Model the process of polypeptide synthesis

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Model the process of polypeptide synthesis, including: (ACSBL079)

• transcription and translation
• assessing the importance of mRNA and tRNA in transcription and translation (ACSBL079)
• analysing the function and importance of polypeptide synthesis (ACSBL080)
• assessing how genes and environment affect phenotypic expression (ACSBL081)

Transcription:

• Process by which genetic information is transferred from a double helical DNA to a single stranded mRNA.
• It is the first step of gene expression.
• Important because transcription produces mRNA which is necessary for carrying out translation, where proteins are produced that are required for the functioning of living organisms.

Process:

The whole process of transcription occurs in 3 stages. They are:

• Initiation:
• Similar to replication, the DNA double helix unwinds forming a transcription bubble.
• The hydrogen bonds between the nucleotide base pairs of the two anti-parallel strands are broken creating two separated strands, a template strand (3’→5’) and a non-template strand (5’→3’).
• There is a particular sequence in the uncoiled DNA which indicates the initiation site of transcription known as promoter.
• RNA Polymerase enzyme binds to the promoter and begins to add complementary bases to the template strand, the process being similar to replication. The only difference here is that in case of mRNA, the nitrogenous base Thymine is replaced by Uracil.
• Elongation:
• RNA Polymerase enzyme moves along the 3’→5’ template strand and adds new nucleotide base pairs complementary to that of the template strand.
• With the addition of new nucleotide base pairs, the RNA strand keeps elongating.
• Termination:
• RNA Polymerase encounters a definite DNA sequence known as the terminator which acts as the stop signal for termination. Once it encounters this sequence, transcription stops.
• The freshly synthesized mRNA is known as nascent mRNA or pre-mRNA (direction is 5’→3’) and will undergo some changes before it enters the translation phase which are known ads post-translational modifications. Some of which are:
• Some sections of the newly created mRNA contain sequences that do not code for proteins. In a process called splicing, these non-coding sequences (also termed as introns) are removed and the coding regions (known as exons) are joined together.
• When the mRNA is exposed to the cytoplasmic environment during its journey to the ribosome, there are chances of it being destroyed by certain cytoplasmic enzymes called ribonucleases. To avoid this, at the 5’ end a 7-methylguanosine cap and at the 3’ end, an extension of around 250 Adenine residues known as Poly (A) tail are added.

Translation:

• Process by which mRNA is transcribed to protein.
• Involves two types of RNA; tRNA and mRNA.
• Takes place in ribosome.
• Before translation begins, nucleotide base pairs are clustered in groups of 3 known as codons. Each codon contains 3 nucleotide base pairs formed by the combination of A, U, G and C.
• Out of possible 64 codons, 61 codons code for different amino acids which are the backbones of the polypeptide/protein synthesized after translation. The other 3 codons act as terminators of the translation process and are termed as stop codons.
• The amino acids are brought by the tRNA molecules.

Process:

The whole process of translations is also divided into 3 steps:

• Initiation:
• As soon as the start codon AUG is identified, two subunits of ribosome, the large subunit and the small subunit associate together.
• The tRNA molecule carries the amino acid for the start codon AUG, Methionine. The tRNA molecule contains an anti-codon region complementary to the codons in the mRNA which allows them to join.
• Elongation:
• The ribosome moves along the mRNA and keeps adding amino acids based on the codon in the mRNA. The amino acid of the second codon forms peptide bond with the amino acid of the first codon and in the same manner as the ribosome moves, each new amino acid forms bond with the previous one forming a polypeptide chain.
• As the peptide chain elongates, the ribosome is divided into three sites. The A site is where new tRNA molecules enter the ribosome, the P site is where peptide bonds are formed and E site is the exit site from which empty tRNA molecules whose amino acids have already bonded with existing amino acids exit the ribosome.
• Termination:
• The polypeptide chain keeps elongating until any of the three stop codons UAA, UAG or UGA are detected. Once the ribosome encounters the stop codon, it triggers a series of events releasing a polypeptide chain.
• The ribosomal subunits dissociate as soon as the polypeptide chain is released.
• The polypeptide chain undergoes different structural modifications to form a functional protein.

Importance of mRNA and tRNA:

• mRNA is considered as the first expression of genes.
• It holds information for precise synthesis of protein and each type of protein to be made from an mRNA strand is decided by the codon arrangements in the strand.
• tRNA molecules are carriers of amino acids which are the backbones of protein molecules.

Importance of polypeptide synthesis:

• For creating proteins that carry out different functions in our bodies. For example, proteins actin and myosin build up our muscles.
• For creating enzymes that control different biochemical pathways happening inside the cells. For example, cellular respiration takes in multiple steps which are controlled by different enzymes.
• Proteins control different characteristics of every living organism. Thus, what we are overall is because of different types of protein expressions. Without polypeptide synthesis, life would have been small and our existence wouldn’t have been very different from what it is now.
• Polypeptide synthesis forms products that are necessary to carry out replication, transcription and translation as well.

Effects of environment in phenotypic expression:

• Expression of phenotype is often controlled by environmental factors.
• For example, natural factors such as light, temperature, nutrient availability, water etc. can affect phenotypic expression of plants.
• An example of how environment affects phenotype is hydrangeas. Hydrangeas are plants that have different flower colours (pink and blue) depending on the PH of the soil they are in (environment). Soils with pH less than 5 (acid) they are blue and soils with pH more than 7 (alkaline) they are pink.

Effects of genes in phenotypic expression:

Interaction between genes can affect phenotypic expressions. Some examples are as follows:

• Dominance:
• In heterozygous genes, if one allele dominates/obstructs the expression of another allele it is known as dominance.
• Example:
  Let, T = allele for tallness of plants (dominant)
  t = allele for dwarfness of plants (recessive)
  The combination Tt will result in the phenotype of the plant to be tall because T is the dominant allele.
• Incomplete Dominance:
• Both alleles in a gene pair are partially expressed creating an intermediate character. None of the alleles are dominant.
• Example:
  Let,
  RR = genotype for red coloured flower
  rr = genotype for white coloured flower
  Flowers having Rr genotype will neither be red nor white rather, the combination of red and white produces pink coloured flower.
• Co-dominance:
• Closely related to incomplete dominance codominance, is when both alleles are simultaneously expressed in a heterozygous gene pair.
• The A and B alleles for blood type can both be expressed at the same time, resulting in type AB blood.
• Epistasis:
• Effect of non-allelic gene pairs on one another.
• Can be of variant types:
• Recessive Epistasis: Complete dominance at both gene pairs; however, when one gene is homozygous recessive, it hides the phenotype of the other gene
• Duplicate recessive epistasis: Complete dominance at both gene pairs; however, when either gene is homozygous recessive, it hides the effect of the other gene
• Dominant epistasis: Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene
• Duplicate dominant epistasis: Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene
• Dominant and recessive epistasis: Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene
• Duplicate interaction: Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene

Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.