DNA replication and protein synthesis

Question 1

Explain why DNA replication is necessary.

Answer 1

DNA copies itself before cell division so that each new cell has the full amount of DNA. This is important for making new cells for passing genetic information from generation to generation.

Question 2

Define the term “semi-conservative replication”.

Answer 2

DNA replication results in one stand from the original DNA double helix and one strand from the new DNA double helix being in the resulting daughter DNA molecule.

Question 3

Draw and annotate a diagram to show the sequence of events in DNA replication.

Answer 3

1) DNA helicase breaks the hydrogen bonds between the two polynucleotide DNA strands. The helix unzips to form two single strands.
2) Each original single strand acts as a template for a new strand. Free-floating DNA nucleotides join to the exposed bases on each original template strand by complementary base pairing (A-T, G-C) .
3) The nucleotides of the new strand are joined together by DNA polymerase. This forms a sugar phosphate backbone. Hydrogen bonds forms between the bases on the original and new strand. The strands to form a double-helix.
4) Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

Question 4

State the roles of the enzyme DNA helicase in DNA replication.

Answer 4

DNA helicase breaks the hydrogen bonds between the two polynucleotide DNA strands.

Question 5

State the role of the enzyme DNA polymerase in DNA replication.

Answer 5

DNA polymerase joins the nucleotides of the new strand together to form a sugar phosphate backbone.

Question 6

Describe how, and explain why, DNA replication occurs by continuous replication of one strand and
discontinuous replication of the other strand.

Answer 6

DNA polymerase moves along the template strand in one direction. It can only bind to the 3’ end of the nucleotide so it travels in the 3’ to 5’ direction. This is not an a problem for the leading strand of DNA because it goes in the 3’ to 5’ direction so it can be continuously replicated. However, the lagging strand of DNA travels in the 5’ to 3’ direction so this results in the DNA having to be produced in sections which then have to be joined. This is called discontinuous replication.

Question 7

Explain the importance of DNA replication conserving genetic information with accuracy.

Answer 7

It makes sure genetic information is conserved (stays the same) each time the genetic information in the cell is replicated.

Question 8

Define the term “mutation”.

Answer 8

A change in the genetic material (a change to the DNA base sequence) which may affect the phenotype of the organisms.

Question 9

Outline how the sequence of bases in DNA can code for the primary structure of a polypeptide chain.

Answer 9

The sequence of amino acids in a polypeptide chain forms the primary structure of a protein.
It is the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein. Each amino acid is coded for by a sequence of three bases (called a triplet) in a gene.
Different sequences of bases code for different amino acids. So the sequence of bases in a section of DNA is a template that’s used to make proteins during protein synthesis.

Question 10

Define the term ‘‘gene’’.

Answer 10

A section of DNA that contains the complete sequences of bases (codons) to code for an entire protein.

Question 11

Define the term ‘‘codon’’.

Answer 11

A three base sequence of DNA or RNA that codes for an amino acid.

Question 12

Define the term “triplet code”.

Answer 12

The genetic code is a sequence of three nucleic acid bases called a codon. Each codon codes for one amino.

Question 13

Define the term“non-overlapping”.

Answer 13

This means that successive triplets are read in order. Each nucleotide is part of only one triplet codon.

Question 14

Define the term ‘‘degenerate’’.

Answer 14

The genetic code is degenerate because there are many instances in which different codons code for the same amino acid.

Question 15

Define the term ‘‘universal’’.

Answer 15

The genetic code is describe as universal because the same specific base triplets code for the same amino acids in all living things e.g. UAU codes for tiersone in all organisms.

Question 16

Explain why the genetic code is a triplet code, the value of it being non-overlapping and the reason for
it being degenerate.

Answer 16

It is a triplet code because it takes a sequence of three bases to code for one amino acid.
It is degenerate because there are 64 possible and only 20 different amino acids, this means that different codons will code for the same amino acid.

Question 17

Describe what is meant by a “start codon” .

Answer 17

The start codon is the codon that signals the start of a sequence that codes for a protein.

Question 18

Describe what is meant by a ‘‘stop codon’’.

Answer 18

Stop codons do not code for any amino acids and they signal the end of a sequence.

Question 19

Outline how mutations can alter the structure of a protein.

Answer 19

A mutation changes the base sequence of DNA and the base sequence defines the order of amino acids in a protein. If the order of amino acids changes the primary structure will change, and this will then affect the higher levels of structure in the protein.

Question 20

Define the term “transcription”.

