2.7 DNA replication, transcription and translation Flashcards
(99 cards)
1
Q
What are the two steps in which cells make proteins from DNA?
A
- 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.
2
Q
Diagram showing an overview of how cells make proteins from DNA
A
3
Q
What happens when a cell divides?
A
Two daughter cells that are identical copies of the parental cell are formed.
4
Q
What is DNA replication?
A
The formation of a new DNA molecule
5
Q
When does replication occur?
A
During the S phase of the cell cycle
6
Q
In most cases, replication results in ___
A
Identical copies of the DNA in the daughter cells.
7
Q
What stays unchanged during DNA replication and what changes?
A
- The chromosome number and genes stay unchanged during DNA replication.
- Only the number of DNA molecules changes.
- It doubles after DNA replication.
8
Q
___ play a key role in DNA replication
A
Enzymes
9
Q
What happens to DNA that means it needs to be unwinded during replication?
A
DNA is normally supercoiled by being tightly wound around histones to form nucleosomes
10
Q
Overview of DNA replication
A
- 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.
11
Q
The two DNA strands of the double helix are ___
A
Anti-parallel to each other
12
Q
Movement of DNA polymerase during replication
A
- 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.
13
Q
In which direction does DNA replication always occur?
A
In the 5’ to 3’ direction
14
Q
Diagram of DNA replication
A
15
Q
Overview of steps of DNA replication
A
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
16
Q
Is DNA replication conservative or semi-conservative?
A
Semi-conservative
17
Q
Why is DNA replication semi-conservative?
A
Because each daughter molecule formed contains one original strand from the old molecule and one newly-synthesized strand.
18
Q
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?
A
DNA in both daughter cells would be radioactive.
19
Q
Sequence of events for DNA replication
A
Unwinding DNA, synthesis of a new DNA strand, winding DNA
20
Q
What is the function of helicase in DNA replication?
A
It is the enzyme that separates the two strands of the DNA double helix during replication.
21
Q
What were Meselson and Stahl trying to figure out?
A
How replication of DNA worked in bacteria
22
Q
What did Meselson and Stahl’s experiments demonstrate?
A
That DNA replication progresses in a semi-conservative way.
23
Q
Diagram of the three possible ways DNA could be replicated
A
24
Q
Describe the first step of Meselson and Stahl’s experiment
A
- 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.
25
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.
26
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.
27
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).
28
Diagram showing the results obtained from the Meselson and Stahl experiment
29
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.
30
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.
31
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
32
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.
33
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.
34
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
35
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
36
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.
37
What is the net product of DNA transcription?
A single-stranded mRNA molecule.
38
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.
39
Diagram showing DNA transcription
40
What is the sense strand?
- The DNA strand that is not transcribed.
| - It has the same sequence of bases as the mRNA molecule, except for thymine being replaced by uracil.
41
What is the sense strand?
- The DNA strand that is not transcribed.
| - It has the same sequence of bases as the mRNA molecule, except for thymine being replaced by uracil.
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What is the antisense strand?
- The transcribed strand.
| - It is complementary to the mRNA molecule.
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Example of a DNA sense sequence
ACT GGC AAT
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Example of a DNA antisense sequence
TGA CCG TTA
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Example of mRNA strand
ACU GGC AAU
46
Diagram showing a single-stranded RNA molecule
47
What step comes after DNA transcription?
Translation
48
Explanation to help remember the difference between transcription and translation
- Transcription is transcribing within one language: the language of nucleic acids.
- Translation is going from one language to another, so translation deals with translating the language of nucleic acids into the language of proteins.
49
A particular triplet of bases in the template strand of DNA is AGT.
What is the corresponding codon for the mRNA transcribed?
UCA
The formation of mRNA follows normal complementary base pairing rules, but you must remember that RNA does not have thymine, but uracil instead.
50
Which enzyme is responsible for uncoiling and separating the DNA strands during transcription?
RNA polymerase
51
Deduce the DNA base sequence of the antisense strand used to generate the following bases on the mRNA: UAG CCA UCC GGC.
ATC GGT AGG CCG
Explanation:
- The antisense strand is complementary to the mRNA strand. By finding the complementary base pair of the sequence of bases on the mRNA, the base sequence of the antisense strand can be deduced.
