S1: DNA Structure Flashcards
What is DNA packaged as in the nucleus?
DNA is present as chromatin (DNA packaged with histone proteins). It is not ‘free’.
What do the minor and major grooves do?
They form receptors for drugs
What are the 3 forms of DNA?
A form
B form
Z form
Describe A-form DNA
A form DNA is more tightly coiled
- tRNA takes this form
- Right handed helices
Describe B-form DNA
- Most common form (sugar phosphate on outside, bases in middle)
- Forms most of the DNA in eukaryotic cells
- Right handed helices
- Has major group (large group of bases exposed)
- Has minor group (small number of bases exposed)
Describe Z-form DNA
- Left handed helix
- Formed when there is alternating purine-pyrimidine bases
- Present in small amounts in cells
Can DNA interconvert between forms?
Yes
Describe DNA structure on a molecular level
- DNA structure consists of repeating sugar-phosphate units linked by 3’ -5’-phosphodiester bonds.
- DNA has a polarity running from 5’ to 3’
- DNA helix is held together by AT and CG base pairs
- Hydrogen bonds between polynucleotide chains
- One strand is complementary in sequence to the other
How many hydrogen bonds are between cytosine and guanine base pairs?
3 hydrogen bonds
bond length 1.8nm
How many hydrogen bonds are between adenine and thymine base pairs?
2 hydrogen bonds
bond length 1.11nm
Which bases are purine and which are pyrimidine?
Purine: Adenine and Guanine
Pyrimidine: Cytosine and Thymine
How many rings do purine and pyrimidine bases have?
Purine: Double rings
Pyrimidine: Single rings
What forms a right handed helix?
2 antiparallel polynucleotide chains forms a RH helix
What form does tRNA fold into?
- A form helix
What is tRNA made from?
Ribonucleotides
What DNA pattern forms left handed DNA?
Left handed or Z DNA is formed by alternating patterns
5’….GCGCGCGCGCGC
5’….GTGTGTGTGTGTG
What is a ‘Holliday Junction’?
- 4 stranded junction
- Formed when you bring DNA from two different chromosomes together
- Important in repairing damage
What is tetraplex DNA?
- Formed at the telomeres (caps at end of DNA which protects the chromosomes)
- Formed by the DNA folding back on itself
- Found at the end of chromosomes
Levels of DNA structure
Primary- Sequence of bases
Secondary- Helical structure (e.g. ABZ)
Tertiary- DNA supercoiling
Quaternary- Interlocked chromosomes
What is primary sequence of DNA found by?
DNA sequencing
What is secondary sequence of DNA found by?
X-ray and chemistry
What is tertiary sequence of DNA found by?
Electron microscopy
Give an example of quaternary structure
When bacteria replicate and chromosomes interlock initially
Describe bacterial DNA
- Circular DNA
- Supercoiled (DNA ribbon itself is twisted in space)
It is supercoiled in bacteria to replicate, supercoil stores energy
What enzyme causes supercoiling?
Enzyme DNA gyrase
which uses ATP
What is chromatin?
DNA is complexed with histones and made into a fibre
Multiple nucleosomes are coiled together, which stack on top of each other.
What is a nucleosome?
Basic building block of chromatin
It is the combined loop of DNA around the histones
What do nucleosomes look like under electron microscopy?
‘beads on a string’
How many histone subunits does DNA double helix wind around?
8 histone subunits (2 molecules each of 2A, 2B, 3 and 4)
What does histone 1 do?
Binds to the outside and linker DNA
What does chromatin condense into?
30nm chromatin
This is then looped and coiled more, using other proteins to form the chromosomes.
What causes mutations in DNA?
DNA damage causes mutations in DNA
- Spontaneous e.g. loss of bases or hydrolysis of C to U
- Chemicals e.g. change of base structure or insert between bases
- Ionising Radiation (X-rays, gamma rays) break DNA chromosomes to cause e.g. leukemia
Why is DNA chemically unstable?
It is joined by sugars whose bonds can be hydrolysed
What is the importance of DNA repair?
- DNA repair maintains genome stability
