DNA Structure

Question 1

Explain a few features of DNA

Answer 1

• a store of genetic information
  -a double helix which contains complementary polynucleotide chains and grooves (minor and major) for drugs
  -DNA is present as chromatin
  -can be damaged by light, chemicals and radiations
  *these can be repaired by DNA repair proteins to
  ensure that genome is maintained as stable and
  accurate
  -a double helix made up of two anti-parallel strands

Question 2

How are the bases bonded and give a few details about them

Answer 2

Guanine has three hydrogen bonds when bonded to cytosine. The bond length is 1.8 nm
Adenine has two hydrogen bonds when bonded to thymine. The bond length is 1.1 nm.
Guanine and Adenine and both purines
Cytosine and Thymine are both pyrimidines.

Question 3

What are the different forms of DNA?

Answer 3

• A-form= when DNA is tightly coiled (e.g. tRNA) and is a right handed helices
• B-form= when DNA is not as tightly coiled but not too loosely coiled; it is the most common form of DNA in eukaryotic cells and contains both major and minor groups
• Z-forms= left handed helices, loosely coiled and has alternating purine a pyrimidine bases; it is present in small amounts.

Question 4

Explain what the term ‘holliday junction’ means

Answer 4

The holliday junction is a four stranded junction that is formed when DNA from two different chromosomes are brought together.
It is important for repairing damage and is useful for homologous recombination during meiosis.

Question 5

How are repeating sugar units in DNA interlinked and is DNA polar?

Answer 5

DNA structure consists of repeating sugar units that are interlinked by 3’,5’ phosphodiester bonds.
The polarity runs from the 5’ end to the 3’ end.

Question 6

What are the different levels to DNA structure?

Answer 6

• Primary- sequence of bases (found by DNA sequencing)
• Secondary- helical structure (A,B&Z) (X-ray + Chemistry)
• Tertiary- DNA supercoiling (electron microscopy)
• Quaternary- interlocked chromosomes (e.g. when bacteria replicate and chromosomes are interlocked initially).

Question 7

How is DNA presented in bacteria?

Answer 7

• found as circular
• it is supercoiled, the DNA ribbon itself is twisted in space by the enzyme DNA gyrase (using ATP) –> needs supercoiling to allow for replication and storing energy
• E-coli chromosomes are organised in 50 independently supercoiled domains.

Question 8

How is DNA presented in a human/eukaryotic cell?

Answer 8

• DNA is complexed with histones and made into a fibre called chromatin
• The double helix is wound around 8 histone subunits (2A,2B,3,4). Histone I binds to the outside and he linker DNA. The combined loop of DNA around the histones are called nucleosomes.
• Multiple nucleosomes are coiled together, which then stack together on top of each other resulting in a fibre of nucleosomes (chromatin)- condensed to a 30 nm chromatin fibre.
• This is then looped and coiled more, using other proteins, forming chromosomes.

Question 9

What are the different types of damage that can occur to DNA?

Answer 9

• spontaneous - loss of bases
• chemical - change of base structure and insertion of new ones
• radiation - UV light produces dimers, ionising radiation which breaks DNA chromosomes.
• DNA is chemically unstable as it’s suars can be easily hydrolysed.