Gene for Beta Chain of Hemoglobin. The beta chain gene is always in the same position, the beta chain locus. However an individual chromosome could carry the Hb A or Hb S allele version at the beta chain locus. Why are they identical? Because they contain the two products of a semi-conservative DNA replication. Homologs need not be identical -- each came from a different source a different parent.
Important: be sure you know the difference between homologs homologous chromosomes and sister chromatids. Many more examples can be found on the web. Try the images on Google for a large assortment. If you do karyotypes on human cells, you will discover that the pattern is different from males and female, as follows:. Both sexes have 22 pairs of chromosomes that look the same regardless of sex, but the 23rd pair is not the same in both sexes.
In females, the 23rd pair consists of 2 large chromosomes that look alike. In males the 23rd pair consists of a large and a small chromosome that do not look alike but act as a pair during meiosis.
The 22 pairs of chromosomes that are the same in both sexes are called autosomes. The remaining pair are called sex chromosomes, and the big one is called the X chromosome and the little one the Y chromosome. So females are XX and males are XY. Overview of Meiosis -- See handout 20A -- if time is short, this will be covered in lecture What is meiosis for? Most of the cells of most higher organisms are diploid.
Humans, for example, have 46 chromosomes, or 23 pairs, in virtually all of their cells. If eggs and sperm also have 46 chromosomes, the next generation, formed from the fusion of an egg and a sperm, would have 92 chromosomes. But clearly the chromosome does not double each generation. So the eggs and sperm, unlike all other cells, must have only 23 chromosomes and be haploid. So there must be a way to make haploid cells from diploid cells. There is, and the process is called meiosis.
During meiosis, one chromosome from each pair is picked at random so that the resulting haploid has 23 chromosomes instead of 23 pairs. Then 2 such haploids fuse, during fertilization, to give you back a diploid with 23 pairs. Why bother with all this? Why sex? After all, you could start the next generation with one complete diploid cell from either parent and save yourself a lot of trouble! Some organisms do reproduce this way, at least some of the time, but most organisms engage in sexual reproduction.
They probably do so because each cycle of meiosis, followed by fusion, allows for a new combination of chromosomes. Crossing over, which occurs at meiosis, also allows for new combinations of genes within chromosomes as well. So it looks like sexual reproduction is useful because it allows reshuffling of the genetic material same argument as for bacteria.
How reshuffling works. Reshuffling Chromosomes. Suppose one person has 2 identical copies of chromosome 1 and 2 identical copies of chromosome 2. Draw these chromosomes in one color, say pink. Another person has 2 copies of chromosome 1 that are the same as each other but different from the copies in the first person, and similarly for chromosome 2.
Draw these chromosomes in another color, say white. The offspring of these two people will have a mixture of "pink" and "white chromosomes. After several generations, it will be possible to get all conceivable combinations of "pink" and "white" chromosomes. Reshuffling genes: In addition to reshuffling whole chromosomes, equivalent parts of chromosomes can be reshuffled or exchanged. Homologous chromosomes pair and can exchange equivalent sections during meiosis by crossing over.
This is equivalent to what happens to bacteria during transformation, transduction, etc. See Sadava figs. Note: the term "genetic recombination" usually refers to reshuffling of genes by crossing over. What happens if there is one pair of homologs? Picture below or on bottom of Handout 20A shows what happens to one pair of chromosomes. It shows all cells at each stage -- before DNA synthesis, after S, after 1st div, and after 2nd div.
DNA synthesis occurs first -- before division. Meiosis is preceded by DNA duplication just as mitosis is. During the S before meiosis or mitosis the cell doubles the DNA content and of chromatids per chromosome.
Products: There are 4 products, each haploid from meiosis , instead of 2 products, each diploid from mitosis. To cut the number of copies of each chromosome from 4 to one requires 2 division, not one. Two divisions of meiosis: The first division of meiosis separates homologs; the second division of meiosis separates sister chromatids.
The first division cuts the chromosome number per cell in half from 2N to N and cuts the DNA content per cell in half from 4c to 2c "c" is defined below. What happens in a cell with one pair of chromosomes is as follows:. What happens to chromosomes per cell during meiosis? Handout 20A top part shows a different view of the stages shown above.
It emphasizes the 'chromosome cycle' -- the number of chromosomes, the number of chromatids, and the DNA content per cell at each stage. Definition of c.
Details of meiosis next time or see handout 19A. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell. Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more. The information on this site should not be used as a substitute for professional medical care or advice.
Contact a health care provider if you have questions about your health. What is DNA? Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange.
In eukaryotes, the genome consists of several double-stranded linear DNA molecules packaged into chromosomes. Each species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells. Human body cells have 46 chromosomes, while human gametes sperm or eggs have 23 chromosomes each. A typical body cell, or somatic cell, contains two matched sets of chromosomes, a configuration known as diploid.
The letter n is used to represent a single set of chromosomes; therefore, a diploid organism is designated 2n. Human cells that contain one set of chromosomes are called gametes, or sex cells; these are eggs and sperm, and are designated 1n, or haploid. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus.
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