DNA, Chromosomes

Question 1

Define gene

Answer 1

Section of DNA that codes for one or more polypeptides and functional RNA

Question 2

The genetic code is degenerate. Explain what this means

Answer 2

Some amino acids are coded for by more than one triplet

Question 3

The genetic code is universal. Explain what this means

Answer 3

The sequences that code for specific amino acids are the same in all organisms

Question 4

The genetic code is non-overlapping. Explain what this means

Answer 4

Each base is only read once

Question 5

What is the difference between introns and exons?

Answer 5

Introns are non-coding sequences of DNA removed in splicing, whereas exons are the polypeptide coding sequences.

Question 6

Define locus

Answer 6

The position of a gene

Question 7

Describe where DNA is found in eukaryotes and prokaryotes

Answer 7

DNA is found in the nucleus in eukaryotes, whereas in prokaryotes DNA is found in the form of a plasmid or loose in the cytoplasm. DNA is not bound to proteins in prokaryotes, whereas in eukaryotes it is bound to proteins called histones.

Question 8

What is difference between the genome and the proteome?

Answer 8

The genome is the complete genetic information of an organism while proteome is the complete set of proteins that COULD BE encoded by a particular genome.

Question 9

What is an allele?

Answer 9

An allele is one of a number of alternative forms of a gene

Question 10

What are homologous chromosomes?

Answer 10

A homologous pair is a pair of chromosomes, one paternal and one maternal, that contain the same gene loci but not necessarily the same alleles.

Question 11

Describe the structure of mRNA

Answer 11

• mRNA is a single polynucleotide strand, with a single helix structure.
• Its nucleotides contains ribose, a phosphate and an organic base (A, C, G or U).
• These nucleotides are joined by phosphodiester bonds.
• There are no hydrogen bonds as there are no bases pairs.

Question 12

Describe the structure of tRNA

Answer 12

• tRNA is a single polynucleotide which forms a “clover” shape.
• Its nucleotides contain ribose, a phosphate and an organic base (A, U, C or G).
• These nucleotides are bonded together with phosphodiester bonds.
• Hydrogen bonds form between complementary bases as base pairing is present in the molecule.
• tRNA contains an amino acid binding site, and an anticodon - a sequence of three bases which are complementary to a codon - an amino acid coding sequence. Each tRNA is specific to one amino acid

Question 13

What is an anticodon?

Answer 13

A sequence of three bases found in tRNA which are complementary to a codon - an amino acid coding sequence.

Question 14

Give two differences between the structure of mRNA and the structure of tRNA.

Answer 14

1. mRNA longer OR Has more nucleotides than tRNA;
2. mRNA is a straight molecule but tRNA is a folded molecule / clover-leaf shaped molecule;
3. mRNA contains no paired bases / hydrogen bonds but tRNA has some paired bases / hydrogen bonds.

Question 15

DNA is a polymer of nucleotides. Each nucleotide contains an organic base. Explain how the organic bases help to stabilise the structure of DNA.

Answer 15

1. Hydrogen bonds between the base pairs holds two strands together
2. Many hydrogen bonds provides strength

Question 16

Compare and contrast the structure of mRNA and tRNA

Answer 16

• Both are single stranded,
• Both contain nucleotides comprising of ribose, a phosphate and one of the bases UCAG.
• mRNA is a single helix, whereas tRNA is clover shaped.
• tRNA contains hydrogen bonds (between paired bases) whereas mRNA does not.
• mRNA is less stable than tRNA.
• tRNA contains a codon and an amino acid binding site, mRNA does not.

Question 17

Contrast the structures of DNA and mRNA molecules to give three differences.

Answer 17

1. DNA double stranded/double helix and mRNA single-stranded;
2. DNA (very) long and RNA short; Accept ‛RNA shorter’ or ‛DNA bigger/longer’
3. Thymine/T in DNA and uracil/U in RNA;
4. Deoxyribose in DNA and ribose in RNA; Ignore ref. to histones Ignore ref. to helix and straight chain alone
5. DNA has base pairing and mRNA doesn’t/ DNA has hydrogen bonding and mRNA doesn’t;
6. DNA has introns/non-coding sequences and mRNA doesn’t;

Question 18

A piece of mRNA is 660 nucleotides long but the DNA coding strand from which it was transcribed is 870 nucleotides long. Explain this difference in the number of nucleotides.

Answer 18

introns present in DNA;