DNA structure Flashcards
DNA structure
Made up of nucleotides
Has different bases
- Purines - has two carbon rings - Adenine and Guanine
- Pyrimidines - has one carbon ring - Cytosine and Thymine
- Pyrimidine - Uracil RNA
Phosphates join onto 5’ carbon
Bases join to 1’ carbon
DNA tends to be a triphosphate
Nucleotide chains - deoxyribonucleic acid
DNA is chain of nucleotides
Joined by covalent bond
Always 5’ to 3’ direction of adding
Makes sugar phosphate backbone
5’ beginning of DNA strand and 3’ is end
Experiment - Erwin Chargaff 1951
Inspired by Avery paper on DNA transforming bacter
Used paper chromatography to find proportions of bases in genomes in numerous species
Conclusions that can be drawn:
- Last 3 columns all roughly equal to 1 so should all be equal and proportional, A=T, C=G and Purines = Pyrimidine
Chargaff’s rules
A% =T% and G%=C%
But AT% not=to GC%
Composition varies between species with some being more AT rich
X-ray crystallography structure of DNA
Maurice Wilkins and Rosalind Franklin - 1952
X pattern means its helix pattern
Is regular pattern, repeating even structure
Distance between spots = distance of one turn = 3.5 nm
James Watson and Francis Crick’s model
Had:
- Structure of nucleotide
- Ratios different nucleotides in DNA
- Crystal structure
Used everyone else’s data to get their idea
Main features of model:
- A-T and G-C hydrogen bonded base pairs
- Anti-parallel strands
- Right-handed double helix
- One helical turn
H-bonding between purine and pyrimidine
2 H bonds between A and T
3 H bonds between C and G
H bond - attraction between delta negative O and delta positive H
1 purine, 1 pyrimidine maintains width of DNA
Mispairing between two purine/two pyrimidines could cause distortion in DNA molecules
Py and Py - too skinny
Pu and Pu - too thick
Key points
Nucleotide = phosphate + deoxyribose + base
5’ - 3’ polarity
2 H bonds =A and T
3 H bonds = G and C
Purines - 2 carbon rings - A and G
Pyrimidines - 1 carbon ring - T and C
Antiparallel, complementary DNA strands
G complementary to C
A complementary to T
Anti parallel, one side 5’ to 3’ other 3’ to 5’
Double helical structure
Right-handed helices
Each phosphate backbone forms a helix so double
Major and minor groove in DNA double Helix
One turn every 10.5 bp, every 3.5 nm
Bases not straight in middle of helix
Major groove - larger indent
Minor groove - smaller indent
Different groves exist so that backbone stays straight
Are major and minor groves at every turn
Structural elements of chromosomes and plasmids - Eukaryotic chromosomes
Linear molecule, held in nucleus of cell
Often many chromosomes in genome
Human genome - 22 pairs of chromosomes + 2 sex chromosomes, range from 5x10^7 to 2.5x10^9 bp
Centromeres help chromosomes segregate at mitosis and meiosis
Centromere - specialised chromosomal region where structure link centromeres to spindle microtubules assemble, direct equal segregation of chromosomes during mitosis and meiosis
Doesn’t have to be in centre of chromosomes
Variety DNA sequences throughout eukaryotes but conserved histones
Origins of replication
Single origin of replication in E coli genome vs ten thousands origins replication in human genome
Bidirectional replication forks
Telomeres protect ends of linear chromosomes
telomere = repetitive DNA at ends of linear chromosomes
Prokaryotic genomes
Bacteria generally have single, circular chromosome
Size - typically few million bp
Plasmids also often found, circular molecule
Size - few thousand bp
Carry variety advantageous genes such as antibiotic resistance cassettes
Passed between cells by conjugation
DNA-binding proteins
DNA-binding domains in protein
- General affinity for DNA
- Or sequence-specific
- Can prefer single-stranded or double-stranded DNA
Roles DNA-binding proteins
- Regulate gene expression
- Cut DNA at specific sequences
- Protect DNA
Example 1 - transcriptional regulators
Proteins bind regulatory sequences near promoters of genes to either stimulate or block transcription
Bend DNA into favourable or unfavourable shape
EG lac operon in E coli - enzyme for breaking down lactose sugar:
- Lac repressor binds DNA, blocks transcription when lactose absent
- Catabolic activator protein binds to DNA and increases transcription when glucose absent
Example 2 - Restriction endonucleases
Enzymes cut DNA at specific sequences 6-10 bp normally palindromic
Originated bacteria to restrict action of viruses
Example 3 - histones
Eukaryotic genome packaged into chromatin
DNA wrapped around proteins - histones
Not sequence specific
