Name: ANSWERS
In the Nucleus Workbook
Year 12 Science
Ruawai College 2014
Use the models and the explanations provided to answer the questions.
1.
What is the name of the membrane that covers the cell?
Cell membrane
2.
Inside the cell membrane is the cytoplasm. What are the names of the organelles that are in
the cytoplasm?
Nucleus, mitochondria, Golgi body, rough ER, smooth ER,
3.
The control centre of the cell is the nucleus. What is the name of the membrane that
surrounds the nucleus?
Nuclear membrane
4.
What is the name of the body that is can be seen inside the nucleus?
Nucleolus
5.
What is the nuclear membrane called in the diagram of the cell nucleus?
Nuclear envelope
6.
What are scattered on the surface of the nuclear membrane?
Nuclear pores
7.
What else is shown inside the nucleus?
Chromosomes, chromatin
8.
What shape are the chromosomes?
Shaped like an X
The nucleus is the control centre of the cell because it contains DNA.
DNA is the genetic material that organises all cell processes.
DNA is scattered throughout the nucleus as chromatin.
Chromatin makes up the chromosomes which are visible before the start of cell division.
9.
What is chromatin made from?
DNA
10.
What are chromosomes made from?
Chromatin
11.
When are chromosomes visible in a cell?
Just before cell division
The nucleolus produces the RNA component of ribosomes, which are the site of protein synthesis.
The nuclear membrane has pores in it to allow substances to move between the nucleus and the
cytoplasm / ER.
12.
What does the nucleolus produce?
RNA
13.
What are the pores in the nuclear membrane called?
Nuclear pores
14.
What do the pores in the nuclear membrane do?
Allow sub stances move between the nucleus and the cytoplasm and ER
DNA (deoxyribonucleic acid) is the very large molecule in chromosomes that carries the genetic code.
Its shape is a double helix – a twisted ladder. The side strands are made of alternating sugar and
phosphate groups. The cross-strands are paired bases.
Each strand of DNA is made up of many repeating units called nucleotides. A nucleotide is made up
of three parts. There are four different nucleotides and the only difference between nucleotides is
the nitrogen base they contain.
15.
What does DNA stand for?
deoxyribonucleic acid
16.
Where will you find DNA?
In chromosomes
17.
What does DNA carry?
Genetic code
18.
What shape is DNA?
Double helix / twisted ladder
19.
What are the side strands of DNA made from? alternating sugars and phosphates
20.
What are the cross-stands of DNA made from? bases
21.
What are the names of the three parts of a nucleotide?
Sugar, phosphate, base
22.
What are the names of the four bases that a nucleotide can have?
Adenine, thymine, guanine, cytosine
In the model of DNA below the base pairs are held together by weak hydrogen bonds.
23.
What does the P stand for?
phosphate
24.
What does the S stand for?
sugar
25.
What does the T stand for?
thymine
26.
What does the A stand for?
adenine
27.
What does the C stand for?
cytosine
28.
What does the G stand for?
guanine
29.
Which base does nitrogen base T pair with?
A – adenine
30.
Which base does nitrogen base C pair with?
G – guanine
31.
How many hydrogen bonds hold the A and T together?
two
32.
How many hydrogen bonds hold the C and G together?
Three
Complete the DNA by writing in the missing
bases.
Deoxyribose is a pentose sugar – this means it
has 5 carbon atoms (numbered)
33.
How many carbons does a pentose sugar have? 5
34.
What base is represented in the diagram?
guanine
35.
What carbon is the base bonded to?
1
Complete the diagram by adding in the numbers
of the carbons of each nucleotide.
Each nucleotide has a 3’ end and a 5’ end.
3’ is called three prime.
5’ is called five prime.
The phosphate groups join the nucleotides
together by bonding the C3 of one sugar with the
C5 of an adjacent sugar.
36.
What do the circles represent in the diagram?
phosphates
37.
What do the letters represent in the diagram?
bases adenine, guanine, cytosine
38.
How many carbons does each nucleotide have?
5
39.
Which carbons are joined to form the chain of nucleotides?
3
The two strands that make up DNA are two
chains made by alternating sugar-phosphate
groups.
One chain runs one way, and the other chain
runs the other way.
The two chains are anti-parallel.
A chain of nucleotides will have a 3 carbon
(shown as 3’, called three prime) at one end, and
a 5 carbon (shown as 5’, called five prime) at the
other.
40.
What carbon is at the top of the nucleotide chain on the LHS of the DNA?
41.
What carbon is at the top of the nucleotide chain on the RHS of the DNA?
5
A nucleotide consists of a nitrogen base + 5 carbon sugar + phosphate group. The carbons in the
sugar are numbered from 1 to 5. The side strands of the ladder are constructed in different
directions. 5’ is called 5 prime. 3’ is called 3 prime.
Looking at (b) in the diagram above:
42.
What carbon starts the left hand side strand of the DNA ladder?
5’
43.
What carbon ends the left hand side strand of the DNA ladder?
3’
44.
In which direction does the left hand side strand of the DNA ladder go? From 5’ to 3’
45.
What carbon starts the right hand side strand of the DNA ladder?
3’
46.
What carbon ends the right hand side strand of the DNA ladder?
5’
47.
In which direction does the right hand side strand of the DNA ladder go? From 3’ to 5’
48.
What do you think ‘antiparallel orientation of strands’ means?
Strands are the same but run in different directions
It is essential that DNA can replicate itself, so that chromosomes can be copied to give the same
genetic code to every new cell that is made.
The genetic code is a sequence of bases along
one strand.
A sequence of three bases, a triplet, codes for
one amino acid.
Amino acids are the building blocks of proteins.
1st
2nd
3rd
49.
What is the code for the first amino acid?
