4. The Genetic Code

Question 1

Recall facts about DNA from Unit 1Which is the correct statement?
DNA is a polymer of nitrogen containing organic bases
or
DNA is a polymer of nucleotides

Answer 1

DNA is a polymer of nucleotides

Question 2

A polynucleotide forms when ….

Answer 2

Phosphodiester bonds form between phosphate and sugar groups as a result of condensation reactions

Question 3

2 DNA polynucleotide DNA strands running in opposite directions (antiparallel) are held together..

Answer 3

by Hydrogen bonding between the nitrogen containing organic bases

Question 4

RNA is..

Answer 4

A single stranded polynucleotide molecule that is generally shorter than DNA

Question 5

What is a triplet and what is it used for

Answer 5

A sequence of three nitrogenous DNA bases, called a triplet, codes for a specific amino acid.

Question 6

What is a gene

Answer 6

A base sequence that codes for a single polypeptide is called a gene.

Question 7

What does Non overlapping mean

Answer 7

The bases in the DNA triplets code are read in sequence and are read only once, (in 1 direction) and the triplets do not share bases.

Question 8

What doe Universal mean

Answer 8

The same specific base triplet codes for the same amino acid in all living organisms

Question 9

Non-coding sequences of bases

Answer 9

1. Some regulatory DNA triplets code for the start and the end of a gene – (these are called the promoter and terminator regions)
2. Much of the DNA in eukaryotic cells does not code for polypeptides. Some non-coding DNA is found within a gene. The non-coding DNA is called an intron, the coding DNA is called an exon
3. Between genes in eukaryotic DNA there may be non-coding DNA which is repeated many times these are called multiple base repeats.

Question 10

There are estimated to be around 3 billion bases in the codes that make up the human genome (all the genes in a cell, including those in the mitochondria and chloroplasts). Of these only 30,000,000 are used too actually code for polypeptides .

1. Calculate the % human DNA that codes for polypeptides.
2. Of the rest of the DNA 25% makes up gene and regulatory triplets – this accounts for how many bases – give your answer in standard form?
3. The rest of the bases are non-coding. What % of Human DNA is non-coding?

Answer 10

1. 1% 2. 742,500,000 = 7.4x10⁸ 3. 100-26 = 74%

Question 11

A few amino acids have…

Answer 11

only 1 triplet code

Question 12

The remaining amino acids have between…

Answer 12

2 & 6 triplet codes each

Question 13

The code is known as degenerate…

Answer 13

as most amino acids have more than one triplet code

Question 14

The triplet code is always read in …

Answer 14

1 direction along the DNA strand

Question 15

The start of a gene is always marked by the same

Answer 15

DNA triplet code TAC. The amino acid coded for by this triplet will be edited out of the polypeptide chain

Question 16

Triplet codes can act as a terminator…

Answer 16

these mark the end of the gene (and polypeptide)

Question 17

The code is non-overlapping…

Answer 17

each base in the sequence is only read once

Question 18

The code is universal

Answer 18

…it is the same in all organisms

Question 19

Explain how a change in a sequence of DNA bases could result in a non-functional enzyme

Answer 19

Change in (sequence of) amino acids/primary structure;

Change in hydrogen/ionic/disulfide bonds;

Alters tertiary structure/active site (of enzyme);

Substrate cannot bind/no enzyme-substrate complexes form;

Question 20

Explain how a change in the DNA base sequence for a protein may result in a change in the structure of the protein.

Answer 20

Change in amino acid/s/ primary structure;

Change in hydrogen/ionic/disulfide bonds;

Alters tertiary structure;

Question 21

The number of amino acids coded for could be fewer than your answer to part

(a)(0.

Give one reason why

Answer 21

Introns/non-coding DNA/ junk DNA;

Start/stop code / triplet;