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DNA Replication/Protein
Synthesis
Topic 2.7
DNA replication
• DNA molecule is unzipped down the middle
by an enzyme Helicase
• Helicase: straightens out the helix and breaks
the hydrogen bonds between nitrogenous
bases
• DNA Ploymerase inserts new nucleotides thus
formimg new complementary strands of DNA
• DNA replication is semi-conservative (one side
of the strand is “conserved”)
DNA Replication
DNA Replication
DNA Replication
DNA Replication
• Meselson and Stahl’s experiment showing
semi-conservative replication.
• Used radioactive isotopes (N15 instead of N14)
• N15 is heavier than N14
• Used density gradient centrifugation to
separate generations of DNA
• Results support semi-conservative replication
Meselson and Stahl
Meselson and Stahl
Protein Synthesis
• DNA provides the instructions for making specific proteins.
• The bridge between DNA and protein synthesis is RNA.
RNA – What is it?
•RNA – Ribonucleic Acid
1. Has ribose sugar instead of
deoxyribose (DNA)
2. Replaces thymine (T) with
uracil (U)
3. Single stranded as opposed to
double stranded DNA
RNA vs DNA
DNA
RNA
5 carbon sugar
5 carbon sugar
Sugar is deoxyribose
Sugar is ribose
Code consists of four different
N-bases
Code consists of four different
N-bases
Cytosine, Thymine, Adenine,
Guanine
Cytosine, Uracil, Adenine,
Guanine
Double stranded molecule
Single stranded molecule
RNA – What does it do?
• RNA has a different function than DNA.
• Whereas DNA provides the instructions for
protein synthesis, RNA does the actual
work of protein synthesis.
• RNA takes from DNA the instructions on
how the protein should be assembled,
then—amino acid by amino acid—RNA
assembles the protein.
Types of RNA
1. Messenger RNA – mRNA
2. Ribosomal RNA – rRNA
3. Transfer RNA - tRNA
mRNA
• Messenger RNA (mRNA)
• Reads the DNA code
inside the nucleus.
• Carries the instructions
to the rest of the cell.
rRna
• Ribosomal RNA (rRNA)
• Contained in the
ribosomes.
• Helps with the assembly of
proteins. Binds to the
mRNA and uses the
instructions to assemble
the amino acids in the
correct order.
Transfer RNA (continued)
• A tRNA molecule consists of a strand of about 80 nucleotides that folds
back on itself to form a three-dimensional structure.
– It includes a loop containing the anticodon and an attachment site at the 3’
end for an amino acid.
tRNA
• Transfer RNA (tRNA)
• Transfers amino acids to ribosomes for protein
assembly.
• Single, folded strand that delivers the proper
amino acid to the site at the right time.
Transfer RNA
• Like other types of RNA, tRNA molecules are
transcribed from DNA templates in the nucleus.
• Once it reaches the cytoplasm, each tRNA is used
repeatedly
• to pick up its designated amino acid in the cytosol (cytoplasm),
• to deposit the amino acid at the ribosome,
• to return to the cytosol and pick up another copy of that amino
acid.
Protein Synthesis (continued)
• There are two parts to protein synthesis.
– Transcription
• Copying DNA in the form of RNA
• Occurs in the Nucleus
– Translation
• Read the RNA and construct the protein.
• Occurs in the Cytoplasm
– In prokaryotes this process happens entirely in the
cytoplasm as there is no nucleus.
Transcription
• During transcription, a DNA strand provides a template for
the synthesis of a complementary RNA strand.
– This process is used to synthesize any type of RNA
from a DNA template.
• Transcription of a gene produces a messenger RNA (mRNA)
molecule.
• During transcription, one DNA strand, the template strand,
provides a template for ordering the sequence of nucleotides in
an RNA transcript
– The complementary RNA
molecule is synthesized
according to base-pairing
rules, except that uracil is
the complementary base
to adenine.
Fig. 17.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Building Amino Acids
• The genetic instructions for a polypeptide chain are
written in DNA as a series of three-nucleotide words.
• Triplets of nucleotide bases are the smallest units of
uniform length that can code for all the amino acids.
• Each triplet is called a codon
• In the triplet code, three consecutive bases specify an
amino acid, creating 43 (64) possible code words.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Each codon specifies which one of the 20 amino acids will be
incorporated at the corresponding position along a polypeptide.
• Because codons are base triplets, the number of nucleotides
making up a genetic message must be three times the number of
amino acids making up the protein product.
– It would take at least 300 nucleotides to code for a
polypeptide that is 100 amino acids long.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Codons / Amino Acides
– 61 of 64 triplets code
for amino acids.
– The codon AUG not
only codes for the
amino acid methionine
but also indicates the
start of translation.
– Three codons do
not indicate amino
acids but signal
the termination
of translation.
Translation
• Translations is the RNA-directed
synthesis of a polypeptide
• In the process of translation, a
cell interprets a series of codons
along a mRNA molecule.
• Transfer RNA (tRNA)
transfers amino acids from the
cytoplasm’s pool to a ribosome.
• The ribosome adds each
amino acid carried by tRNA
to the growing end of the
polypeptide chain.