Chapter 25, 26: Nucleotides, Nucleic Acids,
The Molecules of Heredity
– Each cell of our bodies contains thousands of different
– 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
– 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
The Molecules of Heredity
– By the 1940s, it became clear that
deoxyribonucleic acids (DNA) carry the hereditary
– 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.
• 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.
• made up of nucleotides
• found in all living cells except
• 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
-each nucleotide contains
1) a sugar
2) a base
3) phosphoric acid unit
1) Ribose and Deoxyribose
Bases in Nucleic Acids
Bases for DNA:
A, G, C, T
Bases for RNA:
A, G, C, U
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
• The sequence of the
bases in DNA or RNA form
the primary structure
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
• Double helix: a type of 2° structure of DNA in
which two polynucleotide strands are coiled
around each other in a screw-like fashion.
The DNA Double Helix
• Figure 25.4
• Like a spiral staircase:
hand rail, the
• Hydrogen bonding
occurs between the
A bonds with T
C bonds with G
A bonds with U
C bonds with G
DNA Double Helix
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 fibers are organized into loops, and the
loops into the bands that provide the
superstructure of chromosomes.
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
– DNA is always double stranded; there are several
kinds of RNA, most of which are single-stranded.
• RNA molecules are classified according to
their structure and function.
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
Replication of DNA
The DNA in the chromosomes carries
out two functions:
– (1) It reproduces itself. This process is
– (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
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
• The two strands of DNA in the helix
• When ready to replicate the two
• Bases in the cell will migrate and
bind with their complementary base
to form an exact replica of the
• Replication involves separation of the two
original strands and synthesis of two new
daughter strands using the original strands as
– 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.
• Unwinding the DNA double helix.
– Replication of DNA starts with unwinding of the
– 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.
• Transcription: the process by which information
encoded in a DNA molecule is copied into an
– 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
The information in one DNA strand is
transcribed to a strand of RNA. The
termination site is the locus of termination
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
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
• process whereby a base sequence
of mRNA is used to create a protein
• the mRNA leaves the nucleus and
binds with ribosomes in the
• 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
• Molecular components of reactions
at four stages of protein synthesis:
– 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
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.
Features of the Code
– 64 codons
– 61 code for amino acids.
– 3 (UAA, UAG, and UGA) serve as termination
– 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
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
• The steps
• 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
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.
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
• Not all mutations are harmful.
– Certain ones may be beneficial because they
enhance the survival rate of the species.
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
4. The base sequence along one strand of DNA is A-C-T-G-T.
What would be the sequence on the complementary DNA
5. Describe 3 ways in which RNA differs from DNA
6. What nucleotide bases have double rings?
7. Where is DNA found?