Topic 3.5 Transcription and Translation

Question 1

3.5.1 Compare the structure of RNA and DNA.

Answer 1

DNA

• contains a 5-carbon sugar
• 5-carbon sugar = deoxyribose
• each nucleotide has 1 of 4 nitrogenous bases
• nitrogenous bases are cytosine, guanine, adenine & thymine
• double-stranded

RNA

• contains a 5 carbon sugar
• 5-carbon sugar = ribose
• each nucleotide has 1 of 4 nitrogenous bases
• nitrogenous bases are cytosine, guanine, adenine & uracil
• single-stranded

Question 2

3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

Answer 2

1. an area of DNA of one gene becomes unzipped
   
   • two complementary strands of DNA are now single-stranded in the area of the gene
2. enzyme, RNA polymerase, catalyzes formation of mRNA
   
   • moves along strand of DNA acting as the template
   • RNA nucleotides float into place by complementary base pairing
3. complementary base pairs are the same as in double-stranded DNA, with exception of uracil (replacing thymine) pairing with adenin on the new mRNA molecule

Note:

• only one of the two strands of DNA is ‘copied’; the other strand isn’t used
• mRNA is always single-stranded and shorter than the DNA it is copied from (since it’s a complementary copy of only one gene)

Question 3

3.5.3 Describe the genetic code in terms of codons composed of triplets of bases.

Answer 3

mRNA molecule produced by transcription represents a complementary copy of one gene of DNA. The sequence of nucleotides in the mRNA is usually enough info to make one polypeptide. Message written into the mRNA molecule determines order of amino acids.

• genetic code is written in triplets
  
  • every 3 bases is enough info to code for 1 of 20 amino acids
  • any set of 3 bases that determines identity of one amino acid is called a triplet
  • when a triplet is found in a mRNA molecule, it is called a codon or codon triplet

Question 4

3.5.4 Explain the process of translation, leading to polypeptide formation.

Answer 4

3 types of RNA molecules (all single-stranded and transcribed from a gene of DNA):

1. mRNA: messenger RNA; complementary copy of a DNA gene, has enough genetic info to code for a single polypeptide
2. rRNA: ribosomal RNA; each ribosome is composed of rRNA and ribosomal protein
3. tRNA: transfer RNA; each type of tRNA transfers 1 of 20 amino acids to ribosome for polypeptide formation.

After translation, mRNA will detach from single-strand DNA template and be free-floating in nucleoplasm. Eventually, mRNA will float into cytoplasm through a nuclear pore.

Translation process

1. mRNA locates a ribosome & aligns with it (so that first 2 codon triplets are within boundaries of ribosome)
2. specific tRNA floats in (tRNA anticodon must be complementary to first codon triplet of mRNA molecule) → 1st amino acid is brought into translation process (its identity was originally determined by strand of DNA that the mRNA was transcribed from)
3. 2^nd tRNA brings another specific amino acid while 1st tRNA ‘sits’ in ribosome
   
   • matches 3 anticodon bases with 2nd codon triplet of mRNA
4. enzyme catalyzes condensation reaction between 2 amino acids held by tRNA molecules → forms covalent (peptide) bond
5. bond between 1^st tRNA molecule and its amino acid (connected to 2^nd amino acid) is broken → 1^st tRNA floats into cytoplasm and reloads with another amino acid of same type
6. ribosome with only 1 tRNA inside moves 1 codon triplet down mRNA molecule
   
   • 2^nd tRNA now in original position of 1^st tRNA, creates room for 3^rd tRNA to enter
7. process repeats until final codon triplet; enzyme catalyzes condensation reaction, amino acids bond, earlier tRNA breaks away from its amino acid
   
   • last codon triplet (doesn’t code for amino acid) signals ‘stop’ to the translation process
8. polypeptide breaks away from final tRNA molecule and becomes free-floating polypeptide in cytoplasm of cell

Question 5

3.5.5 Discuss the relationship between one gene and one polypeptide.

Answer 5

Every 1 gene of DNA produces 1 polypeptide.

• polypeptide fromed by amino acids linking together through peptide bonds
  
  • 20 diff. amino acids ∴ wide range of polypeptides are possible
• genes store info (in coded form - triplets of bases known as codons) for making polypeptides
• sequence of bases in a gene codes for sequence of amino acids in a polypeptide
• information in the genes is decoded during transcription and translation, leading to protein synthesis
• Exception: once gene may lead to single mRNA molecule, but mRNA molecule may then be modified in many different ways (i.e. modification may result in production of a different polypeptide during translation portion of protein synthesis)