3.1 genes

What is a gene, allele and a genome?

Gene: a section of DNA that codes for one protein. 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. Genome: the whole of the genetic information of an organism.

Define linked genes. [1 mark]

Genes located on the same chromosomes.

What is the set of alleles that an individual possesses? A.A gene B.A genotype C.A genome D.A genus

B.A genotype

What are Eukaryote chromosomes made up of?

Eukaryote chromosomes are made up of DNA and protein

Explain the consequences of altering a DNA base in the genome of an organism. [8 marks]

-altering a base (in DNA) is a (point) mutation; -only has an effect if base is in a gene; -when mRNA is produced by transcription one mRNA base is different; -one codon in mRNA is different; -one amino acid is different in the polypeptide; -polypeptide produced by translation of mRNA; some base changes do not change the amino acid coded for; -structure of polypeptide /protein may be altered; usually the polypeptide/protein does not function as well; example given: disease: sickle cell anemia; mutation: GAG to GTG; consequence in translation: glutamic acid to valine; consequence for protein: hemoglobin altered so sickle cell formed; consequence for individual: less oxygen can be carried;

Explain how a base substitution mutation, such as GAG to GTG, can lead to a disease like sickle-cell anemia. [2 marks]

-change in the codon (of the mRNA); -tRNA with a different anticodon attaches; -(if codon changed) wrong/different amino acid is joined to peptide/glutamic acid replaced by valine; -distorted hemoglobin molecule alters red blood cell shape/reduces ability to carry oxygen;

A human cell has between 20 000 and 25 000 genes whereas an E. coli cell has approximately 4000 genes. Which of the following statements is true? A.The human genome is larger than the E. coli genome. B.There are more genes on each human chromosome than on the E. coli chromosome. C.The human cell and the E. coli cell produce approximately the same variety of proteins. D.The DNA in both organisms is associated with histones (proteins).

A.The human genome is larger than the E. coli genome.

What is a gene mutation? A. Failure of chromosome pairs to separate properly during cell division B. Changes to genes caused by natural selection C. Changes to the nucleotide sequence of the genetic material D. Changes in karyotypes

C. Changes to the nucleotide sequence of the genetic material

What is the name given to a heritable factor which controls a specific characteristic? A. Allele B. Chromosome C. Gene D. Mutation

C. Gene

What is the difference between the alleles of a gene? A.Their position on the chromosome B.Their amino acid sequence C.Their pentose sugars D.Their base sequence

D.Their base sequence

Define the term allele as used in genetics. [1 mark]

one specific form of a gene (occupying the same gene locus as other alleles of the same gene)

Explain the consequences of a base substitution mutation in relation to the processes of transcription and translation, using sickle cell anemia as an example. [8 marks]

• Sickle cell anaemia is a genetic disease that affects red blood cells in the body. • It is due to a mutation on the Hb gene which codes for a polypeptide of 146 amino acids which is part of haemoglobin (haemoglobin is an important protein component in red blood cells). • In sickle cell anaemia the codon GAG found in the normal Hb gene is mutated to GTG. • This is called a base substitution mutation as adenine (A) is replaced by thymine (T). • This means that when the mutated gene is transcribed, a codon in the messenger RNA will be different. Instead of the normal codon GAG, the messenger RNA will contain the codon GUG. • This in turn will result in a mistake during translation. • In a healthy individual the codon GAG on the messenger RNA matches with the anticodon CUC on the transfer RNA carrying the amino acid glutamic acid. • However, if the mutated gene is present then GUG on the messenger RNA matches with the anticodon CAC on the transfer RNA which carries the amino acid valine. So the base substitution mutation has caused glutamic acid to be replaced by valine on the sixth position on the polypeptide. This results in haemoglobin S being present in red blood cells instead of the normal haemoglobin A. • This has an effect on the phenotype as instead of normal donut shaped red blood cells being produced some of the red blood cells will be sickle shaped. As a result these sickle shaped red blood cells cannot carry oxygen as efficiently as normal red blood cells would. However, there is an advantage to sickle cell anemia. • The sickle cell red blood cells give resistance to malaria and so the allele Hbs on the Hb gene which causes sickle cell anemia is quite common in parts of the world where malaria is found as it provides an advantage over the disease.