7.1 DNA structure and replication Flashcards
structure of a nucleosome
DNA complexed with 8 histone proteins
how nucleosomes help supercoil DNA (DNA –> chromosome pipeline) 5
- nucleosomes (DNA wound around 8 histone proteins) are linked by additional histone protein to form string of CHROMATOSOMES
- string of chromatosomes coil to form a SOLENOID structure
- solenoid structure is condensed to form a 30 NM FIBRE
- fibres form loops, compressed and folded around protein scaffold to form CHROMATIN
- chromatin supercoils during cell dev to form CHROMOSOMES
Where does action of helicase occur
at specific regions (origins of replication), creating a REPLICATION FORK of 2 strands running in antiparallel directions
(helicase: unwinds and seps DNA by breaking H bonds betw base pairs)
what does DNA gyrase do
- reduces torsional strain created by unwinding of DNA by helicase
- relaxes pos supercoils (via neg supercoiling)
what do single stranded binding (SSB) proteins do
- SSB proteins bind to DNA strands after seperation, prevent them from re-annealing
- prevent single stranded DNA from being digested by nucleases
- will be dislodged from strand when a new complementary strand is synthesized by DNA polymerase iii
what does DNA primase do
- generates a short RNA primer on each of the template strands
- the RNA primer provides initiation point for DNA polymerase iii (can extend chain but not start)
how does DNA polymerase iii work
- free nucleotides align w complementary bases
- DNA pol iii attaches to 3’ END of primer
- covalently joins free nucleotides tgt in a 5’ –> 3’ DIRECTION
- moves in opp directions on 2 strands as they r antiparallel
DNA polymerase iii differences on leading and lagging strand
leading
- moves towards replication fork, synthesises continuously
lagging
- moves away from replication fork, synthesises in pieces (okazaki fragments)
what does DNA polymerase 1 do
- lagging strand is synthesised in a series of short fragments = has multiple RNA primers along its length
- DNA pol 1 removes the RNA primers from lagging strand and replaces them w DNA nucleotides
what does DNA ligase do
- joins the okazaki fragments tgt to form a continuous strand
- by covalently joining the sugar-phosphate backbones tgt w a phosphodiester bond
can DNA polymerase initiate replication? why
NO
- it can only add new nucleotides to an existing strand
- RNA primer must first be synthesised to provide an attachment point for DNA polymerase
DNA polymerase adds nucleotide to – end of the primer, extending the new chain in a —- direction
3’ end of primer
replication in a 5’ –> 3’ direction
free nucleotides exist as
deoxynucleoSIDE triphosphates (dNTPs) – have 3 phosphate grps
what cleaves the 2 additional phosphates from free nucleotides
DNA polymerase
uses energy released to form phosphodiester bond with 3’ end
how are okazaki fragments joined
on lagging strand
- fragments are preceded by a primer
- primers are replaced by DNA bases
- fragments joined tgt by combination of DNA pol 1 and DNA ligase
4 examples of non coding DNA
- regulators of gene expression
- introns
- telomeres
- genes for tRNA
non coding dna – regulators of gene expression what are they + 3 examples
DNA sequences that regulate gene expression
- promoters: sequences that occur just before genes, act as a binding point for RNA polymerase enzymes that catalyse transcription
- enhancers: DNA sequences that act as binding sites for protein – INCR rate of transcription
- silencers: DNA sequences that act as binding sites for proteins – DECR rate of transcription
what are introns
DNA base sequences found in eukaryotic genes – get removed at the end of transcriptions
(do not contribtue to the AA sequence transcribed)
what are telomeres
repetitive sequences that protect the ends of the chromosome
- ensure DNA is replicated correctly (every cell div DNA is lost from telomeres)
- protects against chromosomal deterioration during replication
what protects against chromosomal deterioration during replication
telomeres (repetitive sequences at ends of chromosome)
non coding region: genes for tRNA
code for RNA molc – do not get translated into proteins
- fold to form tRNA molc that play impt role in translation
how was X-ray diffraction carried out? (rosalind franklin and maurice wilkin)
- DNA purified, fibres stretched in thin glass tube (make strands parallel)
- DNA targeted by X ray beam – diffracted when contacted an atom
- scattering pattern of X ray recorded and used to investigate molecular structure
inferences from rosalind franklin and maurice wilkin’s investigation of DNA structure by X-ray diffraction 3
- composition
DNA is double stranded - orientation
nitrogenous bases are closely packed tgt inside, phospphate backbone outside - shape
DNA twists at regular intervals (34 angstrom) to form helix (2 strands = double)
what is DNA sequencing
process by which base order of a nucleotide sequence is elucidated
