1. Chapter 25, 26: Nucleotides, Nucleic Acids,
Heredity, Gene
Protein Synthesis
expression, and
Chem 104
K. Dunlap

2. The Molecules of Heredity
– Each cell of our bodies contains thousands of different
proteins.
– How do cells know which proteins to synthesize out of the
extremely large number of possible amino acid sequences?
– From the end of the 19th century, biologists suspected that
the transmission of hereditary information took place in the
nucleus, more specifically in structures called
chromosomes.
– The hereditary information was thought to reside in genes
within the chromosomes.
– Chemical analysis of nuclei showed chromosomes are
made up largely of proteins called histones and nucleic
acids.

3. The Molecules of Heredity
– By the 1940s, it became clear that
deoxyribonucleic acids (DNA) carry the hereditary
information.
– Other work in the 1940s demonstrated that each
gene controls the manufacture of one protein.
– Thus the expression of a gene in terms of an
enzyme protein led to the study of protein
synthesis and its control.

4. Nucleic Acids
• There are two kinds of nucleic acids in cells:
1) ribonucleic acids (RNA)
2) deoxyribonucleic acids (DNA)
• Both RNA and DNA are polymers built from
monomers called nucleotides.
• A nucleotide is composed of:
– a base, a monosaccharide, and a phosphate.

5. Nucleic Acids
• made up of nucleotides
• found in all living cells except
RBC
• deoxyribonucleic acid (DNA)
and ribonucleic acid (RNA)
• DNA is in the nucleus
• RNA is in the cytoplasm
• function in the storage and
transmission of genetic material
•And control and direct all
protein synthesis

6. -each nucleotide contains
1) a sugar
2) a base
3) phosphoric acid unit
1) Ribose and Deoxyribose
RNA
DNA

7. Bases in Nucleic Acids
Bases for DNA:
A, G, C, T
Bases for RNA:
A, G, C, U

8. Pyrimidine/Purine Bases

9. Formation of a Nucleotide
• Alternating phosphate, sugar molecules form the backbone
• The rxn between phosphate and sugar forms an ester bond
with the elimination of water
• The sugar bonds with a base, forming tertiary amine, with the
elimination of water

10. DNA and
RNA Strand
• The sequence of the
bases in DNA or RNA form
the primary structure
DNA

11. Structure of DNA and RNA

13. DNA - 2° Structure
• Secondary structure: the ordered arrangement
of nucleic acid strands.
– the double helix model of DNA 2° structure was
proposed by James Watson and Francis Crick in
1953.
• Double helix: a type of 2° structure of DNA in
which two polynucleotide strands are coiled
around each other in a screw-like fashion.

14. Watson and Crick

15. The DNA Double Helix
• Figure 25.4
Threedimensional
structure of
a DNA
double helix.

16. • Like a spiral staircase:
-the phosphate
sugar backbone
represents the
hand rail, the
bases represent
the steps
• Hydrogen bonding
occurs between the
bases…..
For DNA:
A bonds with T
C bonds with G
For RNA:
A bonds with U
C bonds with G
DNA Double Helix

17. Base Pairing

18. Higher Structure of DNA
– DNA is coiled around proteins called histones.
– Histones are rich in the basic amino acids Lys and
Arg, whose side chains have a positive charge.
– The negatively-charged DNA molecules and
positively-charged histones attract each other and
form units called nucleosomes.
– Nucleosome: a core of eight histone molecules
around which the DNA helix is wrapped.
– Nucleosomes are further condensed into
chromatin.
– Chromatin fibers are organized into loops, and the
loops into the bands that provide the
superstructure of chromosomes.

19. Chromosomes

20. Chromosomes

21. Chromosome

23. Chromosomes
• To accommodate the large
amount of genetic material,
DNA in the nuclei of cells is
coiled around proteins called
Histones

24. Micrograph of a chromosome

25. DNA and RNA
• The three differences in structure between
DNA and RNA are:
– DNA bases are A, G, C, and T; the RNA bases are
A, G, C, and U.
– The sugar in DNA is deoxyribose; in RNA it is
ribose.
– DNA is always double stranded; there are several
kinds of RNA, most of which are single-stranded.

26. RNA
• RNA molecules are classified according to
their structure and function.

27. Genes, Exons, and Introns
Gene: A segment of DNA that carries a base
sequence that directs the synthesis of a
particular protein, tRNA, or mRNA.
– There are many genes in one DNA molecule.
Exon: A section of DNA that, when
transcribed, codes for a protein or RNA.
Intron: A section of DNA that does not code for
anything functional.

28. Structure of tRNA

29. Replication of DNA
The DNA in the chromosomes carries
out two functions:
– (1) It reproduces itself. This process is
called replication.
– (2) It supplies the information necessary
to make all the RNA and proteins in the
body, including enzymes.
Replication begins at a point in the DNA
called the origin of replication or a
replication fork.

30. The central dogma of molecular biology:
– Information contained in DNA molecules is
expressed in the structure of proteins.
– Gene expression is the turning on or activation of a
gene.
DNA
replication
RNA
replication
DNA
mRNA
Transcription
Reverse transcriptase
Translation
prote in

31. DNA
Replication
• The two strands of DNA in the helix
are complementary
• When ready to replicate the two
strands unwind
• Bases in the cell will migrate and
bind with their complementary base
to form an exact replica of the
original

32. DNA Replication
• Replication involves separation of the two
original strands and synthesis of two new
daughter strands using the original strands as
templates.
– DNA double helix unwinds at a specific point called
an origin of replication.
– Polynucleotide chains are synthesized in both
directions from the origin of replication; that is,
DNA replication is bidirectional.

