Download DNA and proteins

The nature of the genetic code
Objectives
Describe and explain
gene expression
Starter
• In pairs draw a mind map to link
the following key terms:
• Allele, base sequence, Chromosome,
DNA, Gene, Genetic code, mRNA,
Nucleus, Nuclear pore, Protein,
Ribosome, RNA,
One of Life’s big questions
What is the genetic code?
DNA and the genetic code
• DNA contains the genetic code which
codes for the synthesis of all
polypeptides
• A gene is a sequence of DNA that codes
for the synthesis of a specific
polypeptide
• It is contained in the nucleus so that the
code is conserved / protected
• Polypeptides are chains of amino acid residues
joined by peptide bonds.
• There are 20 different aa and their sequence
determines the structure and function of the
protein.
• The sequence of bases in a DNA molecule
determines the sequence of aa.
• A genome is the entire sequence of DNA of an
organism (about 25000 genes in the human
genome).
• Each gene occupies a specific locus (position) on a
chromosome and each chromosome consists of one
molecule of DNA.
• The DNA is wrapped around basic histone proteins
(Chromatin)
• In between genes is
non coding DNA
Genes code for polypeptides such as:
Structural proteins (keratin, collagen,
actin, myosin)
Enzymes
electron carriers
Haemoglobin
Antigens, immunoglobulins
Cell surface receptors
Channel proteins
The Genetic Code
Features of the Genetic Code
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•
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It is a triplet code
It is degenerate
It has punctuation
It is widespread but not universal
It is a non overlapping code
The genetic
Code
The Central Dogma of Molecular Biology
Replication
DNA
Transcription
mRNA
Translation
Protein
Principles of Protein synthesis
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DNA stays in the nucleus
Proteins are made on ribosomes in the cytoplasm
SO
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The genetic code must be copied to messenger
RNA (mRNA)
mRNA leaves the nucleus via the nuclear pores
taking the code to the ribosomes
Gene expression occurs in two steps
1. Transcription – the code on part of a
DNA molecule is copied to a mRNA
molecule.
2.Translation – the mRNA takes the code
to a ribosome where it is used to make a
polypeptide.
Transcription
• The process by which the base sequence of a
gene is converted into a complimentary base
sequence of mRNA
• One strand of the DNA (template strand) is
used to make mRNA the other stand is the
coding strand.
• Only part of a DNA molecule is transcribed
at one time.
Transcription
Transcription
• The part of the DNA molecule to be transcribed
unwinds and ‘unzips’ as DNA helicase breaks the H
bonds between the bases
• RNA polymerase catalyses the binding of activated
free RNA nucleotides to the template
• Uracil binds to adenine NOT thymine
• The nucleotides condense together forming
phosphodiester bonds
• The mRNA is complimentary to the template strand
and a copy of the coding strand
mRNA and
the Genetic
Code