DNA Structure Flashcards
DNA Structure - summed up version
what is it a store of?
what is it’s structure? what is it made up of?
how is DNA present in nucleus?
what forms can DNA adopt?
DNA acts as a store for genetic information, this was first shown in studies of pneumococcal transformation and of bacteriophage infection of E. Coli
DNA has a double helix structure, made of complementary polynucleotide chains, minor and major grooves form receptors for drugs
The DNA bases encode genetic information, the central dogma = “DNA makes RNA makes protein”
DNA is present as chromatin in nucleus, it is not “free”
DNA can be damaged by radiation and chemicals, which means DNA has to repair itself.
DNA can adopt a number of different forms, A form, B form and Z form.
DNA A form, B form and Z form
a form - r or L hand? found as? difference in size between major and minor grooves? found as also? shape?
b form - r or l hand? major and minor grooves significance? abundance?
z form - r or L hand? abundance? positioned where? forms where? role?
A form
• right handed helix
• often found in RNA
• difference in size between major and minor groves is
small
• found in tRNA
• it is L shaped, and is a single polynucleotide chain
• has regions that are self complimentary
B form • right handed helix • most common in cells • has major and minor groves which allows proteins and chemicals to read the DNA
Z form • left handed helix • rarest type • usually positioned in front of genes • some proteins can bind to Z DNA • forms when there are repeated sequences: - 5’ GCGCGCGC… - 5’ GTGTGTGT… • has a role in regulating gene expression
General DNA Structure
consists of?
runs from what to what? why?
base pairs? H bonds?
which is stronger?
Consists of a sugar-phosphate backbone
Phosphate bonded to 3C on one side, and 5C on the other
forming 3, 5 phosphodiester linkage
Runs 5’ to 3’
ACGT is the coding element of the DNA
2 antiparallel strands linked by AT and CG pairs
hydrogen bonds between AT and GC
CG bond is stronger as it makes 3H bonds (bond length 1.08nm)
AT bond is weaker as it makes 2 H bonds (bond length 1.11nm)
Complementarity of DNA strands helps in
repair and replication
Unusual DNA Structures
describe holiday junction and role? (4)
describe tetraplex (3)
• Z DNA (described above)
Holliday Junction • made up of 2 chromosomes (2 DNAs) • there is exchange of DNA strands between 2 helices • forming a 4 stranded junction • helpful in DNA repair
Tetraplex
• form at telomeres at the end of chromosomes
• have G rich sequences
• fold and form 4 stranded DNA helix
Levels of DNA structure
what is each level and how can it be analysed?
Primary
• sequence of bases
• can be analyzed by DNA sequencing
Secondary
• helical structures
• can be analyzed by X ray and chemistry
Tertiary
• DNA supercoiling: DNA is twisted on itself
• can be analyzed by electron microscopy
Quaternary
• interlocked chromosomes
Bacterial DNA eg E. coli
structure? (2)
what allows this?
- E. coli DNA is circular and comprises 3 x 106 basepairs
- Is supercoiled: the DNA ribbon is itself twisted in space
- Supercoiling caused by the enzyme DNA gyrase
- Has 50 super coiled domains
Eukaryotic DNA eg Human
how many pairs of chromosomes?
what forms a chromatin? what is this?
how many histones needed for nucleosome? what are these?
what is the interaction and effect of this?
how is nucleosome seen as?
• More base pairs than bacteria
• 23 pairs of chromosomes
• Complexed with histones called chromatins
• Nucleosome forms chromatin
• a nucleosome has a core of 8 histones
• consisting of 2 copies of histones 2A, 2B, 3 and 4
• histones have positively charged tails
- these interact with negatively charged phosphate groups on DNA
- this allows DNA to be wound around histones
• the DNA gets compacted by a factor of 6
• Nucleosome seen as beads on a string
Stability of DNA
explain how DNA can change (4)
what can be hydrolyzed?
example of deanimation
example of anticancer drug and what it does
explain what UV light does and explain what ionising radiation can do
DNA is very unstable
Glycosidic bonds may be hydrolyzed
Base structure can be changed by hydrolysis
this process is called deamination
C can change to U
Chemicals may damage DNA
change base structure: (but also cyclophosphamide)
chemicals may insert between bases (intercalators such as doxorubicin widely used as
anticancer drugs)
Radiation may damage DNA
UV light produces thymine dimers: cross linking of thymine
ionizing radiation (X-ray, gamma ray) break DNA chromosomes to cause leukaemia
DNA damage causes mutations in DNA
3 reasons
Spontaneous- e.g. loss of bases, or hydrolysis of C to U
Chemicals, and radicals generated by oxidative metabolism
-change base structure- (but also cyclophosphamide)
-insert between bases- (intercalators such as doxorubicin
widely used as anticancer drugs)
Radiation
-UV light produces thymine dimers
-Ionising radiation (X-rays, gamma rays) break DNA
chromosomes to cause leukaemia
DNA repair
importance?
what is xeroderma pigmentosum?
what can anticancer drugs do?
Maintains genome stability
More than 100 different enzymes/proteins involved- large investment for the cell
Importance?
Patients with xeroderma pigmentosum- have defect in excision repair that deals with uv damage to DNA. Very prone to skin cancer.
Other cancer prone families have DNA repair defects.
Understanding DNA damage/repair is very important.
Anticancer drugs damage DNA to a point of no repair
intercalators mimic base pairs thus preventing DNA replication
