Necleotides And Nucleic Acids

Question 1

purines

Answer 1

Two carbon ringed bases

Adenine

Guanine

Question 2

Pyrimidines

Answer 2

Single carbon ring bases

Thymine

Cytosine

Uracil (in RNA)

Question 3

Describe formation of DNA (6)

Answer 3

Nucleotides linked together by condensation

Double helix; antiparallel strands

Complementary base Pairs held by weak hydrogen bonds

Phosphodiester bonds between phosphate and pentose

5’ at one end, 3’ at the other

5’ to 3’ leading
3’ to 5 lagging

Question 4

How is DNA structure suites for its role (3)

Answer 4

Polymer; a lot of information stored

Bass sequence; allows coding for information

Double stranded; accurate replication and makes the molecule stable

Question 5

State the three theorised methods of DNA replication

Answer 5

Conservative

Semi-conservative

Dispersive

Question 6

Semi-conservative replication, explain why it is the most efficient form of replication?

Answer 6

Original stand serves as template for a new complementary strand

50% old strand, 50% new

• Allows few errors to be made as original DNA acts as a template.
• Errors can be more easily corrected as original strand is still present
• New strand would see errors more likely to be ignored.

Question 7

Name the enzymes involved in replication

Answer 7

DNA Helicase

DNA polymerase

DNA ligase

DNA Primase

Question 8

DNA Helicase

Answer 8

Separates DNA strands by cutting through HBonda

Uses energy from ATP hydrolysis

Question 9

DNA Polymerase

Answer 9

Catalyses the formation of phosphodiester bonds between nucleotides

Therefore causing deoxyribonucleotides to form DNA strands

Can only move from 5’ to 3’

Question 10

DNA ligase

Answer 10

Links two DNA strands with double strand break

Question 11

Structure of nucleotide

Answer 11

Phosphate links at 5th C on sugar forms- also links with OH group at 3rd C

Pentose sugar( ribose or deoxyribose )

Base; purine or pyrimidines

Question 12

Okazaki fragments

Answer 12

Short piece of DNA created on lagging strand

Question 13

Difference between continuous and discontinuous replication

Answer 13

Continuous;
Moves from 5’ to 3’
DNA polymerase binds to the end
Free nucleotides are added with no breaks

Discontinuous;
3’ to 5’ so DNA cannot bind to end
Free DNA added in sections
Forms Okazaki fragments which are sealed by DNA ligase

Question 14

Why does DNA not catalyse the joining of Okazaki fragments

Answer 14

Enzymes are substrate specific

Nucleotides have a different shape on Okazako fragments compared to normal nucleotides

Question 15

How many different types of codons are possible

Answer 15

64

Question 16

What is the start codon

Answer 16

AUG

Question 17

What are the 3 termination codons

Answer 17

UAA

UAG

UGA

Question 18

Triplet(codon)

Answer 18

Three consecutive nucleotides

Question 19

Gene

Answer 19

Chain of codons with a start and stop.

Question 20

Transcription unit

Answer 20

Genes collectively coding for an end product

Question 21

Briefly describe DNA replication

Answer 21

• DNA unzips via DNA Helicase

New DNA strands formed by free nucleotides forming complementary bonds

DNA polymerase adds nucleotides from 5’ to 3’ on leading strand

3’ to 5’ on lagging strand via Okazaki fragments

Stands rewinds together

Question 22

Antisense

Answer 22

3’ to 5’ strand during transcription

Codes for mRNA therefore complementary to
mRNA and Sense strand

Contains anti-codons which code for the same as antisense

Question 23

Sense strand

Answer 23

5’ to 3’ strand during transcription

Codons code for same amino acids as mRNA

Complementary to antisense

Codes for protein but not used in proteins synthesis

Question 24

Anticodon

Answer 24

Attaches to tRNA

Complementary to mRNA codons

Question 25

Ribosome in Translation

Answer 25

Large and small subunits

Almost same amount of protein

Forms rRNA

Small subunits bind to mRNA during translation

Question 26

rRNA

Answer 26

Maintains structural stability of protein synthesis sequence

Question 27

tRNA

Answer 27

Folded to allow attachment of anti codon

Carries amino acid according to anticodon

Question 28

What structure does the sequence of amino acids form

Answer 28

Primary

Question 29

Describe transcription (6)

Answer 29

1. ) DNA unwinds and unzips using DNA helicase to break hydrogen bonds between base pairs.
2. ) Strands separate into sense (5’ to 3’) and antisense (3’ to 5’) strands.
3. ) Antisense strand acts as a template for the free complementary RNA nucleotides to bind to.
4. ) RNA Polymerase form phosphodiester bonds between RNA nucleotides until the gene ends to form mRNA
5. ) mRNA detaches from DNA. DNA rewinds into helical structure.
6. ) mRNA leaves the nucleus via nuclear pore, into the cytoplasm.

Question 30

Describe translation

Answer 30

1. ) mRNA binds to the smaller subunit in the ribosome at start codon (AUG)
2. ) Complementary tRNAs bring amino acids according to the anticodons
3. ) Maximum of 2 tRNAs can be bound at the same time
4. ) Chain of amino acids form a polypeptide which is primary structure of protein
5. ) Polypeptide chain stops at ‘STOP’ codons; UGA, UAA, UAG.
6. ) Polypeptide chain folds to form secondary and tertiary structures.

Question 31

How is the structure of DNA suited for replication (5) and

Answer 31

• Double stranded with each strand acting as a template
• Complementary base pairs
• H bonds between base pairs can break easily
• purine only bond to pyrimidines
• different hydrogen bonds between AT and GC