2.7 DNA replication, transcription and translation Flashcards
What are the 3 proposed models for how DNA replicates?
- Conservative
- Semi-conservative
- Dispersive
What happens in the conservative replication model?
The parental strands are kept together and the two newly synthesize daughter strands are kept together
What happens in the semi-conservative model of DNA replication?
Two daughter DNA molecules produced from the parent DNA molecule will each have one parental strand and one newly synthesized daughter strand.
What happens in the dispersive replication model?
Strands of the daughter DNA molecule contain pieces of parental and newly synthesize DNA. The original molecule breaks up into nucleotides, each one joins to a complementary nucleotide and new ones join up again.
What is the Meselson and Stahl experiment?
How did they culture 15N DNA?
What were they transferred in?
How did they measure?
What would happen?
They culture of the bacterium E. coli for 14 generations and a medium where the only nitrogen source was 15N. Therefore almost all the nitrogen atoms in the bases of the DNA in the bacteria were 15N.
- Then the bacteria was transferred to a medium in which all the nitrogen was 14N. They then collect the samples of DNA from the bacterial culture from time to time. They extracted the DNA and measured its density using density gradient centrifugation. The DNA solutions were spun at high speeds in a centrifuge. DNA molecules composed of heavy 15N would sink further in the solution than DNA molecule made of 14N.
What were the results of Meselson and Stahl.
Which models do the results support at generation 1? Why?
Which models do the results support at generation 2? Why?
- After the bacterial population had doubled, the DNA band was midway between 15N and 14N bands.
- Results show that DNA or hybrids with half heavy 15N nucleotides and half light 14N nucleotides. = DNA could have replicated either semi conservatively or dispersedly
- after the bacterial population has doubled again, there were two DNA bands. One was lined up exactly with the 14N band and the other was at the midway point.
- results show that one of the DNA strands contained only like 14N nucleotides. Does DNA therefore must have replicated semi conservatively.
What does topoisomerase (DNA gyrase) and DNA Helicase do?
What is DNA Helicase? What does it consists of?
What keeps the strands from reforming?
What is the result fo this process?
First topoisomerase unwind and release the strain on the double helix, a section at a time.
Than DNA Helicase enzyme follows behind, breaking H bonds between the bases.
DNA helicase is an enzyme consisting of 6 globular polypeptides arranged in a donut shape. One strand of the DNA passes through the center of the 5’ strand and the other strand passes outside of the enzyme.
Single stranded DNA binding protein (SSB) attaches to the bases of each nucleotide to separate the strands during replication so it doesnt reform.
Results in two single strand of DNA with exposed nucleotide bases.
How are nucleotides attached to the exposed bases in DNA replication?
What enzyme is involved? What strand does it read? In what direction?
DNA polymerase III reads the parent template strand in a 3’C to 5’C direction and builds the leading strand in a 5’C to 3’C direction.
DNA polymerase can only attach nucleotides to the free OH group on the 3’C.
DNA polymerase catalyses a condensation reaction that joins free phosphorylated nucleotides to exposed bases.
Where do free nucleotides come from?
Food - all food is derived from living organisms so it contains DNA which can be digested into its nucleotides which are stored in the nucleus of our cells
How is the lagging strand built?
They are built in opposite directions of the leading strand.
But it is also built in a 5’C to 3’C direction, away from the replication fork
It is built discontinuously in small sections know as Okazaki fragments
What does DNA ligase do?
It joins the Okazaki fragments together on the lagging strand
What does DNA primase in DNA polymerase I do?
DNA primase: attaches RNA nucleotides to a section of the template strand creating a double stranded attachment site for DNA polymerase III (which can only bind to double strands)
Polymerase I: removes the RNA nucleotides attached by the DNA primers from the newly synthesize strand and replace them with DNA nucleotides
Where in DNA molecules does DNA replication begin? How does eukaryotes and prokaryotes compare? Which way does replication occur?
Begins at sites called origins of replication.
In prokaryotes there’s one origin of replication.
In eukaryotes there are many origins of replication (replication bubbles) - replication occurs in both directions away from the origin.
What are energized/activated nucleotides? What are they called?
When nucleotides are phosphorylated and have 3 phosphate groups.
They are called nucleoside triphosphate (NTPs). Free nucleotides that bonds to the split original strands of DNA have 2 more phosphate groups than a normal nucleotide.
When DNA polymerase III comes to catalyse a condensation reaction to form phosphodiester bonds, hydrolysis of two phosphate molecules provide energize nucleotides which can be incorporated into the growing DNA strand by DNA polymerase III.
DNA - Deoxyribose = dNTPs
- dATP, dTTP, dCTP, dGTP
RNA - ribose = rNTPS
- rATP, rUTP, rCTP, rGTP
What are the steps of the PCR - The polymerase chain reaction?
- Denaturing - Dna sample is heated (95˚C) for 15 seconds to seperate the 2 strands by breaking the hydrogen bond
- Re-annealing - sample is cooled to 54˚C to allow primers to anneal for DNA polymerase III to bind - the primers designate the sequence to be copied
- Extension/Elongation - sample is heated to the optimum temperature for taq polymerase to function (72˚C)
What is taq polymerase?
Basically a substitute for DNA polymerase III because it can withstand the heat of the PCR
Which is the sense and antisense strand of DNA?
sense: same as RNA
antisense: acts as the template and has complementary base sequence to both the RNA and the sense strand
What does the pink and yellow, purple, green represent?
Pink and yellow: RNA molecules
Purple: proteins
Green: site that makes peptide bonds between amino acids, to link them together into a polypeptide
What 3 components that work together to synthesize polypeptides by translation?
- mRNA has a sequene of codons that specifies the amino acid sequence of the polypeptide
- tRNA molecules have an anticodon of three bases that binds to a complementary codon on mRNA and they carry the amino acid corresponding to that codon
- robosomes act as the binding site for mRNA and tRNAs and also catalyse the assembly of the polypeptide
What is the summary of the main events of translation?
What does the accuracy of translation depend on?
complementary base pairing between the anticodon on each tRNA and the codon on mRNA
