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Genes Function:
genes in action
Genes in Action
Chapter 11
Pp 385 - 417
Chapter 11 Gene Function
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TOPIC
The ‘role’ of genes:
Case Study: beta Thalassaemia
11.1
Pgs: 385 – 388
then
Pgs: 397 - 400
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Genes ‘n’ action Case Study:
beta Thalassaemia

various kinds of haemoglobin are found in red
blood cells

each kind consists of four protein chains each
with an iron-containing heam molecule

the gene that controls the production of beta
chains of haemoglobin a (one of the four chains!)
is the hbb gene on chromosome 11

inheritance of the t gene is autosomal

t – normal, t - thalassaemia

absence of beta chains is an inherited disorder
known as beta thalassaemia

Task: Complete Quick Check 1-3
3

HBB gene (Chromosome 11) has 1600bp
consisting of Exons (coding for the proteins) and
Introns.

In all, 146 amino acids are produced in a normal
beta chain

Affected thalassaemia sufferers have a base
substitution in which T is replaced by A

Base substituion affects the seventh codon in
mRNA

Instead of AAG, UAG is produced. UAG codes for
STOP
Quck Check Q’s: 14 & 15
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TOPIC – PROTEIN SYNTHESIS
Genes in Action: Transcription & Translation
11.2
Pgs: 388 – 402
GENE ACTION
When a gene becomes active (to produce
proteins!), it first makes a mobile copy of the coded
instructions that it contains
.
This occurs by a process known as transcription.
This mobile copy of a genetic instruction can leave
the nucleus and move to the cytoplasm where the
instruction is decoded. This occurs by a process
known as translation. So gene action involves two
processes: transcription and translation
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TRANSCRIPTION
Defn: Transcription is the process by which the code
contained in the DNA molecule is transcribed
(rewritten) into a mRNA molecule

transcription involves making a copy of the DNA
template. A special molecule called messenger
RNA (mRNA) is produced.
Consider a DNA template with the base sequence …
(3’) ATGCCTGAAT (5’)..
This DNA acts as a template to guide the formation of a
mRNA molecule with the complimentary base
sequence as follows:
(5’) UACGGACUUA (3’) ..
See Biozone pg 205
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Chapter 11 Gene Function
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Steps in TRANSCRIPTION
(5’) TACGGACTTA (3’) ..
(3’) ATGCCTGAAT (5’)..
Purpose of Transcription!
• to make a complimentary strand of mRNA!
At the start …
• DNA is made of two complimentary strands.
 one is the DNA coding strand (5 – 3)
(5’) TACGGACTTA (3’) ..
(3’) ATGCCTGAAT (5’)..
 the other is the DNA Template strand (3 – 5)
• At the start of Transcription the double stranded DNA
molecule has to unzip and the template copied!
(5’) UACGGACUUA (3’) ..
1: Unzipping DNA…
• enzyme known as RNA polymerase attaches to a
region of DNA in the upstream region of the template
strand
Chapter 11 Gene Function
• The double stranded DNA of the gene unwinds and
exposes the bases of the template strand.
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2. The base sequence of the DNA Template strand guides
the building of a complimentary copy of mRNA. The RNA
polymerase enzyme moves along the DNA template and as
it moves (RNA) nucleotides are brought into place one by
one to form a RNA chain
3. The single stranded RNA molecule called
pre-messenger RNA (pre-mRNA) is fully complimentary
to the original DNA Template molecule.
•the enzyme transcribes only a gene length of DNA at a time
•only RNA polymerase is involved in mRNA synthesis
•It is common to find more than one enzyme transcribing on
the same length of gene
RNA
Polymerase
3
'
5
'
Template
Strand
Elongat
ion
Site
Rewinding
3'
Co
din
g
Str
and
Unwinding
Nascent
5 RNA
'
5'
3'
Movement of polymerase
(but, we are not finished with Transcription!)R
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Pre-mRNA is modified after
transcription
4. The regions of the pre-mRNA that corresponds to the
introns in the coding region of the gene are cut out,
producing a shorter mRNA molecule.
(Splicing of pre mRNA is carried out by a complex
known as splicosome, a complex consisting of RNA
and protein)
5. The final mRNA molecule is chemically capped
with a a methyl “cap” (AUG)
6.
Finally, a long tail of ‘A’ (Adenine’s) called a poly-A tail
is added to produce a fully functional mRNA strand
that is ready for transport out of the nucleus.
Once mRNA is formed, it leaves as a small single
strand through the nuclear pores. Once in the
Cytoplasm the mRNA will engage ribosomes to begin the
next stage in protein synthesis
Quick Check 4 – 8 page 391
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TRANSLATION
Defn: The decoding of the genetic instructions occurs
through the process of translation which takes place in
the cytoplasm.

by the end of this process the mRNA have been
decoded and translated into a protein chain of amino
acids.
Key Parts of Translation (see Table 11.3)
 mRNA
 Nuclear pore
 cytoplasms
 ribosomes
 tRNA (with anti codon)
 Amino acids – (forming polypeptides)
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tRNA
A key player in Translation
tRNA = Transfer RNA

Each tRNA consist of a single strand of 76 coiled RNA
nucleotides

At one end of the tRNA molecule are three bases that
make up an anti-codon (complimentary to a codon)

At the other end of the tRNA molecule is a region that
attached to one specific amino acid

An enzyme, amino acyl tRNA synthetase catalyses
the linking of each amino acid to a tRNA
Chapter 11 Gene Function
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TRANSLATION –

The mRNA leaves the nucleus through the pore and
attaches to sub-microscopic particles known as
ribosomes