ATG
50.
What is the code for the second amino acid?
CTA
51.
What is the code for the third amino acid?
GGC
Amino acids are the building blocks of proteins. All the triplets that code for one complete protein
make up a gene. A molecule of DNA can have thousands of genes. Different sequences of bases
along a gene can produce proteins that are slightly different. Proteins are made by ribosomes.
52.
Are genes made of DNA, or is DNA made of genes?
Genes are made of DNA
53.
Are proteins made of amino acids, or are amino acids made of proteins? Proteins are made
of amino acids
54.
What organelle is responsible for the synthesis (production) of proteins?
Ribosome
It is the base pairing mechanism that allows DNA to replicate itself. Replication occurs in a series of
steps that are controlled by enzymes. Energy is needed for this process and it is supplied by ATP.
The enzyme DNA helicase aids the DNA helix to unwind.
The enzyme DNA polymerase aids in the bonding of new nucleotides to exposed bases.
55.
What has happened to the original stand of DNA helix?
DNA helix has unwound
56.
What is the name of the enzyme that aids the DNA to unwind?
DNA helicase
57.
After the double helix has unwound what happens to the DNA?
DNA unzips after it has unwound
58.
After the DNA has separated (unzipped) what is added to the DNA?
After the DNA has unzipped nucleotides are added to it
59.
What is the name of the enzyme that aids the bonding of nucleotides to the exposed bases?
DNA polymerase helps bond the nucleotides
58.
two
After the nucleotides are added how many stands of DNA are formed?
59.
yes
Are the two copies of DNA identical?
Because of the way the sugar and the phosphate molecules are joined to each other along the DNA
chain, one strand is copied in a straight forward manner, while the other stand is copied in short
fragments in the opposite direction.
60.
What is the name of the strand that is copied in a straight forward manner (left to right)?
Leading strand
61.
What is the name of the strand that is copied in short fragments (right to left)?
Lagging strand
62.
An enzyme is responsible for the strand of DNA unwinding. What is the name of the
enzyme?
DNA helicase
63.
An enzyme is responsible for joining the new bases to the leading strand of DNA. What is
the name of the enzyme?
DNA polymerase
64.
What are the names of the short fragments added to the lagging strand of DNA?
Okazaki fragments
The short fragments that are copied are approximately 1000 bases. These Okazaki fragments are
bonded to form a continuous strand using the enzyme ligase.
Replication occurs in a series of steps, each controlled by enzymes, with energy supplied from ATP.
Two identical DNA molecules result from replication, each having one original strand and one new
strand (known as semi-conservative replication). At the end of replication the two complete DNA
molecules twist into a helix.
65.
What enzyme is responsible for unwinding the double helix shape of the DNA?
DNA helicase
66.
What proteins stabilise the unwound DNA to stop the strands bonding again?
Single strand binding proteins
67.
What enzyme is responsible for adding bases to the leading strand of DNA?
DNA polymerase
68.
What enzyme is responsible for forming a RNA primer (short sequence of bases that are later
removed)?
DNA primase
69.
What enzyme is responsible for forming Okazaki fragments?
DNA polymerase
70.
What replaces the RNA primer?
DNA
71.
What enzyme is responsible for forming the DNA that replaces the RNA primer?
DNA polymerase
72.
What enzyme is responsible for joining the short lengths of DNA?
DNA ligase
The DNA helix is opened at many sites along the whole molecule for simultaneous replication in both
directions - this makes replication very fast.
73.
Fill in the table:
DNA molecule
Strand
ATC GTC ATT
TCG ACA GGT
CAA TTG CAC
Corresponding strand
TAG CAG TAA
AGC TGT CCA
GTT AAC GTG
74.
The four diagrams show the main steps of replication. Beneath each diagram, describe what
is happening.
DNA helix is unwinding to form a ‘ladder’ – uses the enzyme helicase
DNA is unzipping as base pairs separate between the two strands – the hydrogen bonds break
Using complimentary base pairing, nucleotides bond to the exposed bases in each strand – uses
enzyme DNA polymerase
Nucleotides are joined continuously down the entire length of the leading strand
Nucleotides are joined in small sections (Okazaki fragments) down the lagging strand
When replication of each strand is complete, two identical DNA molecules result – each will twist
to form into the double helix
Each molecule consists of one original strand and one new strand
75.
Give the difference in replication for the leading and the lagging strands.
Nucleotides are joined continuously down the entire length of the leading strand
Nucleotides are joined in small fragments down the length of the lagging strand
76.
State what Okazaki fragments are, and how they are joined together.
Okazaki fragments are small sequences of about 1000 bases bonded to the lagging strand in DNA
replication
Okazaki fragments are joined together by the enzyme ligase to form a continuous strand
77.
Explain why replication is described as semi-conservative.
Replication is semi-conservative as one strand in each DNA molecule is from the original molecule
and the other is formed from new nucleotides
78.
Explain why and when DNA replication occurs.
DNA replication occurs before cell division takes place
DNA needs to be replicated so each new cell formed in cell division has a full set of the genetic
code (its chromosomes) so it can grow and carry out essential life processes
79.
Exam Question
DNA replication occurs at different rates during the lifetime of a plant or an animal.
Evaluate this statement, including in your answer:



a description of what DNA replication is
an explanation of how the process occurs
a discussion of why the rate of DNA replication varies during the lifetime of a plant or an
animal
A labelled diagram may be used to support your answer.
80.
Exam Question
Cells carry out DNA replication before they divide.
a.
State the purpose of DNA replication
b.
Explain how the genetic material in a chromosome is replicated, and why the process is
known as semi-conservative. You may draw a labelled diagram(s) to support your answer.
Glossary
Keyword
Definition