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DNA replication, transcription and translation

Topic 2
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7

Biology SL/HL

Before it was believed that an entirely new molecule was produced
using the whole DNA as a template

PCR

➢ Polymerase Chain Reaction, 1970’s
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DNA replication can be carried out artificially in lab

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By replicating DNA segments, scientists can produce huge numbers of these
segments to study and analyse
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Forensic and evolutionary science

➢ Performed in several steps:
1
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Annealing
o

o

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The temperature is lowered to 50 C - 65 C

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A little piece of single-stranded DNA, called a primer, binds
complementary to a base sequence
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The primer fits the first three bases of the DNA strands

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In our cells free nucleotides bind one by one

3
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Repeat
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The process is repeated and more DNA is copied

➢ Thermus aquaticus (Taq) was discovered in 1985
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It is a bacterium occurring in hot springs

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Its enzyme (a specific DNA polymerase) is used in PCR, because it is stable at
high temperatures and it does not denature

RNA types
➢ mRNA, messenger RNA: code for a single polypeptide
➢ rRNA, ribosomal RNA: ribosomes consist of it + ribosomal protein
➢ tRNA, transfer RNA: transfers amino acids to the ribosomes
2

DNA replication, transcription and translation

Topic 2
...
7

Biology SL/HL

RNA nucleotides float into place by complementary base pairing, while RNA
polymerase moves along the strand of DNA acting as the template
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RNA polymerase catalyses the addition of free RNA nucleotides to
the growing mRNA strand

Translation
➢ The mRNA molecule determines the order of the amino acids
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A set of three bases contains enough information to code for one of the 20
amino acids
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Any set of three bases that determines the identity of one amino acid
is called a triplet

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When a triplet is found in an mRNA molecule, it is called a codon or

codon triplet
➢ Once an mRNA molecule has been transcribed, it detaches from the single strand
DNA template and floats free in the nucleoplasm
➢ mRNA floats through one of the nuclear pores and goes to the cytoplasm
➢ tRNA is a single strand of RNA nucleotide, but has areas where the strand has
complementary base paired with itself
➢ tRNA molecules have three bases in the middle loop, which are called anticodons
and determine which of the 20 amino acids will be attached to the tRNA
➢ Translation process:
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The mRNA locates a ribosome and aligns with it, so that the first two codon
triplets are within the boundaries of the ribosome

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A specific tRNA molecule, whose anticodon is complementary to the first
codon of the mRNA molecule, floats in

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While the first tRNA molecule sits in the ribosome holding the first amino
acid, a second tRNA floats in and brings a second specific amino acid, which
is complementary to the second codon
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Two specific amino acids are held side by side

An enzyme catalyses a condensation reaction between the two amino acids
and results in the formation of a covalent bond, called a peptide bond

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The bond between the first tRNA molecule and the amino acid that it
transferred in breaks
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The first tRNA floats away in the cytoplasm and reloads with another
amino acid of the same type

4

DNA replication, transcription and translation
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Topic 2

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