Each ribosome consists of two sub units

Inside the ribosome, the mRNA codon lines up with the
tRNA’s anticodon (they are complimentary

Each loaded tRNA has an amino acid attached

As the mRNA moves through the ribosome, each
codon is ‘read’ (translated)

As read, the mRNA moves one codon at a time and
with each, a tRNA attaches its amino acid to the
growing chain on amino acids!
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Chapter 11 Gene Function
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TRANSLATION –
DECODING INSTRUCTIONS
Biozone 206
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Task: Translating Code
Using the table on the previous page….
1. Write the amino acid sequence from the
following DNA template strand
tac gga gag gca cct act
_____ _____ _____ _____ _____ _____
2. Write the amino acid sequence if a spontaneous
mutation occurred at the sixth nucleotide and
cystosine was transcribed onto the pre mRNA
strand instead of uracil
_____ _____ _____ _____ _____ _____
What kind of mutation is the above?:
•
•
Substitution or frameshift?
Silent, Nonsense or Missense
3. List the amino acid sequence that would result if
an insertion of G occurred between the 11th and
12th base
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_____ _____ _____ _____ _____ _____
Protein Synthesis Summary

Biozone: – Review of Protein Synthesis
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25 000 genes but more than
25 000 characteristics?
‘Alternative Splicing Theory’
394-395
How might one gene produce different proteins at
different stages of development and in different tissues?
Alternative splicing: involves the retention of some
introns and or juggling of exons in the prioduction of
mRNA resulting in the production of different proteins.
1. Intron retention:
2. Exon Juggling
Quick Check 9 - 13
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COMPARING PROKARYOTES &
EUKARYOTES
Similar with some differences:
Characteristic
Prokaryotes
I.E. Bacteria
Eukaryotes
Location of genes
Cytoplasm
nucleus
Survives only a
few minutes
Can survive for
days
Posttranscriptional
modification
Doesn’t occur
Occurs (introns cut
out, cap added,
poly A tail added
Effect of
tetracycline on
transcription
Very effective.
Prevents
transcription
No effect
age of mRNA
Chapter 11 Gene Function
(Nucleus protects!)
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Thalassaemia

Biozone– Inherited Metabolic Disorders

Case Study p397-399

Quick Check 14 - 15
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rRNA
A key player in Ribosomes
rRNA = Ribosomal RNA

Key component of ribosome structure

Stored in nucleus as a nucleolus

Need lots of it

Genes on short arms of chromosomes 13, 14, 15,
21 and 22 code for the production of rRNA

The loci of the above are called nucelar
organiser regions (NORs)

A secondary constriction or narrowing, marks the
position of NOR on each of these chromosomes
(see Figure 11.20
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
Structural and Regulator genes
-
Page 402
-
Genes vary in function:
-
Production of RNA
 All genes produce RNA of some kind (mRNA, rRNA,
tRNA!)
Structural Genes:
 produce proteins that become part of the structure
and function of organisms
Regulator Genes
 Produce proteins that control the action of other
genes
Example: Homeotic Genes in insects; HOX genes in
Mammals.
Actions of Regulator Genes
1.
Some produce DNA – Binding Proteins (directly
switch genes on or off)
2.
Some produce proteins that bind to receptor sites
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on membranes and function as ‘signalliny
proteins
TOPIC
DNA REPLICATION
11.3
Pgs: 403- 415
then
Pgs: 397 - 400
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DNA Replication

Defn: The process by which DNA makes exact copies
of itself is called DNA Replication

When: must occur prior to Interphase

Where: for Eukaryotes – nucleus

What Cells:



Somatic cells (prior to Mitosis)
Germ line cells (prior to meiosis)
Outcome:


Two Double stranded DNA molecules from each one
Each ‘new’ molecule consists on one strand of original DNA and
one copy! This is known as the semi-conservative model
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PROCESS OF DNA REPLICATION
Step One: Unwinding

DNA Unwinds to form a region of single stranded DNA

Unwinding controlled by enzyme called DNA helicase

Occurs at many points along the DNA
Step Two: Copying

Short RNA Primers attach to the open strands and initiate replication

Each ‘open’ strand acts as a template for a new DNA strand

Leading strand is formed one nucleotide at a time;

Individual DNA nucleotides are added at the 3’ end according to base
pairing rules

DNA polymerase is responsible for catalysing this reaction

Lagging strand formedby fragments (chuncks) aided by enzyme DNA
Ligase
Result
 Each new copy of DNA is an exact copy of the original
 Each newdouble stranded molecule contains one of the original DNA
strands and one new DNA strand (semi conservative)
Biozone: 207 - 208
Quick Check 18 & 19
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Chapter 11 Gene Function
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ACTIVE GENES
When are genes active?

Many genes remain active throughout the life of a
person, eg genes controlling enzymes for cellular
respiration.

Some genes are not expressed on a phenotype until
a person is well into adulthood. Eg. Huntington
disease.
Identifying active genes
Defn: Microarrays (DNA Arrays): technique used to
Recognise ‘active’ genes using single stranded DNA
Switching genes off!

Taken From Page 410

RNA interference (RNAi) provides a means of
selectively targeting and silencing genes.

Small interfering RNAs (siRNAs) produced in cells
are the active molecules in gene silencing.

How Does RNA Interference Work? p410

Case Study – Blue Roses at last! p411

Quick Check Questions 20-24 p412

Review Biozone – 203-204 Gene Expression
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Genes in Action Review

Biochallenge Questions 1-5 p413

Chapter Review Questions

Add Key Words to Glossary List

Complete Questions 2-14: p414-417
Chapter 11 Gene Function
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