2.7 DNA replication, transcription and translation Flashcards
The replication of DNA is ……?
Semi conservative. Meaning each molecule produced contains an original strand and a newly synthesised strand. This is because the original molecule serves as a guide or template for the two new strands to be synthesised.
How does the complementary base pairing work when replicating DNA?
With the original strand as a guide, only the complementary base pairs to the ones on the original strand can form hydrogen bonds with each those bases. If a non-complementary one was inserted hydrogen bonding would not occur so it would not be fixed. This ensures that the replicated strand is identical to the strand that was there before.
What is the evidence for semi conservative replication?
Melson and Stahl cultured the bacterium E.coli for fourteen generations in a medium where the only nitrogen source was 15N. Almost all of the bases of DNA in the bacteria were therefore 15N. They transferred the bacteria abruptly to a medium in which all the Nitrogen was 14N. At the temperature used to culture them the generation time was 50 minutes and so the bacteria divided and replicated their DNA once every 50 minutes.
They then collected samples of DNA from the bacterial culture of several hours from the time when it was transferred to the 14N medium. They extracted the DNA and measured its density by caesium chloride density gradient configuration. The DNA could be detected because it absorbs ultraviolet light and so created a dark band when the tubes were illuminated with ultra violet.
They found that the weights were in the middle of 2 14’s and 2 15’s and and therefore it suggested semi conservative replication.
What does helicase do?
Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds.
Before DNA replication can take place the two strands of the molecule need to be separated so they each act as a template for the formation of a new strand. This separation is carried out by helicases a group of enzymes that use energy from ATP.
Helicase is six globular polypeptides arranged in a donut shape. One DNA molecule passes through the inside of the donut and one passes on the outside. Energy from ATP is used to move the helicase along the DNA molecule.
Double stranded DNA cannot be split into two strands while it is still helical. Helicase therefore causes the unwinding of the helix at the same time as it separates the strands.
How does helicase work?
Uses energy from ATP to break hydrogen bonds. Helicase is six globular polypeptides arranged in a donut shape. One DNA molecule passes through the inside of the donut and one passes on the outside. Energy from ATP is used to move the helicase along the DNA molecule.
Double stranded DNA cannot be split into two strands while it is still helical. Helicase therefore causes the unwinding of the helix at the same time as it separates the strands.
What does DNA polymerase do?
DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template.
DNA polymerase moves along the template strand in the 5’ to 3’ direction. adding one nucleotide at a time. Free nucleotides are available in the area where DNA is being replicated. Each time a nucleotide is added to the new strand only one of the four types of nucleotide has the base that can pair with the base at the position reached on the template strand. DNA polymerase brings nucleotides into the position where hydrogen bonds could form, but unless this happens and a complementary base pair is formed the nucleotide breaks away again.
Once a nucleotide with the correct base has been brought into position and hydrogen bonds have been formed between the two bases, DNA polymerase links it to the end of the new strand. This is done by making a covalent bond between the phosphate group of the free nucleotide and the sugar of the nucleotide at the existing end of the new strand.
How can we artificially synthesise DNA?
PCR - Polymerase Chain Reaction
The DNA is loaded into a PCR machine in which a cycle of steps repeatedly doubles the quantity of the DNA.
If DNA is heated to a high temperature, the hydrogen bonds eventually break and the two strands separate. If the DNA is then cooled hydrogen bonds can form so the strands pair up again.
The PCR machine separates DNA strands by heating them to 95 degrees for fifteen seconds. It then cools the DNA quickly to 54 degrees. This would allow re-annealing of parent strands to form double stranded DNA. However short sections of the single stranded DNA called primers are present. The primers bind rapidly to target sequences and as a large excess of primers is present, they prevent re-annealing of the parent strands.
Then what happens is the primer strands are used as templates. The enzyme Taq DNA is used to do this. This enzyme is used because it can resist the brief period at 95 degrees used to separate the DNA strands. The Taq DNA adds about 1000 nucleotides per minute.
BASICALLY
- Heat DNA to 95 so it splits.
- Lower temperature to 54 degrees to allow rebinding of primers
- Then raise temperature so Taq DNA adds nucleotides to these primers to made another DNA strand.
What is transcription? SL
Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.
1) The enzyme RNA polymerase binds to a site on the DNA at the start of a gene.
2) RNA polymerase moves along the gene separating DNA into single strands and pairing up RNA nucleotides with complementary bases on one strand of DNA. There is no thymine in RNA so Uracil is used instead with adenine.
3) RNA polymerase forms covalent bonds between the RNA nucleotides.
4) The RNA separates from the DNA and the double helix reforms
5) Transcription stops at the end of the gene and the completed RNA molecule is released.
The product of transcription is a molecule of RNA with a base sequence that is complementary to the template strand of DNA.
Which is the sense strand? SL Transcription
The DNA strand with the same base sequence as the RNA.
Which is the antisense strand? SL Transcription
The DNA that acted as the template for the RNA and so has the complementary base pairs to it.
What is translation? SL
Translation is the synthesis of polypeptides on ribosomes.
1) An mRNA binds to the small subunit of the ribosome.
2) A molecule of tRNA with an anticodon complimentary to the first codon to be translated on the mRNA binds to the ribosome.
3) A second tRNA with an anticodon complementary to the second codon on the mRNA then binds. A maximum of two tRNA’s can be bound at the same time.
4) The ribosome transfers the amino acid carried by the amino acid on the second tRNA by making a new peptide bond. The second tRNA is then carrying a chain of two amino acids.
5) The ribosome moves along the mRNA so the first tRNA is released, and the second becomes the first.
6) Another tRNA binds with an anticodon complementary to the next codon on the mRNA.
7) The ribosome transfers the chain of amino acids carried by the first tRNA to the amino acid on the second tRNA by making a new peptide bond.
The process continues along the mRNA until the stop codon is released.
What is mRNA?
The RNA that carries the information needed to synthesise a polypeptide. Messenger RNA. At any time a cell will only need to make some polypeptides not all of them and so only some mRNA is transcribed and available for those polypeptides.
What is tRNA?
translation RNA. They are molecules that carry the 3 opposite bases to those on the mRNA and therefore are needed to make the new DNA molecule.
What is rRNA?
Ribosomal RNA
What are codons?
Codons are three bases of RNA. They code for one amino acid. This means amino acids are coded for by three bases. A collection of three bases is a codon.
