4.1 - DNA, Genes and Chromosomes

Question 1

What are the 3 components of nucleotides?

Answer 1

A pentose sugar, a phosphate group and an organic base.

Question 2

Describe the structure of DNA.

Answer 2

• deoxyribose sugar
• a phosphate group
• one of the four organic bases (A,C,G,T)
• It is double stranded, hydrogen bonds between the bases form a helix shape

Question 3

Describe the role of DNA

Answer 3

Carries genetic information, determines our inherited characteristics.

Question 4

Describe the structure of RNA.

Answer 4

• ribose sugar
• a phosphate group
• one of the four bases (A,G,C,U)
• It is single stranded

Question 5

Describe the role of RNA

Answer 5

Tranfers genetic information from DNA to ribosomes for protein synthesis.

Question 6

Which bases are purine and which are pyrimidine?

Answer 6

• Purine (double ring) = adenine and guanine.
• Pyrimidine (single ring) = cytosine, thymine and uracil.

Question 7

How is DNA in eukaryotic cells different form prokaryotic cells?

Answer 7

• Eukaryotic cells = found in the nucleus, long and linear. Associated with histone proteins to form chromosomes. Mitochondria and chloroplasts contain prokaryotic-like DNA.
• Prokaryotic cells = short and circular. Not associated with proteins.

Question 8

What is the genetic code?

Answer 8

The order of bases in DNA. Consists of codons (triplets of bases that code for a particular amino acid).

Question 9

Identify the feature of the genetic code.

Answer 9

• Non-overlapping = each triplet is only read once.
• Degenerate = more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids).
• Universal = same bases and sequences used by all species.

Question 10

What is a gene?

Answer 10

A sequence of bases on a DNA molecule that codes for a specific sequence of amino acids to make a polypeptide. Can also code for functional RNA.

Question 11

What is a locus?

Answer 11

The fixed position on a DNA molecule occupied by a gene.

Question 12

What is an allele?

Answer 12

Different versions of the same gene, found at the same lucus on a chromosome.

Question 13

What are exons and introns?

Answer 13

Exons: regions of DNA that code for amino acid sequences. Separated by one or more inrons.
Introns: regions of DNA that do not code for anything.

Question 14

Where are introns found?

Answer 14

Between exons within genes.

Question 15

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.

Answer 15

• Condensation reaction
• Between phosphate and deoxyribose
• Catalysed by DNA polymerase

Question 16

The two DNA strands of a particular gene contain 168 guanine bases between them. The relationship between the numbers of guanine bases (G), adenine bases (A), thymine bases (T) and cytosine bases (C) in these two strands of DNA is shown in the following equation.
G = 4(A + T) – C
Use this information and your understanding of DNA structure to calculate the maximum number of amino acids coded by this gene.
Show your working.

Answer 16

Correct answer for 2 marks = 70;;
Accept for 1 mark,
A = 42 and T = 42
OR
420 (total bases in gene)
OR
210 (bases in template strand)

Question 17

Name the protein associated with DNA in a chromosome.

Answer 17

Histones

Question 18

Three differences between DNA in the nucleus
of a plant cell and DNA in a prokaryotic cell.

Answer 18

Plant v prokaryote
1. (Associated with) histones/proteins v no histones/proteins;
2. Linear v circular;
3. No plasmids v plasmids;
Do not credit if suggestion that prokaryotic DNA
only exists as plasmids.
4. Introns v no introns;
5. Long(er) v short(er);
Alternatives must be written directly opposite one
another.
Do not award if only half of a mark point is written.
Reference to prokaryotic DNA being single
stranded = max 2.
Reference to prokaryotic DNA not being helical =
max 2.

Question 19

Some DNA nucleotides have the organic base thymine, but RNA nucleotides do not have thymine. RNA nucleotides have uracil instead of thymine.
Give one other difference between the structure of a DNA nucleotide and the structure of an RNA nucleotide.

Answer 19

Deoxyribose in DNA and ribose in RNA;

Question 20

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide.
Give two reasons why.

Answer 20

1. Triplets code for same amino acid
Accept: DNA/code/triplets are degenerate
Reject: codons (as question states within genes)
2. Occurs in introns /non-coding sequence;
Reject: codons (as question states within genes)
Ignore junk DNA
Reject: multiple repeats

Question 21

The nucleus and a chloroplast of a plant cell both contain DNA.
Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus.

Answer 21

In chloroplasts
Must be comparative statements.
Accept alternatives in context of nuclear DNA
1. DNA shorter;
Accept smaller
2. Fewer genes;
3. DNA circular not linear;
Accept DNA in a loop not linear
Accept no chromosomes (in chloroplast) unlike nucleus
4. Not associated with protein/histones, unlike nuclear DNA;
5. Introns absent but present in nuclear DNA;
Ignore references to double and single stranded DNA

Question 22

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells.

Answer 22

Comparisons
1. Nucleotide structure is identical;
Accept labelled diagram or description of nucleotide
as phosphate, deoxyribose and base
2. Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);
3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA
in prokaryotes;
Accept shorter than nuclear DNA/is circular not
linear/is not associated with protein/histones unlike
nuclear DNA;

Contrasts
1. Eukaryotic DNA is longer;
2. Eukaryotic DNA contain introns, prokaryotic DNA does not;
3. Eukaryotic DNA is linear, prokaryotic DNA is circular;
4. Eukaryotic DNA is associated with / bound to protein / histones,
5. prokaryotic DNA is not;