Chapter 24Chapter 24 Nucleotides, Nucleic Nucleotides, Nucleic Acids, and HeredityAcids, and Heredity

The Molecules of The Molecules of HeredityHeredity

• Each cell has thousands of different proteins.

• How do cells know which proteins to synthesize out of 100000s 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

chromosomeschromosomes.

• The hereditary information was thought to reside in genesgenes within the chromosomes.

• Chemical analysis of nuclei showed chromosomes are made up largely of proteins called histoneshistones and nucleic acidsnucleic acids.

The Molecules of The Molecules of HeredityHeredity• By the 1940s, it became clear that deoxyribonucleic acids deoxyribonucleic acids

(DNA)(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.

protein

work

Structure of DNA and RNAStructure of DNA and RNA((based onbased on Nucleic AcidsNucleic Acids))• Two Kinds in cells:

• ribonucleic acids (RNA)

• deoxyribonucleic acids (DNA)

• RNA & DNA: polymers built from monomers (nucleotides)

• A nucleotide is composed of: • 1. a base, 2. a monosaccharide, 3. a phosphate, (e.g. AMP)

1. Purine/Pyrimidine 1. Purine/Pyrimidine BasesBases

Base Pairing:

DNA: A-T;C-GRNA: A-U;C-G

HN

NO

H

N

N

NH2

H

HN

N

H

CH3

Uracil (U)(in RNA only)

Thymine (T)(DNA only)

Cytosine (C)(DNA andsome RNA)

N

N

Pyrimidine

1

2

3

4

5

6

HN

N N

NO

HH2N

Guanine (G)(DNA and RNA)

N

N N

N

NH2

HAdenine (A)

(DNA and RNA)

N

N N

N

HPurine

1

2

3

4

56 7

8

9

O O

O O

HN

NO

H

N

N

NH2

H

HN

N

H

CH3

Uracil (U)(in RNA only)

Thymine (T)(DNA only)

Cytosine (C)(DNA andsome RNA)

N

N

Pyrimidine

1

2

3

4

5

6

HN

N N

NO

HH2N

Guanine (G)(DNA and RNA)

N

N N

N

NH2

HAdenine (A)

(DNA and RNA)

N

N N

N

HPurine

1

2

3

4

56 7

8

9

O O

O O

note:

Nucleosides Nucleosides (base and (base and sugar)sugar)

• compound that consists of D-ribose or 2-deoxy-D-ribose bonded to a purine or pyrimidine base by a -N-glycosidic bond.

HH

HH

OHOCH2

HO OH

O

O

HN

N

anomericcarbon

a -N-glycosidicbond

Uridine

-D-riboside

uracil

1'

2'3'

4'

5'1

NucleoNucleotidestides• a nucleoside w/ molecule of phosphoric acid esterified

with an -OH of the monosaccharide, most commonly either the 3’ or the 5’-OH.

O-

-O-P-O-CH2

OO

H

H

OH

H

HOH

N

N N

N

NH2

5'

1'

3'

Adenosine 5'-monophosphate(5'-AMP)

ATP- a nucleotide

In SummaryIn SummaryNucleoside = Base + Sugar

Nucleotide = Base + Sugar + Phosphoric acidNucleic acid = chain of nucleotides

Structure of DNA and RNAStructure of DNA and RNA

• -bases arranged in various patterns (like A.A.s for Proteins)

GENE ((protein))

•sequence is read from the 5’ end to the 3’ end

start

finish

Primary Structure

DNA - 2° StructureDNA - 2° 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 helixDouble helix:: 2° structure of DNA in which two polynucleotide strands are coiled around each other in a screw-like fashion.

Using Chargaff rules: (A-T; C-G)-X-ray (Franklin, Wilkins)

(R. Franklin, 1920-1958)

Watson, Crick and Wilkins (Nobel Prize 1962)

The DNA Double HelixThe DNA Double Helix-Polynucleotide chains

run anti-parallel

-Bases (hydrophobic)

avoid water & stabilize

d. helix w/ H-bonds (below)

Base PairingBase Pairing

Higher Structure of Higher Structure of DNADNA• DNA is coiled around proteins called histones.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:Nucleosome: a core of eight histone molecules around which the DNA helix is wrapped.

_+_

ChromosomesChromosomesNucleosomes are further condensed into chromatin.chromatin.

Chromatin fibers are organized into loops, and the loops into the bands that provide the superstructure of

chromosomes.chromosomes.