Answer 20

The process of copying sections of DNA base sequence to produce smaller molecules of mRNA which can be transported out of the nucleus via the nuclear pores to the site of protein synthesis.

Question 21

Define the term ‘‘translation’’.

Answer 21

The process by which the complementary code carried by mRNA is decoded by tRNA into a sequence of amino acids. This occurs at a ribosome.

Question 22

State the three types of RNA.

Answer 22

• mRNA
• tRNA
• rRNA

Question 23

Give the structure and function of mRNA.

Answer 23

• Messenger RNA
• A single polynucleotide
• Made in the nucleus during transcription.
• mRNA carried the genetic code from DNA in the nucleus to the cytoplasm where it is used to make a protein during translation.
• In mRNA three groups of three adjacent bases are usually codons.

Question 24

Give the structure and function of tRNA

Answer 24

• Transfer RNA
• A single polynucleotide strand that’s folded into a clover shape.
• H bonds between specific base pairs hold the molecule in this shape.
• Every tRNA molecule has a specific sequence of three bases at one end called an anitcodon.
• They also have an amino acid binding site at the other end.
• It is found in the cytoplasm and carries amino acids that are used to make proteins in translation to the ribosome.

Question 25

Give the structure and function rRNA.

Answer 25

• Ribosomal RNA
• forms the two subunits in a ribosome along with proteins.
• the ribosome moves along the mRNA strand during protein synthesis so the rRNA in the ribosome helps to catalyse the formation of peptide bonds between the amino acids.

Question 26

State the stages in protein synthesis.

Answer 26

Transcription - an mRNA copy of a gene is made in the nucleus.
Translation - amino acids are joined together by a ribosome to make a polypeptide chain.

Question 27

Describe the process of transcription.

Answer 27

1) RNA polymerase attaches to the DNA double helix at the beginning of a gene.
2) The hydrogen bonds between the two strands in the gene break, separating the strands, and the DNA molecule uncoils at that point. One of the strands is then used a template to make an mRNA copy.
3) The RNA polymerase lines up free RNA nucleotides alongside the template strand. Complementary base pairing means that mRNA ends up being a complementary copy of the DNA template strand (except the base T is replaced by U in RNA).
4) Once the RNA nucleotides have paired up with their specific bases on the DNA strand, they’re joined together by RNA polymerase, forming an mRNA strand.
5) The RNA polymerase moves along the DNA, assembling the mRNA strand. The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double helix.
6) When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from the DNA.
7) The mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein sysnthesis can be begin.

Question 28

Describe the process of translation.

Answer 28

1) The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to the ribosome.
2) A tRNA molecule, with an anitcodon that’s complementary to the start codon on the mRNA, attaches itself to the mRNA by complementary base pairing. A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.
3) rRNA in the ribosome catalyses the formation of a peptide bond between the two amino acids attached to the tRNA molecules. This joins the amino acids together.
4) The first tRNA molecule moves away, leaving its amino acid behind.
5) A third tRNA molecule binds to the next codon on the mRNA. Its amino binds to the two amino acids already in place and the second tRNA molecule moves away.
6) This process continues producing a chain of linked amino acids (a polypeptide chain) until there’s a stop codon on the mRNA molecule.
(See diagram in CGP book pg 93)

Question 29

Define the term “sense strand”.

Answer 29

The strand of DNA that runs 5’ to 3’ and contains the genetic code for a protein.

Question 30

Define the term ‘‘anti-sense strand’’.

Answer 30

The strand of DNA that runs 3’ to 5’ and is complementary to the sense strand. It acts as a template strand during transcription.

Question 31

Define the term template stand.

Answer 31

The anti-sense strand of DNA that acts as template during transcription so that the complementary RNA strand formed carry the same code for a protein as the DNA sense strand.

Question 32

Define the term anti-codon

Answer 32

An anti-codon is a sequence of three nucleotides in a region of transfer RNA that recognizes a complementary coding triplet of nucleotides in messenger RNA during translation.

Question 33

Describe the structure of a ribosome and explain the role of rRNA.

Answer 33

• Ribosomes are made up of two subunits, one large and one small.
• These subunits are composed of almost equal amounts of protein and rRNA
• rRNA is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.

Question 34

Draw and label a diagram of tRNA.

Answer 34

• It is composed of a strand of RNA folded in such a way that three bases, called the anticodon, are at one end of the molecule.
• At the other end of the molecule it has a point of attachment for an amino acid.