Quick recap!
 What can you remember from
before you exam?
The Central Dogma
 All organisms, from the simplest bacteria to
ourselves, use the same basic mechanisms of
reading and expressing genes – so fundamental
to life it is known as the “Central Dogma”.
rRNA – Ribosomal RNA
 Is the type of RNA found in ribosomes, during
protein synthesis it provides the site where
polypeptides are assembles.
mRNA- messenger RNA
 mRNA – long single strand, is a mirror copy of one
of the two strands, it is small enough to leave the
nucleus, via the nuclear pores, it acts as a
template for proteins synthesis, it is adapted for
its function by being specific for each protein and
can easily be broken down so is only manufactured
when the protein is needed.
tRNA- transfer RNA
 tRNA – small’ish ~ 80 bases, it is single stranded
folded into a clover leaf shape, with one end
extended further creating the point of
attachment for amino acids.
 There are several different types of tRNA each
able to carry a single amino acid.
 At the opposite end of the tRNA is the anticodon,
this is used to line up the amino acids during
protein synthesis.
Comparing DNA, mRNA and tRNA
DNA
mRNA
tRNA
Double polynucleotide
chain
Single polynucleotide
chain
Single polynucleotide
chain
Large
Medium
Small
Double helix
Single helix
Clover leaf shaped
Deoxyribose
Ribose
Ribose
A, T, C & G
A, C, G & U
A, C, G & U
Found in nucleus
Manufactured in the
nucleus, but found
through out the cell.
Manufactured in the
nucleus, but found
through out the cell.
Quantity is constant in a
cells for a species
(except....)
Varies from cell to cell
and with metabolic
activity
Varies from cell to cell
and with metabolic
activity
Stable
Unstable
In- between stability!
Protein synthesis - overview
 Transcription – production of pre-mRNA
 Splicing – removal of introns from the premRNAto make mRNA
 Translocation – mRNA moving out of the
nucleus
 Translation – where mRNA is used as a
template to which complementary tRNA
molecules attached to amino acids, start to
form a polypeptide chain with the help of a
ribosome.
Transcription in detail
Transcription the steps!
 Task – Using page 224 and you imagination
draw a cartoon strip to show how mRNA is
transcribed.
Posttranscriptional
Modifications
 5’cap – Transcription usually begins with an A or a
G, in eukaryotes the terminal 5’ A or G is removed
then a 5’ – 5’ linkage forms with GTP – this is
called a 5’cap. It protects the 5’ end from
nucleases and phoshatases on its journey out of
the nucleus.
 3’ poly-A tail – At the 3’ end of the mRNA a
special enzyme known as poly-A polymerase adds
about 250 A ribonucleiotides on, again this
protects the mRNA from nucleases.
Posttranscriptional
Modifications
 DNA is made up of sections of exons which code
for proteins an introns which do not.
 If left the introns could disrupt protein
synthesis.
 In eukaryotic cells the introns are removed and
the exons are joined together in a process known
as splicing.
 NOTE: - After the introns have been removed the
exons can be joined together in a variety of
combinations – this means that a single gene can
code for up to a dozen different proteins.
Important point  Mutations can affect the splicing of premRNA
 Certain disorders such as Alzheimer’s are a
result of an error in splicing – why could this
be a problem?
Task
 Add splicing on to your cartoon
strip.
Amino acid activation
 Before amino acids can be used for protein synthesis,
each amino acid has to be linked to a specific tRNA.
 Specific enzymes will only link a particular amino acid
to a tRNA with a particular anticodon.
 This process takes place in the cytoplasm.
Translation in detail
 http://www-
class.unl.edu/biochem/gp2/m_biology/animation/gen
e/gene_a3.html
 http://vcell.ndsu.edu/animations/translation/movieflash.htm
 http://highered.mcgrawhill.com/olc/dl/120077/micro06.swf
Task
 Using page 226 add translation on to your cartoon
strip.
Final stage –
Assembling a protein
 The steps following translation depend on the
protein being made but usually consist of:
 Coiling or folding into α – helix or β – pleated
sheets, into its secondary structure.
 Folding of the secondary structure into a 3-D shape
of the tertiary structure.
 Sometimes the addition of different polypeptide
chains and non-proteins groups to make its
quaternary structure.
Application stuff!
 You have to be able to interpret data from
experimental work investigating the role of
nucleic acids.
 Answer all of the HSW questions from page
228 – 229.
Homework
 Work through the Exam style
questions on page 234 - 235