P3 Preparation

Question 1

Role of tRNA

Answer 1

1. tRNA acts as an intermediate molecule between the codon of mRNA and the a.a sequence of the polypeptide chain
2. tRNA carries the correct a.a from the cytoplasm to the polypeptide chain being synthesised at the ribosome
3. Has shape complementary to aminoacyl-tRNA synthetase for activation of a.a

Question 2

Role of mRNA

Answer 2

1. Starts and terminates translation through start codon (UAC) and stop codons (UAG, UAA, UGA) respectively.
2. Mature mRNAs are exported of the nucleus via nuclear pores and enters the cytoplasm and binds to the ribosomes, forming transcription initiation complex
3. Acts as a template for translation/synthesis of polypeptides
4. mRNA strand codes for a.a sequence of specific polypeptides
5. Allows for excising of introns for alternative RNA splicing
6. Enables a single gene to code for >1 polypeptide, depending on which exons are spliced together to form a continuous coding sequence.

Question 3

Role of rRNA

Answer 3

1. rRNA combines with proteins to form small and large ribosomal subunits. The small and large ribosomal subunits combine to form ribosomes
2. rRNA in the small ribosomal subunit cbp to mRNA during translation
3. rRNA in the large ribosomal subunit cbp to tRNA in the P site and A site
4. A ribozyme, peptidyl transferase, in the large ribosomal subunit, catalyses the formation of peptide bonds between adjacent amino acids

Question 4

How genetic variation may be preserved in a population (Heterozygote Protection)

Answer 4

1. Diploid nature hides genetic variation from selection in the form of recessive alleles in heterozygotes
2. Recessive allele of heterozygotes are not shown in phenotypes, and hence not exposed to natural selection and possible elimination
3. Recessive alleles less favourable than the dominant alleles that may be harmful in the present environment, can persist in a population through their propagation by heterozygous individuals
4. Only recessive alleles present in homozygous recessive individuals will be shown in the phenotype and exposed to natural selection and possible elimination
5. Hence, few recessive alleles can be eliminated from the population in each generation
6. Heterozygote protection maintains a huge pool of alleles that may not be suitable for present conditions, but may bring about benefits when environment changes

Question 5

How genetic variation may be preserved in a population

(Heterozygote advantage - almost strictly exclusive to sickle cell anaemia + malaria situation)

Answer 5

1. If individuals who are heterozygous at a particular locus have greater fitness and reproductive success than homozygous individuals, then 2 or more alleles will be maintained at that locus by natural selection
2. When heterozygotes are favoured over homozygotes, natural selection tends to maintain genetic variation in the population