Table of Contents
ELEMENT 6 – MOLECULAR GENETICS ................................................................................. 1
6.1: Eukaryotic Transcription & Regulation ................................................................................. 1
6.2: tRNA & the Genetic Code..................................................................................................... 4
6.3: Protein Synthesis and Inhibitors .......................................................................................... 5
6.4 & 6.5: Human Genome, Mutations, Genetic Diseases and Epigenetics .................................. 7
6.6 & 6.7: Recombinant DNA technology 1 & 2 ......................................................................... 10
6.9: Mitosis and Meiosis – Karyotypes ...................................................................................... 16
6.10: Abnormalities of Chromosomes ....................................................................................... 17
6.11: Introduction to Single Gene Disorders ............................................................................. 21
6.12: Sex Chromosomes and X-Linked Inheritance .................................................................... 22
6.13: Multifactorial (Polygenic) Inheritance .............................................................................. 23
6.14: Population Genetics......................................................................................................... 26
ELEMENT 6 – MOLECULAR GENETICS
6.1: Eukaryotic Transcription & Regulation
• What is a nucleosome? It’s the first structure in the
packaging of DNA in chromosomes
• How many nucleotides are there in the human genome? 3,200
million
• What is the structure of a nucleosome?
o DNA wrapped around histones
o 2 x H2A, 2 x H2B, 2 x H3, 2 x H4 histones in the core
o 1 x H1 histone, which acts as a clamp on the DNA
o The DNA that wraps around the core histones is called
‘core DNA’
o The DNA that joins up two nucleosomes is called linker
DNA (picture 1)
• Why are histones appropriate molecules to hold DNA
together? Because they’re positively charged and therefore attract
the negatively charged DNA molecules
• Why is the DNA negative? Because of the negatively charged
phosphates in the backbone.
• How are the nucleosomes arranged to create a chromatin? The
nucleosomes are wrapped together to form a solenoid, forming
the ‘chromatin filament’. (Picture 2)
• How does chromatin condense to form a chromosome before
division occurs? The solenoids form loops, each of which
attaches onto the central scaffold of the chromosome (picture
3)
• What 3 things have to happen to mRNA before its ready to leave
the nucleus?
1. Add a 5’ cap of methylated guanine – needed to allow mRNA to bind with ribosome (see
later)
, 2. Add a poly-A tail (added by poly-A-polymerase) – needed to add stability of mRNA in
cytoplasm and increase its lifespan (exonucleases continually chomp away removing
nucleotides one at a time so the longer the tail, the longer the lifespan)
3. Splicing has to occur – to remove the interrupting introns, which don’t code for anything.
• What’s the difference between pre-mRNA and mRNA? mRNA has undergone splicing
• What do RNA polymerases do? They make RNA molecules from the DNA template
• What are the 3 RNA polymerases in eukaryotes, what do they do and where are they found?
RNA polymerase What they make Where they’re found
I rRNA Nucleolus (makes sense because
ribosomes are made in the
nucleolus)
II mRNA Nucleoplasm
III tRNA Nucleoplasm
• In general terms, how does RNA polymerase II know where to bind on the DNA molecule? Because
of the core promoter region, which contains the TATAA box, CAAT/GC box and the basal
transcription apparatus (found within the TATAA box).
• What is the initiator region? It’s the region of DNA on the template strand, which helps define
where the +1 nucleotide is, since the TATAA box is variable in position.
• What is the TATAA box? It’s the same principle as prokaryotic transcription, where the weak T=A
bonds mean that the double helix is unwound here and the RNA polymerase attaches, with the help
of the basal transcription apparatus.
• What does the basal transcription apparatus do and why is it needed? It is a system of
transcription factors that help the RNA pol II know where to bind and start working. It is needed
because in eukaryotes, unlike in prokaryotes, RNA POLYMERASES DO NOT BIND DIRECTLY TO THE
DNA WITHOUT THE HELP OF TRANSCRIPTION FACTORS. Eukaryotic promoter regions are not
recognised by the RNA polymerases themselves, but by a number of transcription factors.
• Describe the basal transcription apparatus.
1. Transcription factor 2D recognises and binds to the
TATAA box
2. Transcription factor 2A stabilises transcription
factor 2D’s binding
3. Transcription factor 2B helps inform where +1 is
4. Transcription factor 2F recruits and binds RNA
polymerase II
5. Transcription factor 2E recruits transcription
factor 2H
6. Transcription factor 2H is a helicase, which
unwinds the DNA double helix and which also
phosphorylates the C-terminal tail of the RNA
polymerase II (kinase activity) allowing the RNA
polymerase to move away from the basal
transcription apparatus and start replicating.
