1.4 DNA and protein synthesis

Question 1

describe the structure of an nucleotide

Answer 1

• Phosphate group ,5 carbon sugar and a nitrogenous base

Question 2

Name the pentose structure in DNA and RNA

Answer 2

DNA = Deoxyribose
RNA = ribose

Question 3

Describe how a poly nucleotide is formed

Answer 3

• Condensation reaction happens between nucleotides to form strong phosphodiester bonds.

Question 4

What is the difference between a pruine base and nitrogenous base

Answer 4

Purine bases - Have two nitrogen containing rings. In DNA these are Adenine and Guanine
Nitrogenous base - Have one nitrogen containing rings. In DNA these are cytosine and thymine and in RNA this is cytosine and uracil

Question 5

40% of a nucleotide in a sample of DNA contained the base guanine. Calculate the proportion of the other nucleotides

Answer 5

40 % guanine : 40% cytosine
10 % adenine : 10% thymine

Question 6

What are some differences in RNA and DNA

Answer 6

• DNA found in nucleus and RNA found in cytoplasm
• DNA molecules are very long molecules where as RNA are relatively short compared to DNA .
  -DNA has a double helix structure of two polynucleotide chains , where as RNA is just polynucleotide strand

Question 7

Why is DNA replication important?

Answer 7

Because when a cell divides into two, both daughter cells must contain a full set of DNA

Question 8

What phase does DNA replication take place ?

Answer 8

Interphase

Question 9

Explain the role in DNA helicase in DNA replication

Answer 9

• Catalyses the breaking of the hydrogen bonds between base pairs to un ravel double helix

Question 10

Why is DNA replication described as Semi conservative ?

Answer 10

Stands from DNA molecules act as templates , new DNA molecule contains one old DNA strand and one new.

Question 11

Describe the process of DNA replication

Answer 11

• DNA helicase attaches to DNA molecule and catalyses the breaking of hydrogen bonds between bases to unwind strand.

-Free nucleotides line up against their complimentary bases on the DNA strand

-Then DNA polymerase moves down the DNA molecule and catalyses the formation of phosphodiester bonds between the free nucleotides and DNA strand. (Condensation)

-DNA ligase joins up the shorter strands of polynucleotides on the lagging strand of DNA.

Question 12

Why is it important that the activated nucleotides lose two phosphate groups when bonded to the DNA

Answer 12

This provides energy for the reaction (ATP)

Question 13

What are the two different isotopes of nitrogen

Answer 13

Nitrogen 14, light
Nitrogen 15, heavy

Question 14

) Describe how the process of DNA replication produces two identical DNA molecules.
(4)

Answer 14

1 - Hydrogen bonds between nucleotides are broken by DNA helicase
2 -Complementary nucleotides bind to the exposed strands.
3 - Hydrogen bonds will only form between cytosine and guanine and thymine and adenine
4 - Leading strand is synthesised continuously but lagging strand requires DNA ligase to join up okazaki fragments

Question 15

What is a gene ?

Answer 15

A sequence of bases on a DNA molecule coding for a sequence of amino acids on a polypeptide chain

Question 16

What are every three bases called on DNA

Answer 16

A codon

Question 17

What do codons read for ?

Answer 17

A specific amino acid,

Question 18

What does the base sequence of a gene determine ?

Answer 18

The amino acid sequence in a polypeptide chain

Question 19

What is known as a degenerate code ?

Answer 19

When more than one triplet can code for the same amino acid.

Question 20

Describe the structure of tRNA and function of tRNA

Answer 20

Structure:
- Single stranded
- Folded to make a clover leaf shape, shape held in place via hydrogen bonds

Function:
- To attach to one of the 20 amino acids and transfer this amino acid to the ribosome to create polypeptide chain
- Specific amino acids attach to specific tRNA binding sites which is determined via the anti codon on tRNA

Question 21

What is meant by an anti-codon?

Answer 21

Three bases which are found on tRNA which are complimentary to mRNA

Question 22

Why are degenerate codes advantageous?

Answer 22

Some single base mutations may still code for the same amino acid.

Question 23

What are start and stop codons?

Answer 23

Some codes do not code for specific amino acids however they code for the start or stop of the amino acid.

Question 24

What is meant by non-overlapping

Answer 24

Each triplet is read separately in threes.

Question 25

Give three differences between RNA and DNA

Answer 25

DNA contains deoxyribose , RNA contains Ribose DNA is double stranded forming a double helix, RNA is single stranded DNA contains thymine whilst RNA contains uracil

Question 26

Give three differences between the process of DNA replication and Transcription

Answer 26

- DNA replication uses DNA nucleotides, transcription uses RNA nucleotides - Replication uses DNA polymerase, transcription uses RNA polymerase - Transcription copies only the template DNA strand, replication copies both strands.

Question 27

Describe the process of Transcription

Answer 27

1 - DNA helicase binds to non coding region of DNA and breaks the hydrogen bonds between the two strands of DNA 2 - Template strand of DNA is exposed 3 - RNA then moves along the template strand and adds complementary nucleotides one at a time. 4 - RNA polymerase reaches the end of the anti sense strand and it detaches , mRNA then leaves via the nuclear pore and goes to ribosome.

Question 28

Describe the process of translation

Answer 28

- Modified mRNA molecule attaches to ribosome in the cytoplasm - tRNA carries amino acid - tRNA molecule with the complimentary anti codon is attracted to complimentary codons in mRNA and hydrogen bonds form between them - tRNA brings correct amino acid and peptide bonds are created between the amino acids - Peptide bond created via condensation reaction - tRNA is released from ribosome when it reaches stop codon

Question 29

Where does this new polypeptide go to get modified

Answer 29

The golgi apparatus

Question 30

What are gene mutations

Answer 30

- A change in the DNA base sequence at a single location

Question 31

Explain a substitution mutation

Answer 31

- Amino acid in polypeptide is altered. - mRNA transcribed contains a different codon - Codons code for a different amino acid therefore produces a different polypeptide - Polypeptide folds in a different shape therefore functioning protein not made

Question 32

How does sickle cell anemia happen?

Answer 32

One single substituion of an amino acid which changes one amino acid in Haemoglobin

Question 33

What is a deletion mutation?

Answer 33

- A base is deleted from DNA, frame shift occurs - All mRNA codons are different - When polypeptide chain is formed, all amino acids are changed and one is completely missing -Polypeptide is folded in the wrong way and a functioning protein is not made

Question 34

What are insertion mutations?

Answer 34

When a new nucleotide is added into the base sequence.

Question 35

Explain insertion mutations

Answer 35

- Changes the amino acid which would have been coded for the original triplet of bases. - Has knock on effect for other base triplets which causes a frame shift