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Download Topic 4 Genetics
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Topic 4 Genetics 4.1.1 State that eukaryotic chromosomes are made of DNA and proteins. [The names of the proteins ( histones) are not required, nor is the structural relationship between DNA and the proteins.] DNA is very long, in order to manage it within a cell, it is wound around histones to consolidate it. The term supercoiled, refers to the DNA tightly coiled up prior to mitosis or meiosis. This is when chromosomes are visible. 4.1.2 Define gene, allele, and genome [Gene: a heritable factor that controls a specific characteristic. ( The difference between structural genes, regulator genes, and genes coding for tRNA and rRNA are not expected at SL] Humans have around 25,000 different genes. Note that another reason different organisms have similar DNA sequences is because all organisms need to make tRNA and rRNA… just like all organisms have the same enzymes and proteins to respire with. If you have the same chemical pathways, you have the same genes. [Allele: one specific form of a gene differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.] You get one set of alleles from your mom, and one from your dad. Which allele that gets expressed depends upon which allele dominates the other…. Sometimes both alleles are expressed ( codominance) Humans have 3 possible alleles for blood type: type A, Type B, and Type O. [Genome: the whole of the genetic information of an organism] Remember the human genome project. The original human genome was determined from a mix of 18 peoples’ DNA. We have currently completely mapped only two individuals DNA: James Watson, and Craig Ventner. It costs about $1 million to map an individuals’ genome. 4.1.3 Define Gene mutation [ The terms point mutation or frameshift mutation will not be used] A mutation is any change in the DNA sequence. I will expect you to know what a point mutation or frameshift mutation is. Mutations can be at the chromosome level. This includes polyploidy, which is the cause of Down’s syndrome, Turner’s syndrome and Kleinfelter’s syndrome. During meiosis, a chromosome fails to separate from its copy. This leaves a gamete with either an extra chromosome, or a missing chromosome. Usually, this results in spontaneous abortion. Other chromosome level mutations are translocations and crossing over. Pieces of the chromosome break off and re-attach elsewhere, or exchange with homologous alleles. Mutations can occur at the base-pair level. Point mutations involve a single change in a nucleotide base. This may or may not result in a different amino acid… which may or may not result in the protein having a different shape. Sicklecell anemia is a point mutation ( see 4.1.4), as is Cystic Fibrosis. Frame shifts involve deletions or additions of base pairs. This throws off the triplet codon code, and can result in dramatically different amino acid sequences. The worst frame shift occurs at the beginning of a gene, since every codon after it is potentially different. Ultra-violet light can cause frame shifts. The light causes adjacent thymines to break their bonds with adenine, and form bonds with each other. Correcting enzymes cut out the adjacent thymines, and form a frame-shift. 4.1.4 Explain the consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia. [GAG has mutated to GTC causing glutamic acid to be replaced by valine, and hence sickle-cell anemia. There has been natural selection occurring in humans in favor of sickle-cell anemia in malarial areas. Only the homozygous condition gives sickle cell anemia. The heterotrophic individuals are more resistant to malaria. The plasmodium protist does not do well in the slightly misshapen red blood cells of the heterotrophic individuals.] Transcription is DNA to RNA, translation is RNA to protein. The base substitution occurring in DNA can affect the amino acid sequence of the protein, and therefore its shape. Shape is everything for a protein.