DNA Structure And Replication Flashcards
Describe what is meant by the central dogma.
- flow of genetic information from DNA —> RNA —> protein
Describe Chargaff’s rules
- amount of each dNTP varies between organisms, but dA=dT and dC=dG in ALL organisms.
How was the function of DNA discovered?
- by 1940s, hereditary material known to reside on one or more chromosomes.
- so seemed logical that protein is genetic material because of diversity needed, but found to be DNA
How was the structure of DNA discovered?
Rosalind Franklin and Maurice Wilkins used X-ray diffraction suggested helix of two strands with a uniform width that stacks ashes with sugar-phosphate on the outside.
If DNA contains more than one chain of nucleotides, what forces hold them together?
- hydrogen bons forming between a purine on one strand, and pyrimidine on the other.
Characterisitics of the double strand of DNA?
- anti-parallel
- complimentary
What does 5’ and 3’ mean
- 5’: free phosphate group on 5’ carbon
- 3’: free hydroxyl group on 3’ carbon
Formula for number of possibilités of nucleotides
If a chain has n nucleotides, 4^n
Humans have 4^250000000 possibilities.
Describe Watson and Crick’s model of DNA double helix
- played great deal of importance on complimentary bases
- suggested a copying mechanism
Models of DNA replication
Dispersive
Conservative
Semiconservative
Where does DNA replication start?
At the ori (specific sequence of bases in the DNA. (Also called the origination of replication)
How does DNA replication start?
- strands separate t the original, and synthesis of new DNA will occur from both parent strands in BOTH directions away from the ori.
Describe the DNA replication process
- helicase unwinds double helix, and strands are kept separated by Single-stranded DNA Binding Protein.
- incoming dNPT hybridized to parental template, forming phosphodiester bond with 3’ end of chain (Bi directional) by DNA polymerase.
What does it mean if DNA replication is bidirectional?
- new DNA needs to be synthesized on both strands on both sides of the ori
- synthesis only occurs in the 5’ to 3’ direction, new strands have to be antiparallel to the template.
Role of topoisomerase
- prevent DNA from getting tangled and relieves pressure in supercooled DNA during DNA replication.
What is the name of the strand where DNA synthesis begins and proceeds normally?
- leading strand
What is the name of the strand where DNA synthesis starts on the other side of the ori strats a short distance away from the ori and works back toward the ori? What does this form?
- lagging strand
- forms Okazaki fragments.
Why is DNA replication called semi-discontinuous?
-because one strand is lagging (formed in Okazaki fragments)
- and the other strand is formed continuously.
What do primers do?
- they provide the 3’ OH group needed for polymerase to build on the existing nucleotides.
Why aren’t primers made of just 1 base pair?
- because its too unstable, so primers are short strands of RNA opposite an as DNA template.
How do primases know when to stop?
- at the end of each Okazaki fragments in a lagging strand.
What is a nick?
Which enzyme fixes this?
- there are no covalent linkages between nucleotides.
- DNA polymerase 1
Which enzyme is the ‘after’ enzyme to fix nicks?
- Ligase
What does X and Y mean it terms of X dépendant and Y synthesizing enzyme
- X: what its using as a template
- Y: What it is making
What is the name of he family of enzymes that hydrolyse a phosphodiester bond? What is it called if it acts within or at the end of the molecule?
- nuclease
- exonuclease (End of the molecule)
- endonuclease (within the molecule)
Role of telomerase
Extends unreplicated end by binding to the 3’ end of the overhanging strand of parental DNA and extends the strand using its own RNA template.
How are millions or billions of base pairs of DNA in such a small space in prokaryotes
DNA is supercoiled
- done by topoisomerase which nick DNA, wind, unwind then reseal DNA.
How do we get 2 meters of DNA into a nucleus that is 5–8
μm in diameter? In eukaryotes?
Chromatin, and wrapping around histone proteins
Histones
- small, basic proteins (basic amino acids)
- sequence is highly conserved among species
- bacteria dont have histones, but they have histones like proteins.
- 5 major types of histones: H1, H2A, H2B, H3, H4
What does the core nucleosome consist of?
- 2 of each histones: H2A, H2B, H3, H4 (8 proteins)
- 146/147 base pairs of DNA
- packing of DNA and histones into nucleosomes yields chromatin fiber of approx 10 nm in diameter.
What types of chromatin exist in interphase?
- hetrochromatin: tightly condensed
- Euchromatin: loosely condensed
