosis reduces chromosome number by half: crossing over, meiosis I, meiosis II, and genetic variation.
Mitosis is used for almost all of your body’s cell division needs. It adds new cells during development and replaces old and worn-out cells throughout your life. The goal of mitosis is to produce daughter cells that are genetically identical to their mothers, with not a single chromosome more or less.
Meiosis, on the other hand, is used for just one purpose in the human body: the production of gametes—sex cells, or sperm and eggs. Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell.
In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar strategies to organize and separate chromosomes. In meiosis, however, the cell has a more complex task. It still needs to separate sister chromatids (the two halves of a duplicated chromosome), as in mitosis. But it must also separate homologous chromosomes, the similar but nonidentical chromosome pairs an organism receives from its two parents.
These goals are accomplished in meiosis using a two-step division process. Homologue pairs separate during a first round of cell division, called meiosis I. Sister chromatids separate during a second round, called meiosis II.
Before entering meiosis I, a cell must first go through interphase. As in mitosis, the cell grows during G1 phase, copies all of its chromosomes during S phase, and prepares for division during G2 phase.
During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length.
For instance, in the image below, the letters A, B, and C represent genes found at particular spots on the chromosome, with capital and lowercase letters for different forms, or alleles, of each gene. The DNA is broken at the same spot on each homologue—here, between genes B and C—and reconnected in a criss-cross pattern so that the homologues exchange part of their DNA.
This process, in which homologous chromosomes trade parts, is called crossing over. It's helped along by a protein structure called the synaptonemal complex that holds the homologues together. The chromosomes would actually be positioned one on top of the other—as in the image below—throughout crossing over; they're only shown side-by-side in the image above so that it's easier to see the exchange of genetic material.
You can see crossovers under a microscope as chiasmata, cross-shaped structures where homologues are linked together. Chiasmata keep the homologues connected to each other after the synaptonemal complex breaks down, so each homologous pair needs at least one. It's common for multiple crossovers (up to 25 !) to take place for each homologue pair 1 .
The spots where crossovers happen are more or less random, leading to the formation of new, "remixed" chromosomes with unique combinations of alleles.
Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis for haploid cells."
The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid—have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids. In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes.
During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes.
[When did the centrosomes duplicate?]
The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate. In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell.
In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells.
The gametes produced in meiosis are all haploid, but they're not genetically identical. For example, take a look the meiosis II diagram above, which shows the products of meiosis for a cell with 2n=4 chromosomes. Each gamete has a unique "sample" of the genetic material present in the starting cell.
As it turns out, there are many more potential gamete types than just the four shown in the diagram, even for a cell with only four chromosomes. The two main reasons we can get many genetically different gametes are:
•Crossing over. The points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. If meiosis happens many times, as in humans, crossovers will happen at many different points.
•Random orientation of homologue pairs. The random orientation of homologue pairs in metaphase I allows for the production of gametes with many different assortments of homologous chromosomes.
In a human cell, the random orientation of homologue pairs alone allows for over 8 million different types of possible gametes7 .
[How do you get that number?]
Dec 30, 2022 · The heavy chain has an extra domain: there are 40 V genes, which are linked to one of 25 D segments, then 6 potential J segments, and one C gene. These rearrangements, although they look something like the RNA splicing that we saw earlier in this course, are happening at the DNA level.
REARRANGE definition: 1. to change the order, position, or time of arrangements already made: 2. to change the order…. Learn more.
Jun 1, 2020 · Definition. DNA replication is a process that occurs during cellular division where two identical molecules of DNA are created from a single molecule of DNA. As a semiconservative process, a single molecule containing two strands of DNA in double helix formation is separated, where each strand serves as a template for the new DNA molecules.
The primary function of B lymphocytes is to produce specific immunoglobulin molecules of high affinity that can react with exogenous antigens. B cells derive from common lymphoid progenitor cells in the bone marrow, where they begin to rearrange their immunoglobulin genes.
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