DNA replication, transcription and translation Flashcards
1
Q
What and when does DNA replication occur?
A
- The formation of a new DNA molecule
- It occurs during the S phase of the cell cycle
- Usually the results are identical copies of the DNA in the daughter cells
- The chromosome number and gene stay unchanged
2
Q
What are the 5 enzymes and their functions that are involved in DNA replication?
A
- Helicase: breaks hydrogen bonds to separate the two strands
- DNA polymerase: replicates DNA by adding new bases to each strand (which are already separated)
- Primase: makes primers which is a chemical molecule that tells DNA polymerase where to start from to add bases
- The primers are made out of RNA
- Ligase: binds the okazaki fragments together
- SSB proteins: bind to the DNA strands to keep them apart to prevent supercoiling
3
Q
What are the first 5 steps of DNA replication?
A
- Because the DNA is supercoiled around histones to form nucleosomes, the DNA must first be unwind so that the enzymes have access
- Helicase unwinds the double helix and separates the two strands by breaking the hydrogen bonds connecting the bases
- Primase begins to produce primers on both strands. It determines where the DNA polymerase should start making new strands of DNA. The original DNA is replicated.
- DNA polymerase links the nucleotides together to form a new strand by using the pre-existing strand as a template
- The DNA polymerase can only create the new strand in the (5’ to 3’) direction. Leading strand (original strand) is from 3’ to 5’
- The leading strand is 3’ to 5’. The lagging are 5’ to 3’. The complementary new strands will be the opposite of each of them.
4
Q
What are the last steps of DNA replication? What does semi-conservative mean?
A
- The opposite strand (lagging strand) needs to keep up with the leading strand, where the unwinding happens
- This means more primers are needed to be placed so the DNA polymerase knows where to add new bases
- Because the lagging strand needs to catch up, it leaves gaps between the Okazaki fragments (short sequences of DNA nucleotides)
- Finally Ligase fills the gaps between the fragments
- In the end there are two sets of DNA double helixes
- DNA replication is semi-conservative because each daughter molecule formed contains one original strand from the old molecule and a newly synthesized strand. Two sets of double helix in the end (4 strands in total)
5
Q
What was the evidence for the semi-conservative theory of DNA strands?
A
- The scientists Meselson and Stahl cultured the E coli bacteria with a nitrogen isotope 15N which was radioactive and could end up in the DNA of the bacteria. In the end all bacterial DNA had 15N in its bases
- The 15N was replaced by the 14N isotope and the bacteria grew for several generations
- Samples were extracted and placed in caesium chloride equilibrium density gradient centrifugation. The SNA could move to different positions based on its density
- DNA with two strands of 15N was heavier than those containing two strands with 14N (lighter isotope)
- After another generation, one strand consisted of a N14 and the other of 15N
- This showed that DNA replication had to proceed in a semi-conservative way
6
Q
Explain the first 3 steps of DNA transcription.
A
- The coding information, the gene (a section of DNA), is copied into the molecule called messenger RNA, mRNA for short. The original DNA is used as a template.
- mRNA follows the same complementary base pairing rules as DNA, except the thymine is replaced by uracil. This is done to ensure that the mRNA is a ‘true’ replication of the original DNA.
- The section of DNA (which is the gene that is required to synthesize a specific protein) is unwound and separated by the RNA polymerase into two separate strands so that it can access the DNA bases.
7
Q
Explain the last 3 steps of DNA transcription.
A
- The RNA polymerase transcribes or copies a sequence of bases from the original DNA into the mRNA. The result is a single-stranded RNA molecule.
- The function of RNA polymerase is to separate the DNA strand and join the ribonucleotides together by phosphodiester bonds to form the mRNA strand. This is how it builds the mRNA strand.
- The part of the DNA strand that is transcribed is called the antisense strand and its bases are complementary to the mRNA molecule.
- The other DNA strand (whose bases are not transcribed) is called the sense strand. The sense strand contains the same bases as the mRNA however contains thymine instead of uracil.
8
Q
Remember!
A
- In DNA it has base T. In RNA it is replaced by base U.
- When a DNA sequence is transcribed to RNA e.g. DNA (ACGTTGC) then in RNA (UGCAACG). Technically it is T but that is replaced by U
- The opposite: if first A then —> U, if first T—> A
9
Q
What is translation?
A
- The synthesis of polypeptides on ribosomes according to the genetic code
- After transcription, the mRNA molecules binds to a ribosome after leaving the nucleus. They bind either to a free ribosome or one of the RER
- Translation occurs in the 5’ to 3’ direction
10
Q
What is a codon and what does it determine?
A
- Each set of three consecutive bases in RNA. Also known as triplet
- Each codon on mRNA corresponds to one amino acid in a polypeptide chain
- The sequence of the codons on the mRNA determines the amino acid sequence of the polypeptide made
- Any changes affecting the base sequence of the DNA may lead to the wrong amino acids being included in the polypeptide
11
Q
How many codons can be formed? What codons act as signals for translation?
A
- From the 20 amino acids that exist, 64 codons can be formed by using 4 bases. Some amino acids are coded for by more than one codon
- AUG codon acts as a signal for protein translation machinery to start
- UAG, UGA and UAA codons act as signals for protein translation machinery to stop
12
Q
What is the genetic code?
A
- It is degenerate, meaning that there are some amino acids that are encoded by more than one codon
- The genetic information in bacteria is translated in the same way in all living organisms, it is universal
- Codons make up the genetic code
13
Q
Be able to deduce which codons correspond to which amino acids.
A
- Look at the genetic code diagram
14
Q
What does the size of the protein depend on?
A
- The length of the mRNA depends on the size of the gene, which is related to the size of the protein
- Three nucleotides (bases) are needed for the ‘start’ codon, which codes for the amino acid methionine. Three nucleotides are needed for the ‘stop’ codon
- Which means 6 nucleotides need to be subtracted from the total amount. Divide the remaining by 3 (3 bases for each codon)
E.g. 465 total - 6(for starting and stopping codons) = 150 + 1 (methionine) = 151 amino acids - In mammals there are 2000 nucleotides, 650 amino acids per protein
15
Q
What are the steps of translation?
A
- Takes place in the cytoplasm of the cell
1. A transfer RNA (tRNA) molecule brings a specific amino acid to the mRNA.
2. tRNA molecules have an anticodon that pairs with a codon of the mRNA so that it is added to the growing polypeptide chain. Complementary base pairing applies.
3. A condensation reaction forms a peptide bond
4. The ribosome moves along the mRNA and a tRNA is released
5. Another amino acid joins, containing the anticodon - A ribosome essentially moves along the mRNA. The amino acids (containing anticodons) keep attaching to the ribosome. These anticodons pair with the bases of the codons of the mRNA
