B3.3 Genetics
ANSWERS
Worksheet Two
Molecular Genetics 1
1. Nucleotide drawn and labelled showing deoxyribose sugar, nitrogenous base and
phosphate group.
A section of DNA drawn showing the nucleotides running anti-parallel.
2. Nucleotides are asymmetrical. This is based on the numbering of the carbon
atoms. The 5’carbon joins to the phosphate group while the 3’ carbon joins to the
next nucleotide. When they are joined together to form DNA, the DNA has a
direction. For base pairing to occur on the opposite side of the DNA, the direction
runs in the opposite direction giving an antiparallel structure, i.e.
3’
5’
5’
3’
3. Enzymes e.g. helicase etc
Structural e.g. hair
Hormones e.g. insulin
Immune system e.g. antibodies
4. This refers to the flow of information from DNA to mRNA to protein.
5. Proteins primary structure is the sequence of amino acids. This is then folded into
either pleated sheet or an alpha helix shape due to hydrogen bonds to form the
secondary structure. The tertiary structure occurs when attraction between the
alpha helix and the pleated sheet structures forms a globular shape. The
quaternary structure occurs if more than one amino acid chain is present.
6. To make an identical copy of the DNA for cell division.
7. Diagram shows DNA replication occuring with the following features: helicase,
replication fork, DNA ligase, DNA polymerase, leading strand, lagging strand,
okazaki fragments.
8. a)
The replication fork is the point where the DNA is unzipped by helicase to
enable free nucleotides to be attached to the parent strand by the enzyme DNA
polymerase to make identical copies.
b)
Nucleotides can only be attached to the template strand in the 5’ to 3’
direction. Therefore on the lagging strand the nucleotides are added in small
sections, called Okazaki fragments, as the DNA is unzipped and joined together
later by DNA ligase.
c)
As DNA runs antiparallel the leading strand runs from the 5’ direction to
the 3’ direction meaning nucleotides can be added continuously. The lagging
strand is the opposite (3’ to 5’) so the nucleotides form Okazaki fragments.
9. Because the process creates two identical strands of DNA each consisting of one
of the original strands and one newly synthesised strand.
10.
Both DNA and RNA consist of repeating units of nucleotides with the bases A, G
and C. However, DNA has T and RNA has U. DNA nucleotides have a
deoxyribose sugar and RNA has a ribose sugar. RNA is shorter and has a variety
of forms and functions e.g. tRNA, mRNA and rRNA. DNA is double stranded
(double helix) and more stable than RNA.
11.
A codon is a series of three nucleotides on mRNA that is complementary to a
triplet on the coding strand of DNA. Whereas an anticodon is complementary to
the codon and is found on the tRNA.
12.
Transcription is the process of copying the DNA code by making mRNA. This is
done by the DNA unwinding and unzipping. The enzyme RNA polymerase
attaches free RNA nucleotides by complementary base pairing to the coding
strand of DNA to produce mRNA. This then leaves the nucleus for translation to
occur.
13.
Both processes involve making a copy of the DNA code in the nucleus.
Transcription makes a copy of the code by producing mRNA with RNA
nucleotides. Whereas DNA replication uses DNA nucleotides to produce an
identical copy. DNA replication uses both sides of the DNA, whereas
transcription only uses the coding strand. The enzymes are also different; DNA
replication uses helicase, DNA polymerase and DNA ligase, whereas
transcription uses RNA polymerase.
14.
Degeneracy refers to the fact that there is more than one codon for each amino
acid. This is an advantage because if there is a mutation (a change in the sequence
of bases) then there is a chance that there would be no effect to the sequence of
amino acids in the protein.
15.
Translation is the process where a protein is formed from mRNA. mRNA moves
into the cytoplasm where tRNA brings the complementary amino acid. These are
joined with peptide bonds to form a polypeptide chain and eventually a protein.
16.
Enzyme
Process(es)
Helicase
DNA replication
Role
Unwinds and unzips the
DNA
DNA polymerase
DNA replication
Attaches DNA
nucleotides to DNA
RNA polymerase
Transcription
Attaches RNA
nucleotides to DNA
DNA ligase
DNA replication
Joins Okazaki
fragments together
17.
Anticodon
A sequence of three nucleotides in a transfer RNA (tRNA),
that codes for an amino acid.
Central Dogma
The transcription of DNA to RNA and then translated into a
protein.
Coding Strand
The strand of DNA that has the complementary sequence to
mRNA.
Codon
Sequence of three mRNA nucleotides coding for one amino
acid.
Complementary The specific pairing of base A with T and base C with G in
base pairing
double-stranded DNA.
Disulphide
Bond between two sulfur-containing amino acids in a folded
Bridge
protein chain.
DNA Ligase
An enzyme involved in the DNA synthesis and repair. Its
function is to join the fragments of DNA together, e.g.
Okazaki fragments.
DNA
An enzyme that adds bases to a replicating DNA strand.
Polymerase
Duplication
A part of the chromosome is present in two or more copies.
Helicase
An enzyme that unwinds and unzips the DNA helix.
Lagging Strand
The strand of DNA that is synthesised discontinuously during
replication (because DNA synthesis can proceed only in the 5´
to 3´ direction).
Leading Strand
The strand of DNA that is synthesised continuously during
replication.
mRNA
(messenger RNA) RNA species that contains the information
to specify the amino acid sequence of proteins and that is
translated on the ribosome.
Nucleotide
A building block of DNA or RNA, consisting of one
nitrogenous base, one phosphate molecule, and one sugar
molecule (deoxyribose in DNA, ribose in RNA).
Okazaki
An Okazaki fragment is a relatively short fragment of DNA
Fragment
created on the lagging strand during DNA replication.
Protein
The formation of a specific protein. Individual amino acids are
Synthesis
joined together in a specific sequence, determined by an
mRNA molecule.
Purines
One of two families of nitrogenous bases found in nucleotides.
Two members are adenine and guanine.
Pyrimidines
One of two families of nitrogenous bases found in nucleotides.
Three members are cytosine, thymine and uracil.
Replication
The replication fork is a structure that forms within the
Fork
nucleus during DNA replication. It is created by helicases,
which break the hydrogen bonds holding the two DNA strands
together.
RNA
An enzyme which helps synthesize mRNA by transcribing the
Polymerase
nucleotide sequence in DNA.
Template
The strand of the DNA double helix that is transcribed by
Strand
complementarily base pairing to make an mRNA.
Transcription
The synthesis of mRNA copy from a sequence of DNA (a
gene); the first step in gene expression.
Translation
The synthesis of a protein using mRNA code.
Triplet
A sequence of three nucleotides on the template DNA strand.
Each triplet represents the code for a particular amino acid.
tRNA
A type of RNA molecule that carries a specific amino acid
and matches it to its corresponding codon on an mRNA during
translation.