Chapter 17_part 1

advertisement

Chapter 17

Nucleotides, Nucleic Acids, and

Heredity

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 .

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.

Nucleic Acids

There 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 Bases

For DNA, the bases are A, G ,C and T

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

Figure 17.1 The five principal bases of DNA and RNA.

Nucleosides

Nucleoside: A compound that consists of D-ribose or 2-deoxy-Dribose bonded to a purine or pyrimidine base by a  -Nglycosidic bond.

uracil O

Base

HN b-

D-riboside

5'

HOCH

2

4'

H

H

O

O

H

3' 2'

HO OH

Uridine

1

N

H

1' a b

-N-glycosidic bond anomeric carbon sugar

Nucleotides

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.

Nucleotides

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

O

CH

3

HN

5'

HOCH

2 O

O

H

H

O

3'

O

P

O

-

O

-

H

H

N

H

1'

Nucleotides

Adenosine 5 ’ triphosphate (ATP ) 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

Example

GTP is an important store energy. Draw the structure of guanosine triphosphate

DNA—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 RNA

Figure 17.2

Schematic diagram of a nucleic acid molecule. The four bases of each nucleic acid are arranged in various specific sequences.

The base sequence is read from the 5 ’ end to the 3 ’ end.

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.

THE DNA Double Helix

Figure 17.4

Threedimensional structure of the

DNA double helix.

Base Pairing

Figure 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 Chromosomes

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 positivelycharged histones attract one another 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.

Superstructure of Chromosomes

Figure 17.8

Superstructure of Chromosomes

Figure 17.8 cont ’ d

Superstructure of Chromosomes

Figure 25.8 cont ’ d

Superstructure of Chromosomes

Figure 25.8 cont

d

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 2-deoxy-D-ribose ; in RNA it is D-ribose .

• DNA is always double stranded ; there are several kinds of

RNA, all of which are single-stranded.

Information Transfer

Structure of tRNA

Figure 17.10 Structure of tRNA.

Structure of rRNA

Figure 17.11 The structure of a typical prokaryotic ribosome.

Ribosome

Figure 25.11 cont ’ d

Different 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 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 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 RNA

Table 17.3 The Roles of Different Kinds of RNA

Download