DNA replication Flashcards
Dawkins definition of a gene?
any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection
Define gene
a unit of information on a chromosome.
Molecular definitions of genes?
- the DNA sequence that is translated into a protein
- the ORF and the control region of the gene including its promoter
DNA is a…
…polymer
What is meant by DNA is a polymer?
meaning it consists of repeated subunits – molecules – in this case nucleotides
the DNA polymer chain is formed by …
the sugar part of one nucleotide being covalently bonded to the phosphate group of the next nucleotide (phosphodiester bond) – and so on…
The strands are not…
…covalently joined together
Nucleotides are added on to a…
…growing chain of DNA by the release of pyrophosphate (PPi) after the addition of a nucleotide
the remaining phosphate group then forms a …
…phosphodiester bond
adjacent nucleotides on a DNA strand are joined by …
…covalent bonds on the sugar phosphate backbone
DNA is a polymer of nucleotides consisting of …
…adenine (A), cytosine (C), guanine (G) and thymine (T)
DNA molecules are…
…double-stranded
The two strands of DNA pair together with…
…complementary sequences forming the pairs
in double stranded DNA A always pairs with …
… T
C always pairs with…
…G
the two strands are paired because of …
…hydrogen bonds (H bonds)
in a molecule of water an oxygen (O) atom shares …
…electrons with 2 hydrogen (H) atoms
oxygen atoms pull electrons towards their …
…nucleus more strongly than hydrogen atoms - more electronegative
The water molecule has a…
…charge difference across the molecule
because the atoms within water molecules have a net charge difference adjacent molecules attract …
…one another and hydrogen bonds form between the H atom of one molecule and the O atom of an adjacent molecule - intermolecular forces
Atoms within water have a…
…net charge difference.
Covalent bonds are…
…strong
Covalent bonds invovle…
…the sharing of electrons.
the bonds between complementary bases of two DNA strands forming the double helix are like those between …
…water molecules
Hydrogen bonds are…
…weak bonds.
DNA molecules can be…
…separated.
the strands can be easily separated because …
…hydrogen bonds are weak - and then be made to reanneal
The seperation and reanneal of hydrogen is important in…
…gene expression (it’s how genes are transcribed)
How are dna molecules separated?
Heating
the double helix can be ‘melted’ forming …
…2 single stranded DNA polymers
Dna strands are…
…antiparallel
each strand of the double helix actually is …
… antiparallel to its complementary strand
DNA is polymerised only in…
…one direction (5’ to 3’)
5’ to 3’ refers to…
…the carbon atoms on the deoxyribose sugar that joins with phosphate groups
Unwinding of the double helix makes…
…two single strands
polymerisation depends upon the presence of a…
… template strand
polymerisation requires…
…dNTPs, a polymerase, Mg2+ and a free 3’ OH end
through each round of cell division each cell must inherit …
…the genetic material of its parent cell – in order to accomplish this the genetic material has to be replicated.
in order for any cell to reproduce it first has to …
…copy its genetic material
the genetic material contains the …
…blueprint for making all the proteins a cell needs - without it the daughter cells cannot function
DNA replicates by …
…copying itself
each strand of the double helix contains the …
…information to make a duplicate strand because DNA molecules pair by complementary base pairing
What are the 3 types of dna replication?
Conservative, Semi conservative, Dispersive.
replication forks can be seen in …
…electron micrographs of chromosomes replicating
in eukaryotes there are multiple …
…replication forks occurring at many paces on the chromosome
in prokaryotes there tends to be…
…one origin of replication only
What are the proteins involved in DNA replication?
DNA is polymerised by a complex of proteins at the replication fork:
1 - DNA polymerase (catalyses the addition of bases to the 3’ end)
2 - Topoisomerase (unwinds the helix to prevent torsion)
3 - Helicase (melts the double stranded molecule)
4 - DNA primase (makes short RNA primers [gives a free 3’ end])
5 - Single stranded binding proteins (prevents premature annealing)
6 - DNA ligase (joins Okazaki fragments on the lagging strand)
Describe the process of DNA replication
1) DNA strands run anti-parallel to one another
2) The enzyme Helicase unwinds the DNA double helix by melting the molecule.
3) DNA polymerase attaches to a ‘leading’ strand
4) DNA polymerase synthesises a new DNA strand in the 5’ to 3’ direction
5) Another DNA polymerase molecule attaches to the ‘lagging’ strand
6) DNA is synthesised in the 5’ to 3’ direction
7) Helicase continues to unwind the DNA double helix
8) Continuous synthesis proceeds on the ‘leading’ strand
9) DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down
10) Synthesis of DNA continues on the ‘lagging strand’
11) DNA polymerase is unable to join up the fragments on the lagging strand
12) This is done by the enzyme Ligase which joins Okazaki fragments on the lagging strand.
replication forks are …
…asymmetrical
DNA polymerase needs …
…small ‘primers’ to start it off – it needs a free 3’ end
Define primer
short sequences of RNA (made by RNA polymerase)
in one direction polymerisation …
…keeps on going but on the so-called ‘lagging strand’ it has to keep being restarted
Direction of synthesis?
Direction of synthesis is 5’ to 3’
Rate of synthesis?
rate ~ 800 dNTPs/second
Error rate of synthesis of dna?
low error rate
DNA polymerase “finds” the …
…correct complementary dNTP at each step in the lengthening process.
DNA polymerase I catalyzes …
…formation of phosphodiester bond between 3’-OH of the deoxyribose (on the last nucleotide) and the 5’-phosphate of the dNTP.
energy required for these reactions is derived from…
…the release of two of the three phosphates of the dNTP.
polymerases have 5’ to 3’…
…exonuclease activity
Purpose of exonuclease activity?
- remove RNA primers
- acts as a proofreading step
primers are removed at the same time that …
…dNTPs are added to the growing chain
Gaps during dna synthesis is closed by…
…DNA ligase
when a mismatch occurs …
…it can be removed by the exonuclease domain of DNA polymerase
Define a mismatch
the wrong dNTP added
