Chapter 12:DNA and RNA

(Molecular Genetics)

1. In recent years, biochemists have found that the DNA of chromosomes is the genetic material that is passed form generation to generation

2. Genes- are sections of DNA molecules

(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

(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

(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)

(B) Watson-Crick Model

For DNA:adenine (A) withthymine (T)

cytosine (C) withguanine (G)

(B) Watson-Crick Model

For RNA:Uracil (U) withthymine (T)

cytosine (C) withguanine (G)

(B) Watson-Crick Model

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

http://www.chem.pitt.edu/~grabowski/Chem_BORIS/dna-rot.mov

Double-helix Structure of DNAhttp://www.chem.pitt.edu/~grabowski/Chem_BORIS/dna-rot.mov

Double-helix Structure of DNA

http://www.chem.pitt.edu/~grabowski/Chem_BORIS/dna-rot.mov

(II) DNA Replication1. DNA, unlike any other

chemical compound, can make exact copies of itself by a process known as replication

2. 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

3. Free nucleotides in the nucleus then bond to the complimentary bases of the DNA strands

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

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

(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

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

(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

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

mRNA Genetic codes

mRNA Genetic codes

http://www.csi-net.net/suzanne/page15.htm

Messenger RNA (mRNA)1. 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

2. The sequence of nucleotides in the mRNA contain the genetic code

3. The genetic code for each amino acid is a sequence of three nucleotides forming a codon

(IV) Protein Synthesis1. In the cytoplasm, the mRNA becomes associated with

a ribosome2. Amino acids in the cytoplasm are “picked-up” by

molecules of transfer RNA (tRNA)3. Each codon on the mRNA bonds with a

corresponding anticodon on a tRNA, which carries a specific amino acid

4. The resulting chain of amino acids is a polypeptide