33. DNA Replication
• Unwinding the DNA double helix.
– Replication of DNA starts with unwinding of the
double helix.
– Unwinding can occur at either end or in the middle.
– Unwinding proteins called helicases attach
themselves to one DNA strand and cause
separation of the double helix.

34. DNA Replication

35. Transcription
• Transcription: the process by which information
encoded in a DNA molecule is copied into an
mRNA molecule.
– Takes place in the nucleus
– Transcription starts when the DNA double helix
begins to unwind near the gene to be transcribed.
– Only one strand of the DNA is transcribed.
– Ribonucleotides assemble along the unwound
DNA strand in a complementary sequence.
– Enzymes called polymerases (poly) catalyze
transcription

36. Transcription
The information in one DNA strand is
transcribed to a strand of RNA. The
termination site is the locus of termination
of transcription.

37. Transcription by RNA
• First step in protein synthesis
• The segment of DNA that contains the necessary information,
unwinds, to expose the bases
• The exposed bases, provide the template for messenger RNA
(mRNA) synthesis

38. Determining the Sequence of
Bases in RNA

39. Transcription

40. RNA in Translation
–
–
–
–
mRNA, rRNA, and tRNA all participate in translation.
Protein synthesis takes place on ribosomes.
A ribosome dissociates into larger and a smaller body.
In higher organisms, the larger body is called a 60S
ribosome; the smaller body is called a 40S ribosome.
– Together the 40S and 60S ribosomes form a unit on which
mRNA is stretched out.
– Triplets of bases on mRNA are called codons.
– The 20 amino acids are then brought to the mRNAribosome complex, each amino acid by its own particular
tRNA.

41. Translation
• process whereby a base sequence
of mRNA is used to create a protein
• the mRNA leaves the nucleus and
binds with ribosomes in the
cytoplasm
• transfer RNA (tRNA) contains and
anticodon, which is a 3 bases
sequence that is complementary to
the codon on the mRNA
• tRNA also carries an amino acid
• the codon of the mRNA determines
the amino acid sequence

42. Translation
– Activation
– Initiation
– Elongation
– Termination

43. • The threedimensional
structure of
tRNA.
tRNA

44. tRNA
Ribosomes
mRNA

45. Protein Synthesis
• Molecular components of reactions
at four stages of protein synthesis:

46. tRNA
– Each tRNA is specific for only one amino acid.
– An amino acid binds to an -OH group of the
appropriate tRNA by an ester bond.
– At the opposite end of the tRNA molecule is a
codon recognition site.
– The codon recognition site is a sequence of three
bases called an anticodon.
– This triplet of bases aligns itself in a
complementary fashion to the codon triplet on
mRNA.

47. Amino Acid Activation
• The activated amino acid is bound to its own
particular tRNA by an ester bond between the
carboxyl group of the amino acid and an -OH
of the tRNA.

48. The Genetic Code

49. Features of the Code
– 64 codons
– 61 code for amino acids.
– 3 (UAA, UAG, and UGA) serve as termination
signals.
– AUG also serves as an initiation signal.
– Only Trp and Met have one codon each.
– More than one triplet can code for the same amino
acid; Leu, Ser, and Arg

50. Features of the Code
• For the 15 amino acids coded for by
2, 3, or 4 triplets, it is only the third letter
of the codon that varies. Gly, for
example, is coded for by
GGA, GGG, GGC, and GGU.
• The code is almost universal: it the same
in viruses, prokaryotes, and eukaryotes;
the only exceptions are some codons in
mitochondria

52. • The steps
of chain
elongation.
Chain Elongation

53. Chain Termination
• Chain termination requires:
• Termination codons (UAA, UAG, or
UGA) of mRNA.
• Releasing factors that cleave the
polypeptide chain from the last tRNA
and release the tRNA from the
ribosome.

54. Information Transfer

55. Mutations and Mutagens
• Mutation: An error in the copying of a
sequence of bases.
– It is estimated that, on average, there is one copying
error for every 1010 bases.
– Mutations can occur during replication.
– Base errors can also occur during transcription in
protein synthesis (a nonheritable error).
– Consider the mRNA codons for Val, which are
CAT, CAC, CAG, and CAA.
– If the original codon is CAT, it may be transcribed onto
mRNA as GUC which codes for Val.
– Other errors in replication may lead to a change in
protein structure and be very harmful.

56. Mutations and Mutagens
• Mutagen: a chemical that causes a base change
or mutation in DNA.
• Many changes in base sequence caused by
radiation and mutagens do not become mutations
because cells have repair mechanisms called
nucleotide excision repair (NER).
– NER can prevent mutations by cutting out
damaged areas and resynthesizing the proper
sequence.
• Not all mutations are harmful.
– Certain ones may be beneficial because they
enhance the survival rate of the species.

57. 1. What are the three components of every nucleotide?
2. Name two types of nucleic acids.
3. Name the base that forms a pair with each of the
following bases:
a) uracil
b) guanine
c) adenine
d) cytosine

58. 4. The base sequence along one strand of DNA is A-C-T-G-T.
What would be the sequence on the complementary DNA
strand?
5. Describe 3 ways in which RNA differs from DNA
6. What nucleotide bases have double rings?
7. Where is DNA found?