DNA replication, transcription and translation

Topic 2.7

Essential idea: in genetic information in DNA can be accurately copied and can be translated to make the proteins needed by the cell.


2.7.U1 The replication of DNA is semi-conservative and depends on complementary base pairing

What is semi-conservative replication?

Semi-conservative replication

What is complementary base pairing?

Complementary base pairing

2.7.U2 Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds

The role of helicases

2.7.U3 DNA polymerase links nucleotides to form a new strand, using pre-existing strand as a template

A basic understanding of DNA replication involves unwinding of the double helix separation of the strands by helicase, followed by formation of complementary strands by DNA polymerase, outlined in the figure below. DNA replication uses complementary base pairing (i.e., adenine bonds thymine while cytosine bonds guanine).

dna structure link dna replication animation
DNA Replication Explained

2.7.U4 Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase

DNA transcription occurs in the nucleus. It involves the unwinding of the double helix and the formation of mRNA complementary to the DNA strand by RNA polymerase. DNA transcription uses complementary base pairing of adenine, thymine, cytosine and guanine (on the DNA) to uracil, adenine, guanine and cytosine (on the nRNA) respectively. It is important to note that eukaryotic RNA needs the removal of introns from pre-mRNA to form mature mRNA.

Place mouse pointer on the figure to view explanation for points 1 to 11.

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DNA Transcription Explained RNA polymerase used RNA binds promoter DNA unwound Antisense strand Free nucleotides RNA nucleotides attached RNA binds promoter Terminator DNA wound PremRNA produced mRNA produced

2.7.U5 Translation is the synthesis of polypeptides on ribosomes

2.7.U6 The amino acid sequence of polypeptides is determined by mRNA according to the genetic code

2.7.U7 Codons of three bases on mRNA correspond to one amino acid in a polypeptide

DNA translation is the process used to synthesize proteins. It occurs in the cytoplasm either on free ribosomes or on the rough endoplasmic reticulum.

Place your mouse pointer on the figure to show the explanation of the process of translation.

dna translation link dna translation animation
Process of translation

2.7.U8 Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA

Relationship between codon and anticodon


2.7.A1 Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by polymerase chain reaction (PCR)

Polymerase Chain Reaction

Polymerase chain reaction (PCR) is used by scientist to copy and amplify minute quantities of DNA. The procedure is outlined in the figure below.

Polymerase Chain Reaction

What is DNA profiling?

DNA profiling involves the use of gel electrophoresis of DNA. In gel electrophoresis DNA fragments move in an electric field and separate from each other according to their sizes. The smallest fragments will move the farthest distance from the origin of the DNA sample. Answer the question in Figure 2 and click on the figure to show the correct answer then double click on the figure to return to the gel electrophoresis. Using Figure 1 and 2 describe how DNA profiling is carried out.

Analysis of DNA profile
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DNA profile explained

Analyse the DNA profiles below to determine and explain which suspect was present at the crime scene.

Place mouse pointer on the figure to see conclusions of DNA analysis.

Analysis of DNA profile

2.7.A2 Production of human insulin in bacteria as an example of the universality of the genetic code allowing gene                      transfer between species

A. Basic technique used in gene transfer

E. coli is used in gene technology because some of its DNA is found on plasmids (smaller circles of DNA). These plasmids can be removed and cleaved by restriction enzymes at target sequences. DNA fragments from another organism can also be cleaved by the same restriction enzyme, and these pieces can be added to the open plasmid and spliced together by ligase. The recombinant plasmids formed can be inserted into new host cells and cloned. When genes are transferred between species, the amino acid sequence of polypeptides translated from them is unchanged because the genetic code is universal.The basic technique in gene transfer is outlined below.

plasmid cloning link plasmid cloning animation
Basic technique in gene transfer

B. Production of human insulin in bacteria

Diagram of the production of human insulin


2.7.S1 Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid

How to use the table of the genetic code?

Use the table below as follows: codon UUU represents amino acid Phenylalanine

Table of the Genetic Code

What is the genetic code?

Genetic Code Explained

2.7.S2 Analysis of Meselson and Stahl's result to obtain support for the theory of semiconservative replication of DNA

2.7.S3 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 amino acid structure

2.7.S4 Deducing the DNA sequence for the mRNA strand

Deducing dna sequence
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