From The Student Room

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Transcription and Translation

---

Compare the structure of RNA and DNA

DNA	RNA
Double stranded polymer	Single stranded polymer
Uses A G C T	Uses A G C U
Pentose sugar is dexoyribose	Pentose sugar is ribose

State one function of messenger RNA and one function of transfer RNA

• mRNA
  • It enters the cytoplasm where it associates with the ribosomes and acts as a template for protein synthesis
• tRNA
  • Forms a clover shaped leaf with one end ending in a c-c-a sequence
  • At this point the amino acid attaches itself
  • There are twenty types
  • Each carries a different amino acid
  • These line up against the mRNA during protein synthesis

Outline DNA transcription in terms of codons composed of triplets of bases

• Transcription is the process by which a complementary mRNA copy is made of the specific region of the DNA molecule which codes for a polypeptide (about 17 base pairs).
• A specific region of the DNA molecule, called a cistron, unwinds.
• This unwinding is the result of hydrogen bonds between base pairs in the DNA double helix being broken. This exposes the bases along each strand.
• Each base along one strand attracts its complementary RNA nucleotide, i.e. a free guanine base on the DNA will attract an RNA nucleotide with a cytosine base. It should be remembered, however, that uracil, and not Thymine is attracted to adenine.
• The enzyme RNA polymerase moves along the DNA adding one complementary RNA nucleotide at a time to the newly unwound portion of DNA. The region of base pairing between the DNA and the RNA is only around 12-based pairs at any one time as the DNA helix reforms behind the RNA polymerase. The DNA thus acts as a template against which mRNA is constructed. A number of MRNA molecules may be formed before the RNA polymerase leaves the DNA, which closes up reforming its double helix.
• Being too large to diffuse across the nuclear membrane, MRNA leaves instead through the nuclear pores. Tin the cytoplasm it is attracted t o the ribosome. Along the MRNA is a sequence of triplet codes that have been determined by the DNA. Each triplet is called a codon.

Describe translation

Describe translation including the roles of mRNA, tRNA, anti-codons and ribosomes leading to peptide linkage formation.

• Translation is the means by which a specific sequence of amino acids is formed in accordance with the codons of mRNA.
• A group of ribosomes becomes attached to the mRNA to form a structure called polysome.
• The complementary anticodon of a tRNA-amino acid is attracted to the first codon on the mRNA.
• The second codon likewise attracts its complementary anticodon.
• The ribosome acts as a framework which holds the mRNA and tRNA amino acid complex together until the two amino acids form a peptide bond between each other.
• Once they have combined, the ribosome will move along the mRNA to hold the next codon-anticodon complex together until the third amino acid is linked with the second.
• In this way, a polypeptide chain is assembled, by the addition of one amino acid at a time.
• Second and subsequent ribosomes may pass along the mRNA immediately behind the first.
• In this way many identical polypeptides are produced simultaneously.
• Once each amino acid is linked, the RNA which carried it to the mRNA is released back into the cytoplasm. It is again free to combine with its amino acid. The ribosome continues along the mRNA until it reaches one of the nonsense codes at which point the peptide is cast off.

Define degeneracy

Having one or more base triplet to code for one amino acid.

Define universal

Found in all living organisms.

Explain the relationship between one gene and one polypeptide and its significance

• Thalassemia - group of blood disorders characterized by a deficiency of haemoglobin, the blood protein that transports oxygen to the tissues.
• Thalassemia is caused by genetically determined abnormalities in the synthesis of one or more of the polypeptide chains that make up the globin part of haemoglobin.
• The various forms of the disorder are distinguished by different combinations of three variable
  • The particular polypeptide chain or chains that are affected
• Whether the affected chains are synthesized in sharply reduced quantities or not synthesized at all
  • Whether the disorder is inherited from one parent (heterozygous) or from both parents (homozygous).

Comments