IB Biology HL 2.7 DNA replication, transcription and translation Notes
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Course
IB Biology Unit 2
Institution
Imperial College London (ICL)
In-depth section 2.7 notes for IB Biology HL students. I took my exams in July 2022 and got a 7 in Biology HL. The notes are a combination of information from lots of different sources with a main focus on the IB Biology Textbook.
2.7 DNA replication, transcription and translation
Objectives:
❏ Understanding:
❏ The replication of DNA is semi-conservative and depends on complementary base pairing
❏ Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds
❏ DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand
as a template
❏ Transcription is the synthesis of mRNA copied from the DNA base sequence by RNA
polymerase
❏ Translation is the synthesis of polypeptides on ribosomes
❏ The amino acid sequence of polypeptides is determined by mRNA according to the genetic
code
❏ Codons of three bases on mRNA correspond to one amino acid in a polypeptide
❏ Translation depends on complementary base pairing between codons on mRNA and anticodons
on tRNA
❏ Applications:
❏ Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase
chain reaction (PCR)
❏ Production of human insulin in bacteria as an example of the universality of the genetic code
allowing gene transfer between species
❏ Skills:
❏ Use a table of genetic code to deduce which codon (s) corresponds to which amino acid
❏ Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative
replication of DNA
❏ Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of
amino acids coded by a short mRNA strand of a known base sequence
❏ Deducing the DNA base sequence for the mRNA strand
❏ Nature of Science
❏ Obtaining evidence for scientific theories: Meselson and Stahl obtained evidence for the semi-
conservative replication of DNA
Semi-conservative replication of DNA:
- When a cell prepares to divide, the two strands of the double helix separate
→
- Each of these original strands serves as a template for the creation of a new
strand
- The new strands are formed by adding nucleotides by DNA polymerase, one by one
and linking them together
- Result is two DNA molecules, both composed of an original strand and
a newly synthesised strand
- DNA replication is referred to as being semi-conservative
- The base sequence on the template strand determines the base sequence on the
new strand
- Only a nucleotide carrying a base that is complementary to the next
base on the template strand can be successfully added to the new
strand
, - This is because complementary bases form hydrogen bonds with each other, stabilising
the structure
- If a nucleotide with the wrong base started to be inserted, hydrogen bonding between bases would
not occur and the nucleotide would not be added to the chain
- Complementary base pairing = the rule that one base always pair with another
- Ensures that the two DNA molecules that result from DNA replication are identical in their base
sequences to the parent molecule that was replicated
- Adenine (A) pairs with thymine (T)
- Cytosine (C) pairs with guanine (G)
- Consequently when DNA is replicated by the combined action of helicase and DNA polymerase:
- Each new strand formed will be identical to the original strand separated from the template
- Two semi-conservative molecules formed will have an identical base sequence to the original
molecule
Meselson and Stahl’s DNA replication:
- The theory that DNA replication was semi-conservative was confirmed by the Meselson-Stahl experiment
in 1958
- Prior to this experiment, three hypotheses had been proposed for the method of replication of DNA:
1. Conservative Model
a. An entirely new molecule is synthesised from a DNA template (which remains unaltered)
2. Semi-Conservative Model
a. Each new molecule consists of one newly synthesised strand and one template strand
3. Dispersive Model
a. New molecules are made of segments of old and new DNA
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