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Higher Human Biology
Unit 1
Human Cells
KEY AREA 3: Gene Expression
Human Cells Learning Intentions
KEY AREA 3 – Gene Expression
1. Gene Expression
2. RNA Structure & Function
3. Transcription
4. Translation
5. Post-translation
3a) Gene Expression
The genetic composition of a cell is called the cell genotype
The cell genotype is determined by the sequence of DNA bases in its genes
The cell phenotype is determined by the proteins that are made when the genes are expressed
Gene expression is how the cell genotype is shown in the cell phenotype (proteins produced)
Only a fraction of the genes in a cell are expressed
3b) Gene Expression
Gene expression is controlled by the regulation of Transcription and Translation
Gene expression is influenced by intra and extra-cellular environmental factors
The order of bases on DNA determines the sequence of amino acids that are made
The sequence of amino acids joined together in polypeptide chains determine the
structure, shape, function of the protein produced
3c) RNA Structure & Function of RNA
RNA is a single strand of RNA nucleotides
RNA nucleotides contain a ribose sugar, a
phosphate and a base
RNA nucleotides contain the bases Adenine,
Uracil, Cytosine, Guanine
mRNA (messengerRNA) carries a copy of a
section of the DNA code for a specific protein
from the nucleus to the ribosome
rRNA (ribosomalRNA) and proteins form a
ribosome
tRNA (transferRNA) carries a specific amino
acid
3d) Transcription



Transcription is the copying of the DNA code for a specific protein into
mRNA
Transcription of DNA into primary and mature RNA transcripts occurs in
the nucleus
Stages in Transcription
1. A promotor region of DNA initiates transcription
2. RNA polymerase enzyme moves to the specific section of DNA and
unwinds and unzips the DNA double helix at that point
3. RNA nucleotides pair with complementary DNA base pairs (A-U, G-C)
forming mRNA
4. RNA polymerase can only add nucleotides to the 3’ end of mRNA
5. RNA polymerase joins the nucleotides together to form a new sugarphosphate backbone
6. The mRNA becomes separated from the DNA template, and is called
the primary transcript of mRNA
3d) Transcription
3e) Transcription

Not ALL nucleotides in a gene play a role in the coding for the amino acids
sequence

Introns are non-coding regions of genes

Exons are coding regions of genes

Introns are found between the Exons

The primary transcript of mRNA which codes for the polypeptide chain of amino
acids is fragmented

Introns are cut out and removed from the primary transcript

Exons are spliced together (RNA splicing) to form mRNA with a continuous
sequence of nucleotides (this is called the mature transcript of mRNA)

The mature transcript of mRNA moves from the nucleus through the cytoplasm
to a ribosome
Transcription Animation (1:52)
Splicing (1:37)
3f) Translation

mRNA is linear and carries the DNA code
in the form of mRNA codons (3 bases=a
codon)

A mRNA codon is complementary to the
triplet of bases on the original DNA strand

tRNA is found in the cytoplasm

tRNA is folded on itself due to hydrogen
bonds forming between bases

tRNA exposes triplets of bases called
tRNA anticodons

tRNA anticodons are complementary to a
mRNA codon and have attachments sites
which bind to a specific amino acid

tRNA pick up specific amino acids in the
cytoplasm and carry them to the ribosome

mRNA codons and tRNA anticodons
translate the genetic code into a sequence
of amino acids
3f) Translation
3f) Stages of Translation
1.
2.
3.
4.
5.
6.
mRNA binds to the ribosome
Each tRNA picks up specific amino acids in the
cytoplasm and carries it to the ribosome
Each tRNA anticodon binds to a complementary
codon on the mRNA lining up the amino acids in a
specific order
Peptide bonds form between the amino acids to form
a growing polypeptide chain.
When the chain is complete it detaches from the
ribosome and will be modified
When the tRNA detaches from its amino acid, it
then collects another
3f) Translation
3f) Translation
3f) Translation
3g) Translation

mRNA codons AUG complementary to tRNA
anticodon UAC codes for the amino acid
methionine (met) AND acts as the START
CODON

mRNA codons UAA, UAG & UGA do not
code for amino acids, but they act as STOP
CODONS

Ribosomes have one binding site for mRNA
and three binding sites for tRNA

Site 2 holds the tRNA carrying the amino
acids and growing polypeptide chain

Site 3 holds the tRNA carrying the next
amino acid

Site 1 discharges the tRNA from the
ribosome once the amino acid has become
part of the polypeptide chain

A string of ribosomes carrying out multiple
translation on the same mRNA strand is
called a polyribosome
Translation animation (2:04)
3h) Post-Translation
Once translation is complete, further modifications
(in addition to folding & coiling) may be required to
enable a protein to perform its specific function
 One gene can create many proteins as a result of
RNA splicing and post translational modification
 Different mRNA molecules are produced from the
same primary transcript depending on which RNA
segments are treated as exons and introns
 Post-translation protein structure can be modified
by: Cutting (cleaving) the polypeptide chain to make it
active
e.g. insulin must have its central section
cut by protease enzymes to make the
protein active
 Combining polypeptide chains
 Adding a carbohydrate
e.g. mucus is a glycoprotein containing
protein & a carbohydrate
 Adding a phosphate
e.g. regulatory proteins need phosphate
added to make them function

Human Cells Questions
KEY AREA 3 – Gene Expression
1. Testing Your Knowledge 1
Page 37
Q’s 1-4
2. Testing Your Knowledge 2
Page 43
Q’s 1-4
3. What you Should Know
Page 43
Q’s 1-15
4. Quick Quiz