2.1.2k The Nature of the Genetic Code

Question 1

Structure of Amino acid on slide 2

Question 2

What does DNA contain

Answer 2

Genes which are instructions for making proteins

Question 3

What is a gene

Answer 3

A sequence of DNA nucleotides that codes for a polypeptide - the sequence of amino acids in a polypeptide forms the primary structure of a protein

Question 4

What do genes do

Answer 4

• Each chromosome in a eukaryotic cell nucleus includes a molecule of DNA
• On each chromosome there are specific lengths of the DNA called genes
• Each gene contains a code that determines the sequence of amino acids in a particular protein or polypeptide

Question 5

Size of genes

Answer 5

Some genes can be as short as 50-100 nucleotides in length, but most are thousands of nucleotides long

Question 6

What is the code for DNA

Answer 6

The code for DNA is a triplet code in which 3 bases code for an amino acid. This is called a codon

Question 7

What is a Triplet code/codon

Answer 7

The 3 nitrogenous bases read along the DNA strand that code for amino acids

Question 8

How many different combinations of triplet codes are there

Answer 8

There are 20 different amino acids & 64 different combinations of triplet codes; therefore some triplets code for the same amino acid.
However, some amino acids only have 1 triplet cod (eg. methionine, tryptophan)

Question 9

What is the advantage of having different codes for the same amino acid

Answer 9

Having diff codes for the same amino acid means a mutation may have no effect on the amino acid coded for

Question 10

see slide 5 for examples of triplet code

Question 11

What is the genetic code

Answer 11

The sequence of base triplets (codons) in DNA or mRNA, which codes for specific amino acids

Question 12

The nature of the genetic code

Answer 12

• Universal
• Degenerate
• Non-overlapping

Question 13

The nature of the genetic code: Universal

Answer 13

The same specific base triplets code for the same amino acids in all living things

Question 14

The nature of the genetic code: Degenerate

Answer 14

There are more possible combinations of triplets than there are amino acids (20 amino acids, but 64 possible triplets)

This means some amino acids are coded for by more than one base triplet

Question 15

The nature of the genetic code: Non-overlapping

Answer 15

In the genetic code, each base triplet is read in sequence, separate from the triplet before it & after it. Base triplets dont share their bases

If a base is added or deleted, it causes a frame shift as every base triplet after that, & hence every amino acid coded for, is changed

Question 16

see slide 7 for using the chart to find protein sequence from a DNA sequence

Question 17

see slide 8-11 for exam qs + ms

Question 18

EXAM Q: Enzymes have active sites in which specific substates fit precisely. Suggest how a genetic mutation may result in an enzyme becoming non-functional

Answer 18

A mutation in the DNA changes the triplet code, meaning different amino acids are incorporated into the protein/enzyme that the DNA codes for. If such a change affects the precise structure of the active site, a substrate may not be able to bind, rendering the enzyme non-functional

Question 19

EXAM Q: Explain why there are likely to be more differences, overall, between base sequences of DNA than between amino acid sequences of proteins

Answer 19

The triplet code for DNA is degenerative, there are 64 different triplets/codons but only 20 amino acids. Therefore, an amino acid can be coded for by more than one codon, so more opportunity for differences in DNA sequence than amino acid sequence