Chapter 10.1 Flashcards
Crossing Over process
While the bivalent is still in prophase I, equivalent lengths of non-sister homologous chromatids are exchanged. Crossing over begins by breaking a covalent bond in the backbone of both strands of the DNA double helix at identical positions on the non-sister chromatids and then causing the helices to switch sides. The position of the chromatids are switched so that, when the backbones are healed (ligated), a chromatid is covalently bonded to the non-sister chromatid. This process creates new combinations of alleles by swapping all the genetic information from the point of exchange onward.
Law of Independent Assortment
Genes found on different chromosomes show independent assortment, meaning that the allele inherited for one gene does not influence which allele is inherited for a different gene. Mendel observed the offspring of parents heterozygous for two traits, for example pea colour and pea shape. He found that the colour of the pea, green or yellow, had no effect on whether the pea would have a round or wrinkled shape. These observations led him to formulate the law of independent assortment , which is true of genes on different chromosomes.
How does Independent Assortment form?
Independent assortment of genes on the different chromosomes occurs because during metaphase I of meiosis, tetrads line up on the equatorial plate with random orientation. There are two possible orientations for a tetrad because there are two poles the chromosomes could be pulled toward. A tetrad has an equal chance of having its maternal copy or its paternal copy facing either pole. As a result, for two heterozygous genes, all four possible combinations of alleles in the haploid cells are equally likely.
Faults of Mendel
Mendel was not aware that some genes are found along the same piece of DNA, even though some of the traits he studied were on the same chromosome. This is because even when genes are on the same chromosome, new allele combinations can be formed by crossing over
Recombinant gametes
Gametes containing a new combination of alleles, different from the two parents
How do genes that are far enough apart on the same chromosome show independent assortment?
This is because crossing over occurs so frequently that they are inherited together and apart with equal frequency, just as if they were on different chromosomes. In Mendel’s peas, pea colour and flower colour are both located on chromosome 1 but are far enough apart that due to crossing over, the alleles are inherited independently.
Main difference between Meiosis and Mitosis
Cells produced by mitosis are genetically identical to each other and to the parent cell. These cells become part of the body or can be used in asexual reproduction to create cloned offspring. The cells produced by meiosis are genetically unique and contain only half the genetic information of the parent cell. These cells eventually form gametes (sperm and egg) for use in sexual reproduction
How do gametes increase genetic diversity?
Gametes increase genetic diversity in offspring by each having a unique combination of alleles and by combining their DNA with DNA from a second parent.
Main similarity between Meiosis and Mitosis.
The processes of mitosis and meiosis are both preceded by the same cell cycle . They both replicate their DNA during interphase and, when preparing to divide, begin to condense their DNA into chromosomes using nucleosome structure for supercoiling . The differences between mitosis and meiosis begin during the process of condensation
First Meiotic event that increases genetic diversity
Prophase I : Crossing over leads to the exchange of DNA between homologous chromosomes, resulting in allele combinations not found in either original chromosome.
Second Meiotic event that increases genetic diversity
Metaphase I : Random orientation of tetrads (pairs of homologous chromosomes) on the equatorial plate results in an independent assortment of maternal and paternal homologues. In other words, each gametic nucleus receives one copy of each type of chromosome, but the version it receives is equally likely to be from the mother or the father.
Third Meiotic event that increases genetic diversity
Anaphase I: Reduction division from the diploid to the haploid number of chromosomes in the nucleus, as a result of homologous chromosomes moving to opposite poles. When the gametes fuse and combine the DNA from two genetically different parents, the resulting offspring possesses the standard diploid number of chromosomes.
