Nucleotides and nucleic acids

Question 1

Explain the structure of a nucleotide

Answer 1

a pentose sugar (ribose or deoxyribose), a phosphate group and a nitrogenous base

Question 2

Name the pentose sugars in DNA and RNA

Answer 2

DNA: deoxyribose
RNA: ribose

Question 3

Describe how polynucleotide strands form

Answer 3

Condensation reactions between nucleotides form strong phoshpodiester bonds

Question 4

Describe the structure of DNA

Answer 4

Double helix of 2 deoxyribose polynucleotide strands, H bonds between complementary base pairs on opposite strands (AT&GC)

Question 5

Describe the structure of RNA

Answer 5

Single stranded. Made up of ribose mononucleotides, linked through condensation reactions

Question 6

Name the complementary base pairs in DNA

Answer 6

2 H bonds between Adenine and Thymine, 3 H bonds between Cytosine and Guanine

Question 7

Name the complementary base pairs in RNA

Answer 7

2 H bonds between Adenine and Uracil, 3 H bonds between Guanine and Cytosine

Question 8

Why is DNA replication described as semiconservative?

Answer 8

Strands of original DNA molecule act as templates, new DNA molecule contains one old strand and one new strand

Question 9

Summarise the process of DNA replication

Answer 9

1. Double helix unwinds and the hydrogen bonds break, catalysed by DNA helicase
2. Complementary base pairing occurs
3. The nucleotides are joined by phosphodiester bonds, catalysed by DNA polymerase

Question 10

What is the role of the enzyme DNA polymerase in DNA replication?

Answer 10

It catalyses the formation of phosphodiester bonds between the free nucleotides to form the new complementary strand of DNA

Question 11

Describe the Meselson and Stahl experiment procedure

Answer 11

• E. coli bacteria was grown in a growth medium containing an
  isotope of nitrogen (15N)
• The bacteria incorporated the isotope into the nucleotides
• The bacteria were allowed to divide and samples were taken
  after each division
• The samples were centrifuged using a salt with a density
  gradient

Question 12

What is the genetic code?

Answer 12

A term used to describe the linear sequence of nucleotides that determine the amino acid sequence in a protein

Question 13

What is meant by a triplet code?

Answer 13

A section of DNA where groups of 3 nucleotides code for one amino acid

Question 14

What is meant by the genetic code being non-overlapping?

Answer 14

The genes are read in order so that the order of the genes determines the order of the amino acids in the polypeptide

Question 15

What is meant by the genetic code being degenerate?

Answer 15

Amino acids can be coded for by multiple different triplets. For example, AGA and CGA both code for the amino acid arginine

Question 16

What is a gene?

Answer 16

A section of the DNA that codes for the sequence of amino acids in a protein

Question 17

Describe the structure of messenger RNA

Answer 17

Long ribose polynucleotide with sugar-phosphate backbone. Nitrogenous bases A, U, G, C,. Single stranded and linear. Codon sequence is complementary to exons of 1 gene from 1 DNA strand

Question 18

State the function of mRNA

Answer 18

Transfers genetic code from DNA in nucleus to ribosomes for translation into a specific polypeptide

Question 19

What is the role of RNA polymerase in transcription?

Answer 19

It catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides to form the mRNA strand

Question 20

Outline the process of transcription

Answer 20

RNA polymerase binds to promoter region on a gene. Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template. Free nucleotides are attracted to their complementary bases. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds

Question 21

Describe the structure of transfer RNA

Answer 21

Single strand folded into clover shape. Anticodon on one end, amino acid binding site on the other

Question 22

State the function of tRNA

Answer 22

Carries amino acids to specific codons on mRNA for translation

Question 23

What is a codon?

Answer 23

Three consecutive nucleotides which code for one amino acid

Question 24

What is an anticodon?

Answer 24

A sequence of three bases on a tRNA molecule which is complementary to a codon on an mRNA molecule

Question 25

What does translation produce and where does it occur?

Answer 25

It produces proteins. It occurs in the cytoplasm on ribosomes

Question 26

Outline the process of translation

Answer 26

1) mRNA travels to a ribosome
2) The mRNA is passed through the ribosome 3 bases (one codon) at a time
3) tRNA molecules with complementary anticodons to the mRNA codons
hydrogen bond
4) The ribosome catalyses the formation of peptide bonds between the
amino acids on the tRNA molecules
5) tRNA molecules leave, the ribosome moves along the mRNA and the
cycle repeats until the polypeptide has been synthesised
● This process requires ATP

Question 27

What is a mutation?

Answer 27

A random change to the base sequence in DNA which results in genetic variants

Question 28

List 3 types of mutations

Answer 28

Insertion, deletion, substitution

Question 29

What is a substitution mutation?

Answer 29

Where one base gets swapped for another base

Question 30

What is a deletion mutation?

Answer 30

Where one or more bases are removed. This results in a frameshift mutation which alters all the codons after the error

Question 31

What is an insertion mutation?

Answer 31

Where one or more bases are added. This results in a frameshift mutation which alters all the codons after the error

Question 32

Give 3 effects of mutations

They aren’t all bad

Answer 32

Harmful, neutral, beneficial

Question 33

What is cancer?

Answer 33

A non-communicable disease which is characterised by uncontrolled cell division. It can be caused by mutations in regulatory genes.