DNA

Question 1

What is a gene

Answer 1

A section of DNA that contains the coded information for making polypeptides and functional RNA. A gene is located at a particular position called a locus on a DNA molecule.

Question 2

What does a gene code for

Answer 2

The amino acid sequence of a polypeptide or a functional RNA (this includes ribosomal RNA and transfer RNA)

Question 3

Which organic bases are found in DNA

Answer 3

Adenine, Guanine, Cytosine and Thymine

Question 4

Describe the reasoning that scientists had when suggesting that there must be a minimum of three bases that code for each amino acid.

Answer 4

1) only 20 amino acids regularly occur in proteins
2) Each amino acid must have its own code of bases on the DNA
3) Only four different bases (A,T,C and G) are present in DNA
4) If each base coded for a different amino acid, only four different amino acids could be coded for.
5) Using a pair of bases, 16 different codes are possible which is still inadequate.
6) Three bases produce 64 different codes, which is more than enough to satisfy the requirements of 20 amino acids.

Question 5

What is a triplet

Answer 5

Three bases which code for amino acids. As there are 64 possible triplets and only 20 amino acids, it follows that some amino acids are coded for by more than one triplet.

Question 6

Describe the key points in the code for an amino acid from triplets

Answer 6

• A few amino acids are coded for only by a single triplet.
• The remaining amino acids are coded for by between two and six triplets each.
• The code is known as the ‘degenerate code’ because most amino acids are coded for by more than one triplet.

Question 7

Describe how triplets function in a DNA molecule

Answer 7

• A triplet is always read in one particular direction along the DNA strand.
• The start of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If this first methionine molecule does not form part of the final polypeptide, it is later removed.

Question 8

What are stop codes

Answer 8

Three triplets do not code for any amino acid. They are called stop codes and mark the end of a polypeptide chain.

Question 9

Describe key features of a DNA code

Answer 9

• the code is non-overlapping which means each base in the sequence is only read once.
• The code is universal. Each triplet codes for the same amino acids in all organisms (with a few exceptions) and this is indirect evidence for evolution.

Question 10

What is an exon

Answer 10

A sequence of DNA that codes for an amino acid sequence

Question 11

What is an intron

Answer 11

A non-coding sequence of DNA

Question 12

Describe the difference between prokaryotic and eukaryotic DNA

Answer 12

• In prokaryotic cells, the DNA is shorter, forms circles and is not associated with protein molecules.
• Prokaryotic cells therefore do no have chromosomes.
• In Eukaryotic the DNA molecules are longer, linear, and are associated with proteins called histones.
• They therefore form chromosomes.
• The mitochondria and chloroplasts in eukaryotic cells also contain DNA which is like the DNA found in prokaryotic cells (short,circular, and not associated with proteins)

Question 13

What is a chromatid

Answer 13

One strand of a replicated chromosome (joined to another chromatid at the centromere)

Question 14

What is a chromosome

Answer 14

A structure consisting of a long, coiled molecule of DNA and its associated proteins (histones) by which genetic information is passed from generation to generation.

Question 15

Describe the structure of DNA within a chromosome

Answer 15

• DNA has a double helix shape
• This helix is wound around proteins (histones) to fix it into position.
• This DNA-histone complex is then coiled.
• This coil, in turn is then looped and coiled again.
• This structure means that a lot of DNA can be condensed into a single chromosome.

Question 16

What are homologous chromosomes

Answer 16

A chromosome pair, one paternal and one maternal, with the same gene loci.

Question 17

What is a locus (in terms of genetics)

Answer 17

The position of a gene on a chromosome.

Question 18

What is the diploid number

Answer 18

The number of pairs of homologous chromosomes found in an organisms cells.

Question 19

What three key things is the genetic code?

Answer 19

Universal
Non-overlapping
Degenerate

Question 20

What is an allele

Answer 20

One of a number of alternative forms of a gene.

Question 21

How are alleles inherited

Answer 21

Each individual inherits one allele from each of its parents.

Question 22

How do alleles differ

Answer 22

Each different allele has a different base sequence, and therefore a different amino acid sequence, and so produces a different polypeptide.

Question 23

Describe how changes in the base sequence of a gene can have serious consequences for an organism (focussing on enzymes)

Answer 23

• Any change in the base sequence of a gene produces a new allele of the gene and this is a mutation.
• This results in a different sequence of amino acids being coded for. This different amino acid sequence will lead to the production of a different polypeptide, and hence a different protein.
• Sometimes this different protein may not function properly or may not function at all.
• When the protein produced is an enzyme, it may have a different shape.
• The new shape may not fit the enzymes substrate.
• As a result the enzyme may not function and this can have serious consequences for the organism.