2.7 DNA replication, transcription and translation Flashcards
What are the two steps in which cells make proteins from DNA?
- Transcription : copying the DNA by synthesising messenger RNA (mRNA) from the DNA base sequences.
- Translation : interpreting the genetic code to synthesise proteins (more specifically polypeptide chains) on ribosomes.
Diagram showing an overview of how cells make proteins from DNA
What happens when a cell divides?
Two daughter cells that are identical copies of the parental cell are formed.
What is DNA replication?
The formation of a new DNA molecule
When does replication occur?
During the S phase of the cell cycle
In most cases, replication results in ___
Identical copies of the DNA in the daughter cells.
What stays unchanged during DNA replication and what changes?
- The chromosome number and genes stay unchanged during DNA replication.
- Only the number of DNA molecules changes.
- It doubles after DNA replication.
___ play a key role in DNA replication
Enzymes
What happens to DNA that means it needs to be unwinded during replication?
DNA is normally supercoiled by being tightly wound around histones to form nucleosomes
Overview of DNA replication
- The first step is to unwind the coils to make the strands accessible to enzymes.
- The enzyme helicase then unwinds the double helix and separates the two DNA strands by breaking the hydrogen bonds between the bases.
- Once the strands are separated and the bases exposed, another enzyme called DNA polymerase can start making new strands of DNA using the two ‘old parent’ strands as templates.
The two DNA strands of the double helix are ___
Anti-parallel to each other
Movement of DNA polymerase during replication
- As the two DNA strands of the double helix are anti-parallel to each other, DNA polymerase proceeds in opposite directions during replication.
- On one strand it moves in the same direction as the replication fork (immediately behind the helicase enzyme), and it moves in the reverse direction on the other strand.
In which direction does DNA replication always occur?
In the 5’ to 3’ direction
Diagram of DNA replication
Overview of steps of DNA replication
1) Original double helix
2) Helicase
3) Free nucleotides
4) Free nucleotides base pair to the original polynucleotide
5) DNA polymerase enzyme joins the sugar-phosphate backbone
6) An enzyme unwinds the double helix
7) Two new double helices each containing one of the original polynucleotides
Is DNA replication conservative or semi-conservative?
Semi-conservative
Why is DNA replication semi-conservative?
Because each daughter molecule formed contains one original strand from the old molecule and one newly-synthesized strand.
Suppose you are provided with an actively dividing culture of E. coli bacteria to which radioactive thymine has been added.
What would happen if a cell replicates once in the presence of this radioactive base?
DNA in both daughter cells would be radioactive.
Sequence of events for DNA replication
Unwinding DNA, synthesis of a new DNA strand, winding DNA
What is the function of helicase in DNA replication?
It is the enzyme that separates the two strands of the DNA double helix during replication.
What were Meselson and Stahl trying to figure out?
How replication of DNA worked in bacteria
What did Meselson and Stahl’s experiments demonstrate?
That DNA replication progresses in a semi-conservative way.
Diagram of the three possible ways DNA could be replicated
Describe the first step of Meselson and Stahl’s experiment
- They cultured E. coli bacteria in the presence of a heavy nitrogen isotope, 15 N.
- DNA contains nitrogen in its nitrogenous bases (ATCG), so the radioactive 15 N would end up in the DNA of the bacteria.
- The result was that all bacterial DNA had 15 N in its bases.
Describe the second step of Meselson and Stahl’s experiment
They then transferred the bacterial culture into a fresh medium where the nitrogen was replaced by 14 N, a lighter isotope, and the bacteria were allowed to grow for several generations.
Describe the third step of Meselson and Stahl’s experiment
- DNA samples were then extracted from successive bacterial generations and subjected to cesium chloride equilibrium density gradient centrifugation.
- This technique allowed the DNA to move to different positions in the centrifuge tube based on its density.
Describe the results of Meselson and Stahl’s experiments
DNA containing one or two strands with 15 N was heavier and showed lower bands than those containing two strands with 14 N (DNA with 15 N in both strands was heaviest).
Diagram showing the results obtained from the Meselson and Stahl experiment
Explain the results of the Meselson and Stahl experiment
- After one generation, i.e. one division of the bacteria, the resulting DNA strand consisted of a double helix, where one strand was made up of 15 N, and the other contained 14 N.
- The band obtained was in between those for DNA with both strands containing 15 N or 14 N only.
- This indirectly demonstrated that DNA replication had to proceed in a semi-conservative way and should involve complementary base pairing to ensure the fidelity of the daughter molecules.
- Results of the successive generations of bacteria further confirmed this theory.
In the experiments performed by Meselson and Stahl, E. coli were grown for many generations in a medium containing 15N, then for one generation in 14N.
What results for the DNA of the last generation showed that replication was semi-conservative?
One strand containing only 15N and one strand containing only 14N.
Which method was used in Meselson and Stahl’s experiment to prove their theory about the semi-conservative nature of DNA replication?
Use of isotopes
What happens in transcription?
- In this step, the coding information is copied or transcribed into a special molecule called messenger RNA (mRNA).
- The DNA functions as a template and the single-stranded mRNA molecule that is made follow the complementary base pairing rules of DNA, with one exception.
Complementary base pairing in DNA vs. RNA
- In DNA, G (guanine) always pairs with C (cytosine) and A (adenine) pairs with T (thymine).
- However, U (uracil) replaces T (thymine) in RNA, so in an mRNA molecule, whenever there is an A in the DNA template, a U will appear in the newly formed mRNA.
Role of complementary base pairing in the transcription process
- It is an extra control mechanism of the transcription process
- It ensures that the mRNA is a true RNA copy of the DNA
Give an example of being able to deduce DNA base sequence from that of the mRNA strand
For instance, if the mRNA sequence is:
GCU CCU GAG UUG
The corresponding base sequence of the DNA strand will be:
CGA GGA CTC AAC
Summary of DNA transcription
- In summary, the section of DNA that contains the required gene is unwound and separated so that RNA polymerase enzymes can access the DNA bases.
- The RNA polymerase then transcribes a sequence of DNA bases into mRNA.
- Hence, the net product of this process is a single-stranded RNA molecule.
What is the net product of DNA transcription?
A single-stranded mRNA molecule.
What is the function of RNA polymerase?
RNA polymerase is responsible for separating the DNA strands of the double helix as well as for joining the ribonucleotides together by phosphodiester bonds to form an mRNA strand.
Diagram showing DNA transcription