DNA Replication and Protein Synthesis

Question 1

Explain why DNA replication is necessary.

Answer 1

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

Question 2

Define the term semi-conservative replication.

Answer 2

DNA replication results in one old strand and one new strand present in each daughter DNA molecule

Question 3

Describe the process of DNA replication in a series of bullet points.

Answer 3

1. DNA helicase (enzyme) breaks 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 C-G.
3. The nucleotides on the new strand are joined together by DNA polymerase. This forms a sugar-phosphate backbone. H bonds from between the bases on the original and new strand.
4. The strands twist to form a double-helix. Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

Question 4

State the roles of DNA helicase in DNA replication.

Answer 4

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

Question 5

State the roles of DNA polymerase in DNA replication.

Answer 5

DNA polymerase joins the nucleotides on the new strand together.

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

1. DNA polymerase always moves along the template strand in the same direction.
2. It can only binds to the 3’ end so travels in 3’ to 5’ direction (Of original strand).
3. This is not a problem for the leading strand of DNA as this travels in the 3’ to 5’ direction- so has continuous replication
4. But lagging strand travels in 5’ to 3’ direction, so polymerase can’t attach the nucleotides continuously- discontinuous replication.
5. This results in DNA being produced in sections which have to be joined

Question 7

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

Answer 7

It makes sure genetic information is conserved each time the DNA in a cell is replicated.

Question 8

Define the term mutation.

Answer 8

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

Question 9

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

Answer 9

1. It’s the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein.
2. Each amino acid is coded for by a sequence of three bases (triplet) in a gene.
3. Different sequences of bases code for different amino acids.

Question 10

What is gene

Answer 10

A gene is a section of DNA that contains the complete sequences of bases (codons) to code for a protein.

Question 11

Define a codon

Answer 11

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

Question 12

Define triplet code

Answer 12

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

Question 13

Define 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 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 universal

Answer 15

The same specific base triplets code for the same amino acids in all living things. e.g UAU codes for tyrosine in all organism

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 codons and only 20 different amino acids- so different codons will code for the same amino acid.

Question 17

Describe what is meant by a start codon

Answer 17

A 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

Don’t code for any amino acids and 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- the primary structure- this then changes the higher levels of protein structure.

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. Occurs at a ribosome.

Question 22

State the three types of RNA.

Answer 22

mRNA, tRNA and rRNA.

Question 23

Describe the structure and function mRNA

Answer 23

1. Is a single polynucleotide made in the nucleus during transcription.
2. mRNA carries the genetic code from DNA in the nucleus to the cytoplasm where it is used to make a protein during translation.
3. 3 groups of three adjacent bases are usually called codons.

Question 24

Describe the structure and function tRNA

Answer 24

1. Transfer RNA- a single polynucleotide strand that’s folded into a clover shape.
2. H bonds between specific base pairs hold the molecule in this shape.
3. Every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon.
4. They also have an amino acid binding site at the other end.
5. It is found in the cytoplasm and carries amino acids that are used to make proteins (translation) to the ribosome.

Question 25

Describe the structure and function rRNA

Answer 25

Ribosomal RNA- forms the two subunits in a ribosome along with proteins. 2. The ribosome moves along the mRNA strand during proteins synthesis. 3. 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. DNA gene copied into mRNA- H bonds break 2. Free/activated RNA nucleotides line up by complementary base pairing to one template/sense strand 3. Catalysed by RNA polymerase 4. mRNA moves out of nucleus through nuclear pore and attaches to ribosome

Question 28

Describe the process of Translation

Answer 28

1. mRNA moves to ribosome 2. tRNA molecules bind to mRNA 3. Anticodons match/bind to codons (tRNA anticodons bind to RNA codons) 4. Specific amino acid attaches to tRNA 5. Peptide bonds form between the amino acids- catalysed by the rRNA in the ribosome

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 antisense 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 strand

Answer 31

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

Question 32

Describe and explain the difference between the functions of RNA polymerase and DNA polymerase

Answer 32

1. RNA polymerase makes RNA and is used in transcription. One strand of DNA used and one strand formed 2. DNA polymerase is used in DNA replication, which is semi-conservative so both strands are used. Before cell division