Nucleic Acids - DNA Structure and Replication

Question 1

What (3) are nucleotides made up of?

Answer 1

1) Nitrogenous base
2) Phosphate group
3) Pentose sugar (Deoxyribose sugar)

Question 2

Which nitrogenous bases are purines?

Answer 2

Adenine and guanine

Question 3

What nitrogenous bases are pyrimidines?

Answer 3

Thymine and cytosine

Question 4

What is the difference between purines and pyrimidines?

Answer 4

Purines have 2 rings whereas pyrimidines have 1 ring

Question 5

How can the structure of DNA be described?

Answer 5

• double stranded
• antiparallel strands
• double helix
• has a major and minor groove
• sugar-phosphate backbone

Question 6

What chemical bonds hold DNA together?

Answer 6

• Covalent phosphodiester bonds between 5’ of one sugar and 3’ of another
• Hydrogen bonds between base pairs

Question 7

How many hydrogen bonds between adenine and thymine?

Answer 7

2 hydrogen bonds

Question 8

How many hydrogen bonds between guanine and cytosine?

Answer 8

3 hydrogen bonds

Question 9

What are the differences between DNA and RNA?

Answer 9

• the sugar in DNA is deoxyribose and in RNA it is ribose
• Bases in DNA are thymine, in RNA it is replaced by uracil
• Double stranded DNA and single stranded RNA
• DNA stores genetic material whereas RNA has many purposes

Question 10

What are the 3 types of RNA?

Answer 10

• Coding
• Structural
• Regulatory

Question 11

What is the role of coding RNA ?

Answer 11

• Messenger mRNA is translated into proteins, some mRNA is polycistronic
• RNA is also how some viruses store their genetic material e.g. HIV

Question 12

What is the role of structural RNA?

Answer 12

• Ribosomal rRNA = make up ribosomes
• Transfer tRNA =carries amino acids to the ribosome in translation

Question 13

what is the role of regulatory RNA

Answer 13

• microRNA = involved in post-transcriptional regulation of gene expression
• Xist (X inactivation specific transcript) = switches off one copy of the X chromosome in XX cells creating a Barr body

Question 14

How is DNA packaged?

Answer 14

• DNA (-ve charge) bound to histone proteins (+ve) to form nucleosomes (10nm thick)
• Nucleosomes coiled to form chromatin fibres (30nm thick)
• chromatin fibres assembled on a protein scaffold to form a chromosome

Question 15

How does semi-conservative replication occur?

Answer 15

1) DNA helicase breaks hydrogen bonds to unzip the helix, single stranded DNA binding proteins keep it unzipped, topoisomerase stops ‘overcoiling’
2) The enzyme primase attaches an RNA primer which acts as a start point to attach nucleotides.
3) DNA polymerase III adds nucleotides in a 5’ to 3’ direction starting from the primer.
4) Pol III has proofreading activity. If any wrong bases are incorporated, DNA polymerase can immediately remove the wrong base using 3-5 exonuclease activity.
5) On the lagging strand, new primers must reattach every time helicase unzips the next bit of DNA, new DNA polymerase adds bases until the next primer, results in lots of separate DNA fragments called okazaki fragments
6) RNA primers are removed and replaced with DNA by DNA polymerase I, Pol I has 5-3 polymerase and exonuclease activity
7) DNA ligase stitches the okazaki fragments together.

Question 16

How are DNA errors detected and repaired?

Answer 16

Pol III misincorporates 1 in 10^5 bases, proofreading activity of polymerase fixes 99%
The other 1% can be detected by mismatch repair which fixes 99%. (The repaired DNA can develop mutations later)

*I in 10^9 mutations obtained during DNA replication remain