Lecture 19: The Genetic Code

Question 1

What is the difference between a nucleoside and a nucleotide?

Answer 1

A nucleoside consists of a sugar (either ribose or deoxyribose) bound to a base (a, g, c, t, u)
A nucleotide consists of a sugar bound to a base and one or more phosphate groups

Question 2

What is the properties of rRNA?

Answer 2

It is found in the ribosomes
It is relatively stable-once produced it remains in the cell in a functional state for reasonably long times
rRNA is produced by specialised regions of certain chromosomes-it is the condensation of these chromosomal regions which form the nucleolus

Question 3

What is the properties of tRNA

Answer 3

• Involved in protein synthesis by bringing the amino acids to the site of protein synthesis (the ribosomes)
• they are decoders of the information
• relatively stable- one produced it remains in the cell in a functional state for reasonably long times
• quite small nucleotides- 70-90 nucleotides

Question 4

rRNA and tRNA are non-specific, explain.

What is the evidence that they are non-specific

Answer 4

Both rRNA and tRNA are involved in protein synthesis. However they don’t code for a given proteins
Evidence that they are non-specific
-rabbit mRNAglobin injected into a frog oocyte induces production of rabbit globin. Synthesis takes place on frog ribosomes using from tRNA molecules
-in vitro proteins synthesis-the protein produced is determined by the mRNA. These mehhh

Question 5

In contrast mRNA is very specific, explain

Answer 5

It is much less stable than rRNA or tRNA

• bacterial mRNA is very unstable with half-life of minutes
• turnover of mRNA allows for nuclear control of cell function -cell only manufactures protein if it is needed
• complementary to one strand of DNA-the template strand
• codes for the amino acid sequence of a protein
• has very little secondary structure- if it did, it would complicate the process of protein synthesis

Question 6

Is the genetic code overlapping or not?

Answer 6

A mutation involving a single base change suits in only a single amino acid being changed.
This the genetic code is non-overlapping

Question 7

Each triplet of bases coding for an amino acid is called a?

Answer 7

Codon.

Note: the code is the base sequence if the mRNA

Question 8

What are the properties of the genetic code

Answer 8

1. It is a triplet code, non-overlapping and reads linearly.
2. The code is degenerate, that is, most amino acids are coded for by more than one codon
3. The code is almost UNIVERSAL, that is the code is the same whether its in viruses, bacteria, plants and animals
4. The code is read from 5’ -> 3’
5. AUG is the START codon
6. There are 3 STOP codons
   UAA
   UAG
   UGA