1. Chapter 12:DNA and RNA
(Molecular Genetics)

3. In recent years, biochemists have found that the DNA of chromosomes is the genetic material that is passed form generation to generation
Genes- are sections of DNA molecules

4. (I) DNA Structure
A very large molecule consisting of thousands of smaller, repeating units known as nucleotides
DNA is found in the nucleus of the cell

5. (A) DNA Nucleotide A DNA nucleotide is composed of three parts:
1. A phosphate group
2. A deoxyribose (5-carbon sugar) molecule
3. A nitrogenous base of either adenine, thymine, guanine, or cytosine

6. (B) Watson-Crick Model
Watson and Crick developed a model of the DNA molecule
In this model, the DNA molecule consists of two complimentary chains of nucleotides in a “ladder” type organization
The four nitrogenous bases of the DNA molecule bond together in only one way:
adenine (A) with thymine (T)
cytosine (C) with guanine (G)

7. (B) Watson-Crick Model
For DNA:
adenine (A) with
thymine (T)
cytosine (C) with
guanine (G)

8. (B) Watson-Crick Model
For RNA:
Uracil (U) with
thymine (T)
cytosine (C) with
guanine (G)

9. (B) Watson-Crick Model

10. Double-helix Structure of DNA
Double-helix Structure of DNA
Each “step” of the ladder consists of nitrogenous bases bonded together by weak hydrogen bonds
The two chains of the DNA molecule are twisted to form a spiral, or double-helix

11. Double-helix Structure of DNA
Double-helix Structure of DNA

12. Double-helix Structure of DNA
Double-helix Structure of DNA

13. (II) DNA Replication
DNA, unlike any other chemical compound, can make exact copies of itself by a process known as replication
In replication, the double-stranded DNA helix unwinds; the two strands then separate, or unzip, by the breaking of the hydrogen bonds between pairs of bases
Free nucleotides in the nucleus then bond to the complimentary bases of the DNA strands

14. Replication produces two identical DNA molecules that are exact copies of the original molecule

15. Replication produces two identical DNA molecules that are exact copies of the original molecule

16. (III) RNA (Ribonucleic acid)
RNA is a nucleic acid, like DNA, composed of nucleotide building blocks
There are three major differences between the structure of DNA and RNA:
1. In RNA, ribose is substituted for deoxyribose
2. uracil (U) is substituted for thymine (T)
3. RNA consists of only a single strand of nucleotides

17. RNA Nucleotide A RNA nucleotide is composed of three parts:
1. A phosphate group
2. A ribose (5-carbon sugar) molecule
3. A nitrogenous base of either uracil, thymine, guanine, or cytosine

18. (III) RNA (Ribonucleic acid)
1. In RNA, ribose is substituted for deoxyribose
2. uracil (U) is substituted for thymine (T)
3. RNA consists of only a single strand of nucleotides

19. Genetic Code
A genetic code contains the information for the sequence of amino acids in a particular protein
This code is present in mRNA molecules and is three bases long. This is known as a codon
Ex: UAG - is a codon

20. mRNA Genetic codes

21.
mRNA Genetic codes

22. Messenger RNA (mRNA)
When portions of DNA molecules unwind and separate, RNA nucleotides pair with complimentary bases on the DNA strand. This forms a mRNA that is complimentary to the DNA strand
The sequence of nucleotides in the mRNA contain the genetic code
The genetic code for each amino acid is a sequence of three nucleotides forming a codon

23. (IV) Protein Synthesis
In the cytoplasm, the mRNA becomes associated with a ribosome
Amino acids in the cytoplasm are “picked-up” by molecules of transfer RNA (tRNA)
Each codon on the mRNA bonds with a corresponding anticodon on a tRNA, which carries a specific amino acid
The resulting chain of amino acids is a polypeptide