Chapter 17Chapter 17Nucleotides, Nucleic Acids, and Nucleotides, Nucleic Acids, and

HeredityHeredity

The Molecules of HeredityThe 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 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 nucleic acidsacids.

The Molecules of HeredityThe Molecules of Heredity• By the 1940s, it became clear that deoxyribonucleic deoxyribonucleic

acids (DNA)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.

Nucleic AcidsNucleic AcidsThere are two kinds of nucleic acids in cells:• Ribonucleic acids (RNA)• Located elsewhere in the nucleus and even outside

the nucleus (cytoplasm)• Deoxyribonucleic acids (DNA)• Present in the chromosomes of the nucleic of

eukaryotic cells

Both RNA and DNA are polymers built from monomers called nucleotides. A nucleotide is composed of: • A base, a monosaccharide, and a phosphate

Purine/Pyrimidine BasesPurine/Pyrimidine Bases

• Figure 17.1 The five principal bases of DNA and RNA.

For DNA, the bases are A, G ,C and TFor RNA, the bases are A, G, C and U

NucleosidesNucleosidesNucleoside:Nucleoside: A compound that consists of D-ribose or 2-

deoxy-D-ribose bonded to a purine or pyrimidine base by a -N-glycosidic bond.

sugar

Base

NucleotidesNucleotides

Nucleotide:Nucleotide: A nucleoside in which a molecule of phosphoric acid is esterified with an -OH of the monosaccharide, most commonly either at the 3’ or the 5’-OH.

NucleotidesNucleotidesDeoxythymidine 3’-monophosphate (3’-dTMP),

NucleotidesNucleotidesAdenosine 5Adenosine 5’’--

triphosphatetriphosphate (ATPATP) serves as a common currency into which energy gained from food is converted and stored.

Table 17.1 The Eight Nucleosides and Eight Nucleotides in DNA and RNA

ExampleExample GTP is an important store energy. Draw the structure of

guanosine triphosphate

DNA—Primary (1°) StructureDNA—Primary (1°) Structure

For nucleic acids, primary structure is the sequence of nucleotides, beginning with the nucleotide that has the free 5’ terminus.◦ The strand is read from the 5’end to the 3’end.◦ Thus, the sequence AGT means that adenine (A) is the

base at the 5’ terminus and thymine (T) is the base at the 3’ terminus.

Structure of DNA and RNAStructure of DNA and RNA

Figure 17.2Figure 17.2

Schematic diagram of a Schematic diagram of a nucleic acid molecule. The nucleic acid molecule. The four bases of each nucleic four bases of each nucleic acid are arranged in various acid are arranged in various specific sequences. specific sequences. The base sequence is read from the 5’ end to the 3’ end.

DNA—2° StructureDNA—2° Structure

Secondary 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 helixDouble 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 HelixTHE DNA Double HelixFigure 17.4 Three-dimensional structure of the DNA double helix.

Base PairingBase PairingFigure 17.5 A and T pair by forming two hydrogen bonds. G and C pair by forming three hydrogen bonds.

The complemetary base pairs

Superstructure of Superstructure of ChromosomesChromosomes

DNA is coiled around proteins called histoneshistones..• 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 one another and form units called nucleosomes.

Nucleosome:Nucleosome: A core of eight histone molecules around which the DNA helix is wrapped.

• Nucleosomes are further condensed into chromatinchromatin..• Chromatin fibers are organized into loops, and the

loops into the bands that provide the superstructure of chromosomes.chromosomes.

Superstructure of Superstructure of ChromosomesChromosomes• Figure 17.8

Superstructure of Superstructure of ChromosomesChromosomes

Figure 17.8 cont’d

Superstructure of Superstructure of ChromosomesChromosomes

Figure 25.8 cont’d

Superstructure of Superstructure of ChromosomesChromosomes

Figure 25.8 cont’d

DNA and RNADNA and RNAThe three differences in structure between DNA and RNA are:• DNA bases are A, G, C, and TT; the RNA bases are A, G,

C, and U.U.• the sugar in DNA is 2-deoxy-D-ribose2-deoxy-D-ribose; in RNA it is D-D-

riboseribose..• DNA is always double strandeddouble stranded; there are several

kinds of RNA, all of which are single-stranded.single-stranded.

Information TransferInformation Transfer

Structure of tRNAStructure of tRNAFigure 17.10 Structure of tRNA.

Structure of rRNAStructure of rRNA• Figure 17.11 The structure of

a typical prokaryotic ribosome.

RibosomeRibosome Figure 25.11 cont’d

Different Classes of RNADifferent Classes of RNA Messenger RNA( mRNA): produced in the process

called transcription and they carry the genetic information from the DNA in the nucleus directly to the cytoplasm.◦ Containing average 750 nucleotides◦ Not-long lived

Transfer RNA (tRNA): transport amino acid to the site of protein synthesis in ribosomes◦ 74-93 nucleotides per chain◦ Contains cytosine, guanine, adenine, uracil and

amodified nucleotide called 1-methylguanosine

Different Classes of RNADifferent Classes of RNA Ribosomal RNA (rRNA) Ribosomes: small spherical

bodies located in the cells but outside the nuclei, contain rRNA◦ Consists of about 35% protein and 65% ribosomal

RNA Small Nuclear RNA (snRNA): found in the nucleus of

eukaryotic cells. ◦ 100-200 nucleotides long, neither subunit tRNA or

rRNA◦ To help with the processing of the initial mRNA

transcribed from DNA into a mature form

Different Classes of RNADifferent Classes of RNA Micro RNA (miRNA): another type of small RNA

◦ 20-22 nucleotides long ◦ Important in the timing of an organism’s

development.◦ They inhibit translation of mRNA into protein and

promote the degredation of mRNA Small Interfering RNA (siRNA): eliminate expression

of an undersirable gene, such as one that causes uncontrolled cell growth or one that came from a virus◦ Has been used to protect mouse liver from hepatitis

and to help clear infected liver cells of the disease

Roles of Different kinds of Roles of Different kinds of RNARNA

Table 17.3 The Roles of Different Kinds of RNA