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Transcription & Translation (2.7/7.2/7.3)

Gene Expression PHET

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Transcription and Translation

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Epigenetics and Disease Article

Lick your Rats ActivitySome mother rats spend a lot of time licking, grooming, and nursing their pups. Others seem to ignore their pups. Highly nurtured rat pups tend to grow up to be calm adults, while rat pups who receive little nurturing tend to grow up to be anxious. It turns out that the difference between a calm and an anxious rat is not genetic—it's epigenetic. The nurturing behavior of a mother rat during the first week of life shapes her pups' epigenomes. And the epigenetic pattern that mom establishes tends to stay put, even after the pups become adults.

Epigenetics Video: 13 minutes

2.7 DNA replication, transcription and translation

Nature of science:

Obtaining evidence for scientific theories—Meselson and Stahl obtained evidence for the semi-conservative replication of DNA. (1.8)


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 sequences 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 and skills:

Application: Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR).

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

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

Skill: Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA.

Skill: 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 known base sequence.

Skill: Deducing the DNA base sequence for the mRNA strand.


The different types of DNA polymerase do not need to be distinguished.


Aim 8: There are ethical implications in altering the genome of an organism in order to produce proteins for medical use in humans.

Essential idea: Cell respiration supplies energy for the functions of life.

7.2 Transcription and gene expression

Nature of science:

Looking for patterns, trends and discrepancies—there is mounting evidence that the environment can trigger heritable changes in epigenetic factors. (3.1)


Transcription occurs in a 5’ to 3’ direction.

Nucleosomes help to regulate transcription in eukaryotes.

Eukaryotic cells modify mRNA after transcription.

Splicing of mRNA increases the number of different proteins an organism can produce.

Gene expression is regulated by proteins that bind to specific base sequences in DNA.

The environment of a cell and of an organism has an impact on gene expression.

Application and skills:

Application: The promoter as an example of non-coding DNA with a function.

Skill: Analysis of changes in the DNA methylation patterns.


RNA polymerase adds the 5´ end of the free RNA nucleotide to the 3´ end of the growing mRNA molecule.

Theory of knowledge:

The nature versus nurture debate concerning the relative importance of an individual’s innate qualities versus those acquired through experiences is still under discussion. Is it important for science to attempt to answer this question?

Essential idea: Information transferred from DNA to mRNA is translated into an amino acid sequence.

7.3 Translation

Nature of science:

Developments in scientific research follow improvements in computing—the use of computers has enabled scientists to make advances in bioinformatics applications such as locating genes within genomes and identifying conserved sequences. (3.7)


Initiation of translation involves assembly of the components that carry out the process.

Synthesis of the polypeptide involves a repeated cycle of events.

Disassembly of the components follows termination of translation.

Free ribosomes synthesize proteins for use primarily within the cell.

Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.

Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.

The sequence and number of amino acids in the polypeptide is the primary structure.

The secondary structure is the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding.

The tertiary structure is the further folding of the polypeptide stabilized by interactions between R groups.

The quaternary structure exists in proteins with more than one polypeptide chain.

Application and skills:

Application: tRNA-activating enzymes illustrate enzyme–substrate specificity and the role of phosphorylation.

Skill: Identification of polysomes in electron micrographs of prokaryotes and eukaryotes.

Skill: The use of molecular visualization software to analyse the structure of eukaryotic ribosomes and a tRNA molecule.


Names of the tRNA binding sites are expected as well as their roles.

Examples of start and stop codons are not required.

Polar and non-polar amino acids are relevant to the bonds formed between R groups.

Quaternary structure may involve the binding of a prosthetic group to form a conjugated protein.

Jessica Clark,
Mar 20, 2014, 9:22 AM