3. Replication
• Makes duplicate copies of DNA in the
process of replication
• DNA replication involves separating the
DNA molecule into 2 strands.
• Each strand than serves as a pattern, or
template, to make a new DNA molecule.
• The result is 2 identical double-stranded
molecules of DNA
4. Replication (con’t)
• Because each of these double-stranded
molecules of DNA consists of a single
strand of old DNA (the template) and a
single strand of new, replicated DNA (the
complement), this is called semi-
conservative replication.
5. Steps
1. Helicase unwinds DNA, creating a replication
fork. Proteins bind to the single strand to keep
the DNA from recombining.
2. RNA primase starts replication at special base
sequences by adding RNA primers.
3. DNA polymerase attaches to the RNA primers
and begins elongation. (Elongation = the adding
of DNA nucleotides to the complement strand)
6. Steps (con’t)
4. The leading complementary strand is assembled
continuously as the DNA uncoils.
5. The lagging complementary strand is assembled
in short pieces called Okazaki fragments. The
pieces are joined together by DNA ligase.
6. The RNA primers are replaced by DNA
nucleotides.
8. Protein Synthesis
• Proteins can function as:
– Enzymes
– Repairing agents for the cell
• Proteins determine the shape and
function of organisms.
• Each gene is responsible for one protein.
So, the message of the DNA code is
information for building proteins.
9. Protein Synthesis (con’t)
• How do genes code for proteins? THE
GENETIC CODE!!!
• Genetic code: sequence of nitrogen bases
along one of the two strands that codes
for the synthesis of proteins.
• The nitrogen bases set up in sets of 3 that
are known as codons.
10. Protein Synthesis (con’t)
• Because the sequence of 3 nitrogen bases forms
the code for an amino acid, the DNA code is
often called the triplet code.
• 64 combinations are possible when a sequence
of 3 bases is used. So, 64 different codons are
in the genetic code.
• 61 of the codons code for amino acids, and the
other 3 code for protein synthesis to start or
stop.
11. Protein Synthesis (con’t)
• More than one codon can code for an amino
acid; however, for any one codon, there can
only be one amino acid.
• The genetic code is universal. Universal means
that the codons represent the same amino acids
in all organisms.
• Protein synthesis: process that describes how
enzymes and other proteins are made from
DNA.
12. Protein Synthesis (con’t)
• Steps of Protein Synthesis:
– Transcription
– RNA processing
– Translation
• There are 3 types of RNA:
– mRNA (messenger): provides the instructions for
assembling amino acids into a polypeptide.
– tRNA (transfer): delivers amino acids to a ribosome
for their addition into a growing polypeptide chain
– rRNA (ribosomal): combines with proteins to form
ribosomes
13. Transcription:
From DNA to RNA
RNA is transcribed from a DNA
template by a process that
resembles DNA replication.
14. Steps of Transcription:
• Initiation
– The “start transcribing” signal is a
nucleotide sequence called the promoter,
located in the DNA at the beginning of the
gene. Promoter is a specific place where
RNA polymerase attaches.
– RNA polymerase attaches to the promoter
– Start of RNA synthesis
15. Steps of Transcription (con’t)
• RNA Elongation
– During the 2nd
phase of transcription,
elongation, the RNA grows longer.
– As RNA synthesis continues, the RNA peels
away from DNA and allows the DNA strand
to rejoin.
16. • Termination
– The 3rd
and final phase, RNA polymerase
reaches the terminator sequence of bases in
DNA.
– This sequence signals the end of the gene and
the polymerase molecules detaches from the
RNA molecule and the gene.
Steps of Transcription (con’t)
17. RNA Processing
The RNA copy that is formed is called
mRNA.
The mRNA carries the information for
making the protein chain to the ribosomes,
where it goes through RNA processing.
18. Translation
Conversion between different languages –
from nucleic acid to protein.
Occurs on the ribosomes and uses tRNA.
End product is a very long protein chain.
19. tRNA
• Composed of about 75 nucleotides
• Each tRNA recognizes only one amino
acid.
• The amino acid becomes bonded to the
top of the tRNA.
• Located on the bottom of the tRNA are 3
nitrogenous bases, called an anticodon,
which pairs with mRNA codons during
translation.
20. Steps of Translation
• Initiation
– 1st
phase brings together:
• mRNA
• 1st
amino acid with its attached tRNA, and
• The 2 subunits of a ribosome
21. Steps of Translation (con’t)
• Elongation
– Amino acids are added one by one in a 3-step
process
• Step 1
– Codon recognition
• Step 2
– Peptide bond formation
• Step 3
– Translocation
22. • Termination
– Elongation continues until a stop codon
(UAA, UAG, or UGA) is reached.
– Polypeptide is released.
– Ribosome splits into subunits and falls off.
Steps of Translation (con’t)
