Download DNA, Transcription and Translation

CONTINUITY AND
CHANGE
DEOXYRIBONUCLEIC ACID,
(DNA).
DNA is the genetic material of all cellular organisms. In
eukaryotes it is found in the nucleus. However, there is
some DNA in the mitochondria of animal and plants cells and
the chloroplasts of plant cells.
• The Building Blocks.
• DNA is a macro-molecule, (large) which is made up of a
series of chemical building blocks called nucleotides. Each
nucleotide consists of 3 very different and separate
components:
• a phosphate group (P),
• a five-carbon sugar, (S), (deoxyribose),
• and one of four nitrogen-containing bases: adenine (A),
guanine (G), thymine (T) and cytosine (C).
Bases
The 4 bases can be classified into 2 groups.
• The purines, which have a double ring structure. These are
adenine & guanine.
Adenine
Guanine
• The pyrimidines, which have a single ring. These are thymine
and cytosine.
Cytosine
Thymine
Uracil
DNA - Nucleotides
Nucleotides join together to form a polynucleotide chain in
which consecutive nucleotides are attached by
phosphodiester bonds, (a type of covalent bond) between
the number 3 carbon in the sugar of one nucleotide and the
phosphate of the next.
Nucleotides joined
Nucleotide Structure
DNA – Nucleotides con’t
DNA is composed of two chains of nucleotides linked together in a ladderlike arrangement with the sides composed of alternating deoxyribose sugar
and phosphate groups and the rungs being the nitrogenous bases as
indicated by the diagram below.
The two strands are held together by hydrogen bonds (electrostatic
attraction). Two hydrogen bonds hold adenine to thymine. Three bonds
attach cytosine to guanine as indicated in the diagram above.
Note:
* The 2 ends of the strands
are called the 5’ & 3’ ends.
The 5’ has a free phosphate
and the 3’ has a free hydroxyl
group.
Hydrogen
bonds
* The DNA strand is referred
to as antiparallel, meaning
one strand runs 5’ to 3’ and
the other 3’ to 5’.
DNA Structure
Chromosomes
• Chromosomes are found in the
nucleus. They consist of DNA and
therefore, carry the genes. They
change shape at various stages of
the cell’s life.
• The DNA molecule is very long. In a
human nucleus of 6μm, the DNA will
be 1.8m long.
• In eukaryotes the DNA is coiled
around proteins called histones.
When the DNA is coiled it is called
a nucleosome.
• Note: the coiling protects the DNA
and gives it the appearance of a
string of beads. When the cell is
about to divide the chromosomes
condense. The nucleosome folds in a
regular manner producing supercoils.
DNA REPLICATION
Refer to the following websites:
http://207.207.4.198/pub/flash/24/menu.swf
Refer to the second one.
http://www.wiley.com/college/pratt/047139387
8/student/animations/dna_replication/index.
html
Transcription
•
•
•
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Transcription = rewriting or copying of information
from DNA to ribonucleic acid ( RNA).
Occurs in the nucleus.
RNA is a single stranded molecule containing the
sugar ribose, phosphate and the bases Adenine,
Guanine, Cytosine and Uracil.
The synthesised RNA is complementary to one strand
of DNA. In RNA Thymine is replaced by Uracil.
The RNA molecule produced is called messenger
RNA (mRNA), because it acts as a messenger,
carrying the copy of the code from the nucleus into the
cytoplasm.
Transcription con’t
A Typical Gene.
Promoter
Exon
1
Intron
1
Exon
2
Intron
2
Exon
3
Intron
3
Exon
4
Messenger RNA synthesis is controlled by RNA
polymerase. A typical gene consists of:
• A Promoter Region – it has a specific
sequence recognised by RNA polymerase that
initiates transcription.
• Exons – the coding segments of the gene.
• Introns – the non-coding segments of the
gene.
Transcription con’t
Synthesis
• RNA polymerase moves along the template strand of DNA,
adding nucleotides to the growing RNA strand.
Note: RNA polymerase reads the template strand from the
5’ end to the 3’ end but builds the new mRNA molecule from
it’s 3’ end to 5’ end.
• The RNA molecule produced is called the Primary
Transcript.
• It is then modified by enzymes that cut out the introns and
join the remaining pieces together, forming the shortened
mRNA molecule.
• The RNA molecule is complete when the ends are chemically
modified. It is chemically capped (a Methyl Cap) at the 5’
end and a tail of Adenines - A’s, (a poly - A tail) is added to
the 3’ end.
• The mRNA the leaves the nucleus, entering the cytoplasm.
Translation
Translation = Translation refers to the reading
of the instructions contained in the mRNA and
the assembly of the polypeptide product.
• mRNA travels into the cytoplasm, where a ribosome
attaches to it. Once the ribosome has attached to the
mRNA molecule, translation begins and the mRNA molecule
is read in the 5’ to 3’ direction.
• The information contained in the mRNA for protein
synthesis is read in a series of 3 base or triplet codes called
CODONS. The sequence of codons code for the sequence
amino acids that make up the polypeptide. Included in these
3 base codes are start and stop codons.
• The codon AUG codes for Methionine and is use to start
translation.
• The codons UAA, UAG and UGA are 3 codons that signal
stop, hence no more amino acids are added to the
polypeptide.
Translation con’t
• There are 20 amino acids commonly found in organisms.
• Of the 20 there are only 2 that are specified for by only 1
codon, (Tryptophan – UGG and Methionine – AUG).
• The other 18 can be specified for by 2 or more codons, (eg.
CCU, CCC, CCA & CCG all specify for Proline)
Base sequence in
DNA template strand
GCT
CGA
AAA
TTT
TCC
AGG
TAG
ATC
GCU
AAA
UCC
UAG
Ala
Lys
Ser
Stop
Transcription
(copying)
Base sequence in mRNA
Translation
(change language)
Amino acid sequence in
Polypeptide
translation
Translation con’t
• Transfer RNA, (tRNA), are short singlestranded molecules approx. 80 nucleotides
long, which fold to form a cloverleaf shaped
structure. There are many different types
of tRNA.
• tRNA acts as transport molecules, bringing
the correct amino acid to the ribosome for
assembly into the polypeptide. To be able to
do this there must be 2 binding sites on the
tRNA molecule.
1) The binding site that binds to the codon,
known as the anticodon where the
nucleotides on the tRNA molecule are
complementary to the mRNA, and
2) The amino acid attachment site where
the amino acid binds to the tRNA molecule
for transport to the ribosome.
tRNA Molecule