- A pairs with T, and U pairs with A (remember mRNA has U instead of T).
- C pairs with G and vice versa.
52
What is the next step in the process from DNA to protein after transcription?
Translation
53
What happens once the gene has been transcribed into mRNA?
- It is the mRNA molecule that forms the template for protein synthesis.
- Thus, on leaving the nucleus, the mRNA binds to one or several ribosomes and allows the translation process to begin.
- This happens with the involvement of either free ribosomes in the cytoplasm or ribosomes on the rough endoplasmic reticulum.
54
What is translation?
The synthesis of polypeptides on ribosomes according to the genetic code.
55
What is each set of three consecutive bases in RNA known as?
A codon or triplet
56
What is each codon translated into?
Each codon is translated into one amino acid in a polypeptide chain.
57
Diagram showing the mRNA sequence can be divided into codons
58
What does the sequence of the codons on the mRNA determine?
The amino acid sequence of the polypeptide made.
59
Since the DNA of the gene directs the synthesis of mRNA, any changes affecting the base sequence of the DNA may ___
Lead to the wrong amino acid(s) being included in the polypeptide made.
60
How many amino acids are there in total?
20
61
How many codons can be formed using the 4 bases?
64
62
Coding of amino acids- reword?
- 64 codons can be formed by using the 4 bases.
- Therefore, some amino acids are coded for by more than one codon, accounting for the degeneracy of the genetic code.
- There are also specific codons that act as a signal for the protein translation machinery to start (AUG) or to stop (UAG, UGA, UAA).
63
What is the genetic code?
- The genetic code is degenerate, which means that there are some amino acids that are encoded by more than one codon.
- The genetic code is also universal.
- This means that the genetic information in bacteria is translated in the same way as that of elephants, sequoia trees, or any other living organism.
64
When was the genetic code first worked out?
In the 1960s
65
Table showing the genetic code (The table is read from the left to the right (first base, then second, and finally, third base). So, for example, if the mRNA begins with AUG–CCC this would be translated into the amino acids methionine (Met) and proline (Pro))
66
Although you don't need to know the names of all the amino acids and their abbreviations, you must be able to ___
- Read the genetic code.
- Deduce which codon(s) corresponds to which amino acid.
- Deduce the sequence of amino acids coded by a short mRNA strand of known base sequence.
67
Diagram of the genetic code in a circular version. The arrows indicate how you can find out which
amino acid is coded by GCU.
68
Explanation of how to interpret the genetic code in a circular version (previous flashcard)
- If you want to find out which amino acid is coded by GCU, you start off in the centre of the circle with the G.
- Then, you move outwards and find C (the second circle) and lastly U (the last circle).
- The amino acid coded by GCU is alanine.
69
What does the length of the mRNA depend on and what is it directly related to?
- The length of the mRNA depends on the size of the gene and is directly related to the size of the protein that is made.
- For example, if a mRNA is 456 nucleotides in length, it can code for a protein that is 151 amino acids long.
70
Why is it that if an mRNA is 456 nucleotides in length, it can code for a protein that is 151 amino acids long?
- The explanation for this is that three nucleotides are needed for the start codon, which codes for the amino acid methionine.
- Three nucleotides are needed for the stop codon, which leaves 450 nucleotides.
- Divide by 3 (each set of three nucleotides forms one codon) and you get 150 codons = 150 amino acids, so the total length of the protein is 151 amino acids (when you add the amino acid methionine).
71
Diagram showing the schematic form of a tRNA molecule with its anticodon, and the 3' end, where the amino acid attaches itself.
(tRNA)
72
What is the average length of an mRNA molecule in mammalian cells?
- About 2,000 nucleotides
| - This translates roughly to a protein of around 650 amino acids in length.
73
Diagram showing a simplified model of the synthesis of a small protein
74
Where does translation take place?
In the cytoplasm of a cell
75
How is a polypeptide chain formed during translation?
- A transfer RNA (tRNA) molecule brings a specific amino acid to the mRNA.
- tRNA molecules have an anticodon that pairs with a codon of the mRNA, ensuring that the correct amino acid is added to the growing polypeptide chain.
