Biology

1. Central Dogma

2. Maintenance and expression of genetic information
Central Dogma:
DNA RNA Protein

3. Review
• Nucleic Acids
• Structure of DNA

4. 4
Nucleotides
Nucleic acids consist of nucleotides that have a
sugar, nitrogen base, and phosphate
nucleoside
Sugar
Base
PO4

5. Nitrogen-Containing Bases
N
N
N
N
H
NH2
N
N
O
CH3
O
H
H
N
N
N
N
O
H
NH2
H
N
N
NH2
CH3
O
H
N
N
O
CH3
O
H
H
adenine (A) thymine (T)
guanine (G) cytosine (C) uracil (U)

6. Sugars
O OHCH2
OHOH
HO HO O OHCH2
OH
ribose deoxyribose
(no O)

8. Nucleosides in DNA
Base Sugar Nucleoside
Adenine (A) Deoxyribose Adenosine
Guanine (G) Deoxyribose Guanosine
Cytosine (C) Deoxyribose Cytidine
Thymine (T) Deoxyribose Thymidine

9. Nucleosides in RNA
Base Sugar Nucleoside
Adenine (A) ribose Adenosine
Guanine (G) ribose Guanosine
Cytosine (C) ribose Cytidine
Uracil (U) ribose Uridine

10. Nucleotides in DNA and RNA
DNA
dAMP Deoxyadenosine monophosphate
dGMP Deoxyguanosine monophosphate
dCMP Deoxycytidine monophosphate
dTMP Deoxythymidine monophosphate
RNA
AMP adenosine monophosphate
GMP guanosine monophosphate
CMP cytidine monophosphate
UMP uridine monophosphate

11. Structure of Nucleic Acids
 Polymers of four nucleotides
 Linked by alternating sugar-phosphate bonds
 RNA: ribose and A, G, C, U
 DNA: deoxyribose and A,G,C,T
nucleotide nucleotide nucleotide nucleotide
P sugar
base
P sugar
base
P sugar
base
P sugar
base

12. Nucleic Acid Structure
3,5-phosphodiester bond
O
N
N
NH2
O
CH2OP
O
O
-
O
-
OH
O
N
N
NH2
CH2OP
O
O-
OH
O
N
N
AMP
CMP
3
5

13. Discovering the structure of DNA
Structure was discovered in 1953 by James
Watson and Francis Crick

14. DNA Replication
• DNA in the chromosomes replicates itself
every cell division
• Maintains correct genetic information
• Two strands of DNA unwind
• Each strand acts like a template
• New bases pair with their complementary
base
• Two double helixes form that are copies of
original DNA

15. 15
DNA Unwinds
G- -C
A- -T
C- -G
T- -A
G-C
A-T
C-G
T-A

16. 16
DNA Copied with Base Pairs
Two copies of original DNA strand
G-C G-C
A-T A-T
C-G C-G
T-A G-A

17. b

31. TRANSCRIPTION
• Refers to the transfer of the genetic code from
a molecule of DNA to an intermediary
molecule called ribonucleic acid (RNA).
• involves the production of a special kind of
RNA known as messenger RNA (mRNA).

32. Steps in Transcription
• The process begins when the two strands of a
DNA molecule separate, a task directed by the
enzyme RNA polymerase.
• After the double helix splits apart, one of the
strands serves as a template, or pattern, for the
formation of a complementary mRNA molecule.
• Free-floating individual bases within the cell bind
to the bases on the DNA template using
complementary base pairing.
• The individual bases then link together to form a
strand of mRNA.

33. • In eukaryotes, the mRNA strand undergoes an
additional step before the next stage of protein
synthesis can occur.
• The mRNA strand consists of coding regions
called exons separated by regions called introns.
• The introns do not contribute to protein
synthesis.
• Special enzymes in the nucleus remove the
introns from the mRNA strand.
• The remaining exons then link together to form
an mRNA strand that contains the entire code for
making a protein.

34. • Once transcription is complete and the
genetic code has been copied onto mRNA, the
genetic code must be converted into the
language of proteins.
• That is, the information coded in the four
bases found in mRNA must be translated into
the instructions encoded by the 20 amino
acids used in the formation of proteins.
• This process is called translation.

36. TRANSLATION
• takes place in cellular organelles called ribosomes
• In eukaryotes, mRNA travels out of the nucleus
into the cell body to attach to a ribosome.
• In prokaryotes, the ribosome clasps mRNA and
starts translation before these strands have
finished transcription and separated from the
DNA.
• In both eukaryotes and prokaryotes, the
ribosome acts like a workbench and clamp that
holds the mRNA strand and coordinates the
activity of enzymes and other molecules essential
to translation.

37. Steps in Translation
• A tRNA with an attached methionine binds to
the small ribosomal sub-unit.
• The initiation complex binds to an mRNA
molecule. The first codon bound is always the
start codon.
• The large ribosomal sub-unit binds to the
small ribosomal sub-unit.
• The methionine tRNA binds to the P site of the
large sub-unit.

38. • The second codon base pairs with the anti-
codon of a tRNA molecule, which enters the A
site of the large sub-unit.
• The catalytic site on the large sub-unit
catalyzes the formation of a peptide bond
between the amino acids using the energy
stored in the tRNA meth bond. The dipeptide
remains attached to the second tRNA.
• The start codon drops off the ribosome.

39. • The ribosome moves one codon to the right
on the mRNA. The tRNA bearing the newly
formed dipeptide moves to the P site and the
A site is emptied.
• The next tRNA base pairs with the third codon
and moves into the A site.
• A peptide bond is formed between the
dipeptide and the new amino acid, forming a
tripeptide that remains attached to the third
codon.

40. • This process repeats until a “stop” codon is
reached. The finish peptide is released from
the ribosome.
• The mRNA is released from the ribosome.
• The ribosomal sub-units separate.