4.1+Chromosomes,+genes,+alleles+and+mutations

4.1 Chromosomes, genes, alleles & mutations 4.1.1 State that eukaryote chromosomes are made of DNA and proteins

Eukaryotic chromosomes are found in the nucleus. They contain DNA and proteins. As you can see on the image below: 

[[image:CB_Chromosomes.gif width="228" height="231"]]
4.1.2 Define gene, allele and genome

Gene: A heritable factor that controls a specific characteristic (such as eye color), consists of a length of DNA and occupies a position on a chromosome know as a locus.

Allele: Refers to one specific form of gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene: 

Genome: The total genetic material of an organelle, cell or organism. the gene pool refers to the total of the genes carried by the individual member of a population.

4.1.3 define gene mutation

A permanent change in the sequence of base pairs in the DNA that makes up a gene.

e.g. A mutation occurs in the Manx cat where some are born without tails.



4.1.4 explain the consequences of a base substitution mutation in relation to processes of transcription and translation, using the example of sickle cell anemia media type="youtube" key="9UpwV1tdxcs" height="349" width="425" Mutation and DNA: >>> not true.
 * Old opinion: Mutation which involves a smaller number of nitrogenous bases in the DNA will be less significant than one where a large number of bases is changed
 * Example: Sickle-cell anemia: only 1 base leads to a protein with one amino acid changed and >>> cause disease of sickle-cell anemia

Hemoglobin
 *  Made of 4 polypeptide chains: 2 α and 2 β
 * When an A to T base substitution occurs, GAG has GTG causing glutamic acid to be replaced by valine and hence sickle -cell
 * Common hemoglobin: HbS (normal level of Oxygen) >< 'normal' Hemoglobin HbA (makes it crystallize at low oxygen levels)
 * HbA >>> the erythrocyte change from a biconcave shape into a sickly-cell shape >>> less efficient at transporting oxygen as keeping their stickle shape

Symptoms of Sickle cell anemia:
 * Acute anemia >>> physical weakness; cause damage to the heart and kidney >>> might cause death (in homozygous individually)
 * <span style="color: blue; direction: ltr; font-family: Calibri; font-size: 16px; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">The gene for sickle cell anemia is co-dominant with the normal allele although the latter is expressed more strongly in the heterozygous individual.
 * <span style="color: blue; direction: ltr; font-family: Calibri; font-size: 16px; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">Plasmodium ( cause malaria) cannot reproduce in erythrocytes with HbS >>> help to reduced chance of contracting malaria.

<span style="color: blue; font-family: Calibri; font-size: 16px; font-weight: bold; margin: 0in 0in 0in 0.375in; text-decoration: underline;">Natural selection and effect on young generation >>> Test before having children
 * <span style="color: blue; direction: ltr; font-family: Calibri; font-size: 16px; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">Sickle cell trait is more common than malaria infested area such as west Africa.
 * <span style="color: blue; direction: ltr; font-family: Calibri; font-size: 16px; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">Carriers often don’t know if they possess the sickly cell allele >>> may pass in to their children. 2 carriers have 25% chance of the child having the disease.
 * <span style="color: blue; direction: ltr; font-family: Calibri; font-size: 16px; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">If the pregnant mom and the father are both carrying sickle cell, they should test the child using amniocentesis or chorionic villi sampling. If the child have sickle cell >>> may decide to discontinue the pregnancy
 * <span style="color: blue; direction: ltr; font-family: Calibri; font-size: 16px; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">>>> Ethical issues relating to screening of fetuses and abortion of those found of have a genetic disease

<span style="color: blue; font-family: Calibri; font-size: 16px; font-weight: bold; margin: 0in; text-decoration: underline;">TOK: There is a clear casual link in frequency of the sickle-cell allele is correlated with the prevalence of malaria.

There is natural selection in favor of the sickle-cell allele in malarial areas (cause severe anemia in the homozygous >>> frequencies of the sickle - cell and the normal hemoglobin alleles: balance the twin risks of anemia and malaria