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FLC > DNA Structure > Flashcards

DNA Structure Flashcards

1
Q

Describe the structural features of DNA

A
  • Double helix structure
  • Complementary polynucleotide chains
  • Major and minor grooves act as receptors for drugs
  • DNA has polarity
  • Runs in 5’-3’ direction
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2
Q

Why is DNA more stable than RNA?

A

Each phosphodiester has a negative charge
- repels OH- group hydrolytically attacking the phosphate
backbone
- DNA doesn’t have OH on 2nd C so not hydrolysed easily

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3
Q

How is DNA stored in the nucleus?

A

As chromatin

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4
Q

Describe the 4 levels of DNA structure and how they can be viewed in a lab

A

Primary structure
- Sequence of bases
(found by DNA sequencing)

Secondary structure
- helical structure in forms A.B, and Z
(found by Xray and chemistry)

Tertiary structure
- DNA supercoiling
(viewed by electron microscopy)

Quartenary structure
- interlocked chromosomes

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5
Q

Describe the A form structure of DNA

A

Tightly coiled
Right hand helix
e.g. tRNA

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6
Q

Describe the structure of B form DNA

A

Most common form
Major and minor grooves present
Right hand helix

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7
Q

How are right hand helices formed?

A

Two anti parallel polynucleotide chains are held by hydrogen bonds. The strands are complementary in sequence

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8
Q

Describe the Z form DNA structure

A

Left hand helix

formed by alternating purine and pyrimidine bases

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9
Q

Is DNA structure stationary?

A

No, DNA can interconvert between forms

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10
Q

What are Pyrimidine bases?

A

Single ring structures

e.g. Cytosine, Thymine and Uracil

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11
Q

What is meant by Purine bases?

A

Double ring structures

e.g. Guanine and Adenine

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12
Q

How are complementary base pairs formed?

A

2 Hydrogen bonds between A and T

3 Hydrogen bonds between C and G

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13
Q

How is the DNA backbone linked?

A

Repeating sugar phosphate units are linked by 3’ - 5’

phosphodiester bonds

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14
Q

How are the DNA base pairs stored in chromosomes?

A

Humans have 3 billion base pairs organised into 23 pairs

of linear chromosomes

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15
Q

How is DNA so compact?

A

DNA is complexed with histones and made into chromatin

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16
Q

What is a nucleosome?

A

A length of DNA coiled around a core of histones - the

building blocks of chromatin

17
Q

Describe the structure of DNA histone complexes formed

A

DNA is wrapped around 2 molecules of each 2A, 2B, 3 and 4 histone subunits
Histone 1 binds to the outside and to linker DNA

18
Q

How is a holiday junction formed?

A

4 stranded junction formed when DNA from 2 different

chromosomes are brought together

19
Q

What is tetraplex DNA?

A

Formed at telomeres by folding DNA back in itself

20
Q

What is the benefit of tetraplex DNA?

A

Protects ends of chromosomes joining to another

21
Q

Describe the structure of a tRNA molecule

A

tRNA folds into an A form helix made of ribonucleotides

22
Q

List the features of bacterial DNA

A
  • circular
  • supercoiled DNA
  • organised into 50 independently supercoiled domains
23
Q

What is the benefit of supercoiled DNA?

A

Supercoiling stores energy

24
Q

What enzyme is used to cause supercoiling?

A

Gyrase using ATP

25
Q

What does DNA damage cause?

A

mutations

26
Q

How is DNA damaged?

A

~ Spontaneously
e.g. base loss, hydrolysis of bases

~ Chemically
e.g. change of base structure (chemotherapy)
       base insertion (intercalator)

~ Radiation
e.g. UV light - produces T dimers
Ionisation (Xrays/Gamma) - break chromosomes

27
Q

Why is DNA chemically unstable?

A

DNA is joined together by sugars which can be easily

hydrolysed

28
Q

What is the benefit of DNA repair?

A

Maintains genome stability

29
Q

Explain why Xeroderma pigmentosum patients are prone to skin cancer

A

Defect in excision repair dealing with UV DNA damage

FLC (34 decks)

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