- Again, common complementary base pairing rules apply.
76
Important things to remember about translation (separate)
- DNA → mRNA → protein.
- The order of nucleotides defines the order of codons, which defines the order of amino acids, which is the primary structure of a protein.
- tRNA (anticodon) pairs with mRNA (codon).
- Translation occurs in the 5' to 3' direction.
- All interactions between DNA and RNA, and mRNA and tRNA, follow complementary base-pairing rules.
- DNA: A pairs with T, and C pairs with G.
- RNA: A pairs with U, and C pairs with G.
- The genetic code is universal!
77
What is the role of tRNA in translation?
It brings amino acids to the ribosome.
78
What does PCR stand for?
Polymerase
Chain
Reaction
79
What is the purpose of PCR?
To amplify small fragments of DNA.
80
What practical applications does PCR have?
- The discovery of this technique has revolutionised medical science, forensic science and molecular biology.
- PCR has enabled scientists to clone genes, to work with minute amounts of DNA found at crime scenes, identify the dead and, perhaps most extraordinarily, sequence the DNA of extinct species and other life forms.
81
When and by whom was the PCR technique for amplifying DNA developed?
By Kary Mullis in 1983, earning him a Nobel prize for his work.
82
Give an overview of how PCR works
- PCR is a technique that can make billions of copies of one molecule of DNA by repeatedly copying a specific stretch of that DNA.
- The technique uses the cyclic heating and cooling of a DNA sample in the presence of primers (small fragments of RNA), DNA nucleotides and Taq polymerase to amplify the DNA.
83
PCR allows the ___ production of ___
Rapid
Multiple copies of DNA using Taq DNA polymerase.
84
What is Taq polymerase and what is its habitat?
- A DNA polymerase isolated from a bacterium, Thermus aquaticus.
- It lives in hot water springs at temperatures between 50 °C and 80 °C.
- Most of its proteins are thermostable, which means they can operate at higher temperatures than other organisms.
85
Why is Taq polymerase suitable for PCR?
- As PCR uses high temperatures, it requires a special DNA polymerase that can withstand these higher temperatures.
- Taq polymerase does not denature at high temperatures (between 50 to 80 degrees Celsius).
86
Diagram showing a simplified version of the technique of PCR
87
Which components are needed for PCR?
Primers, Taq polymerase and DNA nucleotides
88
The genetic code is ___
Universal
89
Because the genetic code is universal, a gene for a human protein is translated by using the same codons as a ___
Bacterial gene
90
What does placing a gene from one organism into a different organism result in?
A transgenic organism
91
What does placing a gene from one organism into a different organism result in?
A transgenic organism
92
How are transgenic microorganisms, such as bacteria, made and what can they be used for?
- By inserting a gene from another organism into the plasmid of their cells.
- Plasmids are small circular DNA molecules in some prokaryotic cells that can be used as transfer molecules.
- The organism can then be used as a biofactory for many of today's therapeutic drugs.
93
Diagram showing how plasmids can be used to produce different proteins such as antibiotics and vaccines
94
What do people with diabetes require to treat their disease?
Insulin
95
What is insulin and what is its function?
- A hormone produced by the beta cells of the pancreas.
| - It regulates glucose uptake and the conversion of glucose to glycogen in the liver.
96
Describe the history of how insulin was obtained
- Insulin used to be extracted from the pancreas of cattle or other farm animals because the structure of this hormone in those animals is very similar to that of human insulin.
- However, the purification process was not very efficient and some patients developed allergic reactions to the animal insulin.
- In 1982, the human gene for insulin was transferred to E. coli, resulting in the production of human insulin.
- Now diabetics can be treated with human insulin, which causes fewer problems than insulin extracted from other species.
97
Why is the production of human insulin in bacteria a reality today?
Because of the universality of the genetic code, which allows gene transfer between species
98
Which organelles are used in transferring genes from one organism to another?
Plasmids
99
How does DNA polymerase form each new strand of DNA during replication?
By joining nucleotides together with covalent bonds between the sugar and phosphate.
(Before this, free DNA nucleotides pair up with their complementary bases in each template strand.)
