Chapter 12Nucleic Acids

A. Nucleic Acids• Macromolecules composed of subunits

called nucleotidespolymers made of monomers

• Nucleotides:“building blocks” for DNA (deoxyribonucleic acid) & RNA (ribonucleic acid)

Cont. Nucleotides

• Based on sugar (2 general types of nucleotides)a. Ribotides ribose nucleotidesb. Deoxyribotides deoxyribose nucleotides

Cont. Nucleotides1. Nucleotides composed of 3 subunits:

a. Pentose sugar (C5)

RNA: ribose (R) DNA: deoxyribose (D)

Cont. Nucleotidesb. Phosphate group (H2PO3)– Derived from phosphoric acid (H2PO3 - inorganic

compound)

c. Nitrogen base– DNA: adenine (A) , cytosine (C), guanine (G),

thymine (T)– RNA: adenine, cytosine, guanine, uracil (U)

c. Nitrogen bases are 1 of 2 types: – Pyrimidine- C, T, U (consist of single-ring) – Purines- A, G (consist of double-ring)

Cytosine

Adenine

Thymine Uracil

Guanine

B. Ribose and DNA Nucleotides

*** NOTE: Uracil only in ribose series; thymine only in deoxyribose series!!!

RNA Nucleotides DNA Nucleotides

Adenine – ribose – O – P Adenine – deoxyribose – O – P

Cytosine – ribose – O – P Cytosine – deoxyribose – O – P

Guanine – ribose – O – P Guanine – deoxyribose – O – P

Uracil – ribose – O -- P Thymine – deoyxyribose – O -- P

C. Nucleotides can be synthesized into:

1. Energy carriers ATP, CTP, GTP, TTP, UTP2. Coenzymes (carry H2)

FMN (Flavin Mononucleotide)FAD (Flavin Adenine Dinucleotide)NAD (Nicotinamide Adenine Diphosphate)

NADP (Nicotinamide Adenine Dinucleotide Phosphate)

3. Genetic Systems polymers called DNA & RNA

DNA Nucleotides

A-T or A-U(2 H-bonds)

G-C(3 H-bonds)

D. DNA (Watson-Crick Model); proposed 19531. Genetic material composed of 2 long chains of nucleotides2. A=T ; C=G (*base pairs rule pyrimidine always pairs with purine3. 2 chains run in opposite directions, connected by nitrogen bases to form double helix. 4. Hydrogen bonds between bases

Cont. DNA (Watson-Crick Model)

5. DNA forms genes genetic information that passes from parent to offspring

E. RNA (essential to protein synthesis)1. Contains ribose instead of deoxyribose2. Contains uracil instead of thymine3. Nucleotides in long chains, in single strands unless

RNA is the only nucleic present in the organism4. THREE types of RNA:

a. messenger RNA (mRNA)b. transfer RNA(tRNA) Essential in protein synthesis!c. ribosomal RNA (rRNA)

RNA vs DNA

Chapter 12: DNA Replication

***RECAP: DNA replication occurs in S phase of cell cycle

A. DNA Structure 1. Double-stranded polymer composed of paired

nucleotides2. Because A combines with T, C with G, SEQUENCE of

bases varies to produce different genes3. Once nucleotide sequence is fixed, sequence in

other strand is also determined

B. DNA Replication (S phase)1. Requires a team of enzymes (DNA

polymerase)2. Steps in replication:

a. One enzyme unwinds portion of DNAb. Another enzyme breaks H-bonds between paired

nucleotides and separates strandsc. New nucleotides are added to each original strand

in proper direction according to original strandd. Phospates of new nucleotides bond to sugarse. New strand attached to original

DNA Replication

Cont. DNA Replication

3. Semiconservative Replication (validated!) each new DNA molecule contains ½ of original

Cont. DNA Replication

4. Conservative Replication (negated) one DNA molecule = 2 “old” strands one DNA molecule = 2 “new” strands

5. Each new cell formed after M phase (mitosis) in cell cycle contains same set of chromosomes & genes as parent cell (each DNA ½ old, ½ new)

C. Replication Forks 1. DNA is a giant molecule!

2. If replication began at one end, process would take TOO LONG!3. Replication forks multiple sites of DNA replication

Cont. Replication Fork4. Replication different along each strand (done by DNA POLYMERASE!)

a. Leading strand synthesis continues in ONE direction • Nucleotides added one at a time- in order

b. Lagging strand short, discontinuous segments of DNA synthesized• Segments then joined by DNA ligases

5. Result: Synthesis of 2 new DNA molecules behind the replication fork as it moves along original DNA molecule

Chemical Evidence for Watson-Crick Model

CELL TYPE A (%) G (%) C (%) T (%)

Human 30.4 19.6 19.9 30.1

Ox 29.0 21.2 21.2 28.7

Salmon sperm 29.7 20.8 20.4 29.1

Wheat germ 28.1 21.8 22.7 27.4

E coli 26.0 24.9 25.2 25.9

Sea urchin 32.8 17.7 17.3 32.1

1.Erwin Chargaff- analyzed nuclei of different cell types

*** SIGNIFICANCE: The concentration of specific bases (A & T)/ (C & G) approached a 1:1 ratio (A=T) and (C=G)

Upon analysis of many tissues from many different species, Chargaff concluded that the amounts of A & T and C & G were ALMOST EXACTLY EQUAL!

Cont. Watson-Crick Model

2. Watson and Crick reasoned that these the 1:1 ratio was due to PAIR BONDING between the 2 bases that were equal (A-T) and (C-G)

3. Watson and Crick reasoned that these specific bases would bond

according to SIZE always 1 pyrimidine paired with 1 purine– Size of these base pairings was proven experimentally by the x-

ray diffraction work of Wilkins and Franklin

4. Watson and Crick used the 1:1 ratio of A-T and C-G to propose a mechanism of replication or making an exact copy of itself– Proposed in 1953 that the method of replication of DNA would

be SEMI-CONSERVATIVE– Each new strand molecule retains ½ of the original DNA molecule

Cont. Watson-Crick Model5. Watson-Crick Model for the structure of DNA in 1953

revolutionized Biology a. Nature of the gene

b. Explanation of mutationsc. Variations w/in species sequence of nucleotides make specific types of genesd. Cell division and reproductione. Abnormal cell division cancer

Meselson- Stahl confirmed semi-conservative replication

1928- Griffith’s Transformation

Avery, MacLeod, McCarty 1944- DNA was transforming agent

1952- Chase & Hershey’s Experiment