Nucleic acids

Question 1

What is the structure of a nucleotide?

Answer 1

A phosphate group, pentose sugar and nitrogen containing base

Question 2

Name the pentose sugars in DNA and RNA

Answer 2

DNA: deoxyribose
RNA: ribose

Question 3

State the role of DNA in living cells

Answer 3

Base sequence of genes codes for functional RNA and amino acid sequence of polypeptides

Genetic information determines inherited characteristics

Question 4

State the role of RNA in living cells

Answer 4

mRNA: Complementary sequence to 1 gene from DNA with introns spliced out. Codons can be translated into a polypeptide by ribosomes

rRNA: Component of ribosomes

tRNA: Supplies complementary amino acids to mRNA codons during translation

Question 5

How do polynucleotides form?

Answer 5

Condensation reactions between nucleotides form strong phosphodiester bonds

Question 6

Describe the structure of DNA

Answer 6

Double helix of 2 polynucleotide strands

H-bonds between complementary base pairs on opposite strands:

A + T
G + C

Question 7

Which bases are purine and which are pyrimidine?

Answer 7

A and G = 2 ring purine bases

T and C and U = 1 rings pyrimidine bases

Question 8

Name the complementary base pairs in DNA

Answer 8

2 H bonds between A and T

3 H bonds between G and C

Question 9

Name the complementary bases in RNA

Answer 9

2 H bonds between A and U

3 H bonds between G and C

Question 10

Relate the structure of DNA to its functions

Answer 10

Sugar phosphate backbone and many H bonds provide stability

Long molecule stores lots of information

Helix is compact for storage in nucleus

Base sequence of triplets codes for amino acids

Double stranded for semi-conservative replication

Complementary base pairing for accurate replication

Weak H bonds break so strands separate for replication

Question 11

Describe the structure of mRNA

Answer 11

Long ribose polynucleotide

Contains uracil instead of thymine

Single stranded and linear

Codon sequence is complementary to exons of 1 gene from 1 DNA strand

Question 12

How is mRNA suited to its functions?

Answer 12

Breaks down quickly so no excess polypeptide forms

Ribosome can move along strand and tRNA can bind to exposed bases

Can be translated into a specific polypeptide by ribosomes

Question 13

Describe the structure of tRNA

Answer 13

Single strand of about 80 nucleotides

Folded into a clover shape

Anticodon on one end, amino acid binding site on the other

Anticodon binds to complementary mRNA codon
Amino acid corresponds to anticodon

Question 14

Order DNA, mRNA and tRNA according to increasing length

Answer 14

tRNA
mRNA
DNA

Question 15

Why is DNA replication described as semiconservative?

Answer 15

Strands from one original DNA molecule act as a template molecule

New DNA molecule contains 1 old strand and 1 new strand

Question 16

Outline the process of semiconservative DNA replication

Answer 16

1. DNA helicase breaks H bonds between base pairs
2. Each strand acts as a template
3. Free nucleotides from nuclear sap attach to exposed bases by complementary base paring
4. DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand
5. H-bonds reform

Question 17

Describe the structure of ATP

Answer 17

Nucleotide with an adenine with 3 phosphate groups

Question 18

Explain the role of ATP in cells

Answer 18

Energy released is coupled to metabolic reactions

Phosphate group phosphorylates compounds to make them more reactive

Question 19

How is ATP resynthesised in cells?

Answer 19

ATP synthase catalyses condensation reaction between ADP + Pi

During photosynthesis and respiration

Question 20

Why is water a polar molecule?

Answer 20

O is more electronegative than H so attracts the electron density in the covalent bond more strongly

Question 21

State 4 biologically important properties of water

Answer 21

Metabolite / solvent for chemical reactions in the body

High specific heat capacity

High latent heat of vapourisation

Cohesion between molecules

Question 22

Explain why water is significant to living organisms

Answer 22

Solvent for polar molecules during metabolic reactions

Enables organisms to avoid fluctuations in core temperature

Cohesion-tension of water molecules in transpiration stream

Question 23

What are the inorganic ions and where are they found in the body?

Answer 23

Ions that do not contain carbon atoms

Found in cytoplasm and extracellular fluid

May be in high or very low concentrations

Question 24

Explain the role of hydrogen ions in the body

Answer 24

High concentration of H+ = low pH

H+ ions interact with H bonds and ionic bonds in tertiary structure of proteins which can cause them to denature

Question 25

Explain the role of iron ions in the body

Answer 25

Forms Haem groups

Haem group has binding site to transport 1 molecule of O2 around body in the bloodstream

4 Haem groups per haemoglobin molecule

Question 26

Explain the role of sodium ions in the body

Answer 26

Involved in co transport for absorption of glucose and amino acids in the lumen of the gut

Involved in propagation of action potentials

Question 27

Explain the role of phosphate ions in the body

Answer 27

DNA
ATP
NADP
cAMP