Biological molecules - NUCLEIC ACIDS AND DNA REPLICATION

Question 1

Why is DNA said to be degenerative, universal and non-overlapping?

Answer 1

• Degenerative: Amino acid can be coded for by more than one DNA triplet
• Universal: DNA triplets code for the same amino acids in [all] organisms
• Non-overlapping: DNA triplets read only once (in groups of 3)

Question 2

Describe the structure of DNA (6 marks)

Answer 2

• Polymer of nucleotides
• Nucleotides are made of phosphate, deoxyribose sugar + base
• Nucleotides joined by phosphodiester bonds in a condensation reaction
• 4 bases: purines (2 rings) paired with pyrimidine (1 rings) [guanine + cytosine, adenine + thymine]
• Bases are joined by hydrogen bonds
• Antiparallel strands
• Coiled into a double helix

Question 3

Why does DNA form a double helix?

Answer 3

• Stable configuration; not straining bonds
• Negative phosphates repel each other on the outside
• Bases lie flat in the centre, filling the space

Question 4

What is the function of DNA?

Answer 4

• Instructions for making proteins and RNA’s
• Information storage (preserved for a long time without changes)

Question 5

How does the structure of DNA relate to its function?

Answer 5

Double stranded - allows splitting and replication to make 2 identical double helixes//allows for errors to be identified during copying (decreased mutation)

Bases are inside the negative backbone - protects bases from undergoing chemical reactions ( decreased mutation)

Question 6

How is RNA formed?

Answer 6

By a condensation reaction between 2 nucleotides, resulting in the removal of water and the formation of a phosphodiester bond.

Question 7

What is the function of messenger RNA?

Answer 7

Copy of the code for 1 protein
- transferring genetic info to the ribosome (short term, can make more)

Question 8

What is the function of ribosomal RNA?

Answer 8

protein synthesis and rRNA’s

Question 9

What is the function of transfer RNA?

Answer 9

Transferring the correct amino acids to the ribsomes for protein synthesis

Question 10

Describe the structure of tRNA

Answer 10

• Folded to form ‘clover-leaf’ shape
• Held together by hydrogen bonds

Question 11

Describe the structure of mRNA

Answer 11

• Linear (straight chain)
• Relatively short
• Every 3 bases are referred to as codons (codes for an amino acid)

Question 12

Why does DNA need to be replicated?

Answer 12

Cell division - every cell needs to contain a a copy of the DNA - precedes

Question 13

What is the first step in DNA replication?

Answer 13

• 2 strands are separated by DNA helicase, which breaks the hydrogen bonds

Question 14

What is the second step in DNA replication?

Answer 14

Free DNA nucleotides are attracted to the template strand and form hydrogen bonds.

Question 15

What is the 3rd step of DNA replication?

Answer 15

Complementary base pairing:
Adenine with Thymine
Cytosine with Guanine

Question 16

What is the fourth step of DNA replication?

Answer 16

DNA Polymerase catalyses the formation of phosphodiester bonds between nucleotides

Question 17

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication (PPQ)

Answer 17

Watson and Crick

Question 18

Describe the function of DNA helicase (PPQ)

Answer 18

Catalyses the destruction of the hydrogen bonds, separating the DNA strands.

Question 19

Describe the function of DNA polymerase (PPQ)

Answer 19

Catalyses the formation of phosphodiester bonds between the nucleotides.

Question 20

Contrast the structures of ATP and a nucleotide found in DNA to give two differences (PPQ)

Answer 20

• ATP only contains adenine, whereas a nucleotide in DNA could adenine, thymine, cytosine and guanine.
• ATP contains 3 phosphate groups, whereas a nucleotide in DNA only has 1 phosphate group.

Question 21

Give 3 ways in which prokaryotic DNA is different from eukaryotic DNA (PPQ)

Answer 21

• Prokaryotic DNA is circular, eukaryotic is linear
• Prokaryotic DNA is coding (exons), whereas eukaryotic DNA is non coding (introns)
• Eukaryotic DNA is bound to histone proteins within the cell

Question 22

Which part of the DNA molecule contains nitrogen? (PPQ)

Answer 22

The nitrogenous base

Question 23

Which scientists found evidence for semi-conservative replication? (PPQ)

Answer 23

Meselson and Stahl

Question 24

Describe how, after the parent DNA strands separate, the second strand of DNA is formed (PPQ)

Answer 24

• Semi conservative replication
• Complementary pairing
• Hydrogen bonding of nucleotides
• Due to DNA polymerase

Question 25

Give two differences between the structure of DNA and the structure of RNA (PPQ)

Answer 25

• DNA contains thymine, whereas RNA contains uracil
• DNA has a deoxyribose sugar, whereas RNA has a ribose sugar

Question 26

What is the role of the single stranded DNA fragments in semi conservative replication? (PPQ)

Answer 26

• Act as the template strand
• Determines order of nucleotides

Question 27

What is the role of the DNA nucleotides in semi conservative replication? (PPQ)

Answer 27

• Forms complementary base pairs

Question 28

A polypeptide has 51 amino acids in its primary structure.
What is the minimum number of DNA bases required to code for the amino acids in this polypeptide?

Answer 28

153

• 51 amino acids = 51 codons
• In each codon there are 3 bases
• Hence 3 x 51