Ø In order: D, A, B, F, Polymerase II, E, H
Ø Mnemonic: DAB For Paul’s
Efficient Healing
• What is the CAAT/GC box and what does it do? Its a
promoter region where more transcription factors bind to help stabilise the basal transcription
apparatus, and is usually present in ‘housekeeping genes’ (these are genes present in every cell that
make proteins that do boring maintenance work for the cell)
,• Do all protein-coding genes have TATAA boxes and/or GC&CAAT boxes? No, and hence why we still
do not know how many protein coding genes we have, because to figure out how many we have we
use enzymes which recognise theses boxes.
• What feature of the eukaryotic
promoter region is present when the cell
needs high levels of transcription? You
get other transcription factors about
1000bps downstream that bind to their
specific binding positions. The whole thing
then folds over and allows interactions
between this region and the core
promoter region (with the BTA in it), to
allow much more rapid, efficient
transcription. This involves a mediator
complex to facilitate the interaction
between the two (in pale blue)
• What feature confers cell specificity, as
well as helping further with
trancription? Enhancer regions
• What are enhancer regions? These are a series of nucleotide sequences, that if present, are
recognised by transcription factors that bind to it, making the whole thing fold over onto the
promoter region, helping it.
• How do enhancer regions confer cell specificity, with the exmaple of haemoglobin expression at
different stages in life? The enhancer regions regulate multiple genes, by allowing one to be
expressed at a time, depending on which one it interacts with. As a foetus, foetal haemoglobin is
expressed as the enhancer region interacts with the ε gene. As a baby, infant haemoglobin is
expressed as the enhancer region interacts with the γ gene. As an adult, adult haemoglobin is
expressed as the enhancer region interacts with the β gene.
• How do you get tissue specific gene regulation? Through the methylation of DNA molecules (usually
cytosine). This stops the appropriate transcription factor from binding. If the DNA is methylated
differently in different tissues, you have tissue-specific regulation
• What is imprinting? It’s the process by which these methylation patterns are laid down in the DNA
of different tissues as an embryo.
• What happens to these patterns during cancers? They get messed up à not regulated
, • What process is responsible for the change of
DNA being in the dense chromosome formation to
becoming loose chromatin? Acetylation of
histone tails by acetyl transferase enzymes. This
reduces the positive charge on the histones and
therefore weakens the bond between the DNA and
histones, so the solenoid structure disappears.
• What are control elements? They are little
sections of DNA that lie just before the enhancer
regions and before the promoter regions, where
hormone/receptor complexes bind and regulate
transcription.
• What is the mechanism of steroid hormone
aciton? See below
6.2: tRNA & the Genetic Code
• Why is the genetic code only ‘almost’ universal? Because chloroplasts and mitochondria have slight
deviations from the norm
• What are codons that specify the same amino acid called? Synonyms. For example, UAU and UAC
both code for Tyrosine, so they are synonyms of each other.
• On which side of the tRNA molecule is the amino acid attachment site? The 3’ end
• What modified bases are there on the tRNA molecule?
1. Dihydrouracil (UH2) – found on the dihydrouracil loop
2. Thymine (odd for RNA) – found on the thymine-pseudouracil-cytosine loop
3. Pseudouracil (Ψ) - found on the thymine-pseudouracil-cytosine loop
4. Inosine (I) – found in the anticodon, increasing the number of codons a tRNA can recognise
• What is inosine? A modified adenine
• What do these modified bases ensure? They make sure the right amino-acyl tRNA synthetase
enzyme binds to the tRNA, and the inosine increases the number of codons a tRNA can recognise.
• What do amino-acyl tRNA synthetase enzymes do? They attach the amino acid to the amino acid
attachment site, activating it. This converts ATP à AMP, as it’s a very high-energy reaction, forming
a high-energy bond.
• What’s an aminoacyl tRNA? A tRNA molecule with an
activated amino acid attached
• What is the wobble hypothesis? It’s the hypothesis dictating
how tRNA molecules bind to their codons. It ensures
there is specificity even though there isn’t one tRNA per
codon (i.e. some tRNA molecules bind to numerous different
codons, but each of these codons will code for the same
amino acid). It states that:
