D1- Explain how eukaryotic chromosomes store a unique genotype, discussing how the processes of independent assortment, mutation and crossing over contribute to genetic variation amongst individuals
DNA molecules store the genetic blueprint of an animal, in other words that is called the genotype, or genome. The genome is stored around protective histone proteins (nucleosomes) and packaged into one or more chromosomes. They are coiled and packed very tightly. Inside a DNA molecule there are many different sections that each code for a particular characteristic/trait. This is called a gene. In eukaryotic organisms, DNA is held in the nucleus of the cell, and two copies of each ...view middle of the document...
A DNA strand will split into two single strands, and the junction at where it splits into two is called the replication fork. When DNA has split into two single strands it will join on with free nucleotide bases to create two new strands. The leading strand of DNA is ordered in the 5’ to 3’ direction and is copied almost continuously, but the lagging strand is made in sections, which are called Okazaki fragments
The triplet code refers to the number of bases that code for each amino acid. And, the name tells it, there must be a minimum of three bases in total to make enough combinations. If there are two bases there wouldn’t be enough combinations, so that is why three is just enough. The genetic code refers to the way in which A,C, T and G base sequences in DNA (or U in RNA) instruct for the combination in amino acids.
Gregor Mendel established the basic rules of inheritance of genetically determined characteristics. He stated that genes that are present will always express the trait, these are called the dominant genes. Some genes will only express if there are two copies of the same gene present, these are called recessive. Heterozygotes will process two different versions of the same gene, so this would be a dominant and a recessive gene, and homozygotes will process two similar versions of the same gene, like TT, or tt. This concept can be explained with the Punnett Square, named after R.C Punnett, who created it to represent inheritance patterns
Independent assortment is the random separation and assortment of non-homologous chromosomes, during meiosis. For a diploid cell with two pairs of homologous chromosomes, independent assortment gives four possible types of gamete. The theoretical number of combinations is 2, to the power of the number of pairs of homologous chromosomes: 2 ²=4 for two pairs, so therefore for the 23 pairs of human chromosomes it would be 2²³= 8388608. This means that independent assortment alone is the source of significant variation. But when this is combined with recombination and the random fusion of gametes, the potential for genetic variation is enormous.
Each member of a homologous pair of chromosomes has exactly the same genes and therefore determines the same characteristics, for example eye colour and blood group. However, the alleles of these genes may differ, for example they may code for brown or blue eyes, or blood group A or B. The random distribution, and consequent independent assortment, of these chromosomes therefore produces new genetic combinations.
Stage 1: one of the pair of chromosomes includes the gene for eye colour and carries one allele for brown eyes and one for blue eyes. The other chromosome carries the gene for blood group and also carries both the allele for blood group A and the allele for blood group B. There will be two possible arrangements, A and B, of the two chromosomes at the start of meiosis. Both are equally likely, but each...