Author: Kim Edgell
Document comments: Created by the \'abHTML to RTF .Net\'bb 5.8.2.9

Nucleic Acid homework 2: Replication 
Name: ______________________________ %:______________
Q1.The diagram shows part of a DNA molecule.
 
(a)     How many nucleotides are shown in the diagram above?
___________________________________________________________________(1)
(b)     Name the type of bond labelled X in the diagram.
___________________________________________________________________(1)
(c)     The enzymes DNA helicase and DNA polymerase are involved in DNA replication.
Describe the function of each of these enzymes.
DNA helicase _______________________________________________________
___________________________________________________________________
___________________________________________________________________
DNA polymerase _____________________________________________________
___________________________________________________________________
___________________________________________________________________(2)
(d)     Adenosine triphosphate (ATP) is a nucleotide derivative.Contrast the structures of ATP and a nucleotide found in DNA to give two differences.
1. _________________________________________________________________
___________________________________________________________________
2. _________________________________________________________________
___________________________________________________________________(2)
Q2.Draw and label a single DNA nucleotide

(2)
(b)     Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA.
1. _________________________________________________________________
___________________________________________________________________
___________________________________________________________________
2. _________________________________________________________________
___________________________________________________________________
__________________________________________________________________(2)
(c)     Replication of mitochondrial DNA (mtDNA) is different from that of nuclear DNA. The replication of the second strand of mtDNA only starts after two-thirds of the first strand of mtDNA has been copied. A piece of mtDNA is 16 500 base pairs long and is replicated at a rate of 50 nucleotides per second.
Tick (✓) the box that shows how long it would take to copy this mtDNA.
 
A | 330 seconds |
B | 440 seconds |
C | 550 seconds |
D | 660 seconds |
(1)

Q3.(a)     Name two enzymes involved in the semi-conservative replication of DNA.
1. _________________________________________________________________
2. _________________________________________________________________(2)
(b)     Sometimes, damage occurs during DNA replication. One enzyme involved in repairing damage to DNA is called ATR.
ATR works as follows.
•   ATR phosphorylates other enzymes involved in repairing DNA.
•   ATR also phosphorylates substrates required to repair DNA.
When ATR phosphorylates other enzymes, these enzymes become able to bind to their substrates. Use your knowledge of enzyme structure to suggest why.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________ (2)
(c)     The enzyme-catalysed reactions activated by ATR only occur if the substrates have been phosphorylated.
Use your knowledge of energy changes in enzyme-catalysed reactions to suggest why.
___________________________________________________________________
___________________________________________________________________ (1)
(d)     Sometimes, a mutagenic agent causes DNA to break. A different enzyme called ATM binds to the broken DNA. This leads to the activation of a protein coded for by a tumour suppressor gene. The effect of ATM binding is to stop cell division until DNA is repaired.
A mutation could result in a person having non-functional forms of the gene that produces ATM.
What can you predict about the possible effects of having a non-functional form of ATM?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________ (3)

Q4.(a)     Explain why the replication of DNA is described as semi-conservative.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
 (2)
(b)     Bacteria require a source of nitrogen to make the bases needed for DNA replication. In an investigation of DNA replication some bacteria were grown for many cell divisions in a medium containing ^14N, a light form of nitrogen. Others were grown in a medium containing ^15N, a heavy form of nitrogen. Some of the bacteria grown in a ^15N medium were then transferred to a ^14N medium and left to divide once.
DNA was isolated from the bacteria and centrifuged.
The DNA samples formed bands at different levels, as shown in the diagram.
 
(i)      What do tubes A and B show about the density of the DNA formed using the two different forms of nitrogen?
______________________________________________________________
______________________________________________________________(1)
(ii)     Explain the position of the band in tube C.
______________________________________________________________
______________________________________________________________
______________________________________________________________(2)
(c)     In a further investigation, the DNA of the bacterium was isolated and separated into single strands. The percentage of each nitrogenous base in each strand was found. The table shows some of the results.
 
| Percentage of base present
DNA sample | Adenine | Cytosine | Guanine | Thymine
Strand 1 | 26 | | 28 | 14
Strand 2 | 14 | | |
Use your knowledge of base pairing to complete the table.(2)
Q5.(a)     Give the two types of molecule from which a ribosome is made.
___________________________________________________________________
___________________________________________________________________(1)
(b)     Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
__________________________________________________________________ (3)
(c)     The table below shows the base sequence of part of a pre-mRNA molecule from a eukaryotic cell.
Complete the table with the base sequence of the DNA strand from which this pre-mRNA was transcribed.
 
| | | | | | | | | DNA
A | C | G | C | A | U | U | A | U | pre-mRNA
(1)
(d)     In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA.
Explain why.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________(2)
Q6.(a)     DNA is a polymer of nucleotides. Each nucleotide contains an organic base.Explain how the organic bases help to stabilise the structure of DNA.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________(2)
(b)     Triplets of bases in a DNA molecule code for the sequence of amino acids in a polypeptide. The genetic code is frequently written as the three bases on mRNA that are complementary to a triplet on DNA. Table 1 shows what different combinations of bases on mRNA code for. The names of amino acids are abbreviated. For example, ‘Ala’ stands for alanine.
 
First base | Second base | Third base
| Guanine (G) | Adenine (A) | Cytosine (C) | Uracil (U) |

G

GGG    Ala
 GGA    Gly
 GGC    Gly
 GGU    Gly

GAG    Glu
 GAA    Glu
 GAC    Asp
 GAU    Asp

GCG    Ala
 GCA    Ala
 GCC    Ala
 GCU    Ala

GUG    Val
 GUA    Val
 GUC    Val
 GUU    Val

G
A
C
U

A

AGG    Arg
 AGA    Arg
 AGC    Ser
 AGU    Ser

AAG    Lys
 AAA    Lys
 AAC    Asn
 AAU    Asn

ACG    Thr
 ACA    Thr
 ACC    Thr
 ACU    Thr

AUG    Met
 AUA    Iso
 AUC    Iso
 AUU    Iso

G
A
C
U

C

CGG    Arg
 CGA    Arg
 CGC    Arg
 CGU    Arg

CAG    Gln
 CAA    Gln
 CAC    Hist
 CAU    Hist

CCG    Pro
 CCA    Pro
 CCC    Pro
 CCU    Pro

CUG    Leu
 CUA    Leu
 CUC    Leu
 CUU    Leu

G
A
C
U

U

UGG    Trp
 UGA    stop
 UGC    Cyst
 UGU    Cyst

UAG    stop
 UAA    stop
 UAC    Tyr
 UAU    Tyr

UCG    Ser
 UCA    Ser
 UCC    Ser
 UCU    Ser

UUG    Leu
 UUA    Leu
 UUC    Phe
 UUU    Phe

G
A
C
U

Suggest one advantage of showing the genetic code as base sequences on mRNA, rather than triplets on DNA.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________(1)
(c)     What name is given to a group of three bases on mRNA that codes for an amino acid?
___________________________________________________________________(1)
(d)     Use information from Table 1 to explain why the genetic code is described as degenerate.
___________________________________________________________________
__________________________________________________________________ (2)
(e)     Suggest the role of the mRNA base triplets UGA, UAG and UAA.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________(2)
(f)      Table 2 shows the sequence of mRNA bases forming part of a single gene.
 
Table 2
Base on DNA template | | | | | | | | |
Base on mRNA | G | U | G | U | A | C | U | G | G
Encoded amino acid | | |
Complete Table 2 to show the base sequence of the DNA template from which this mRNA was transcribed and the encoded amino acid sequence.(2)