Chapter 12

DNA: The Molecule of Heredity

Objectives

Analyze the structure of DNA Determine how the structure of

DNA enables it to reproduce itself accurately.

What is DNA?

Genetic materialNecessary for protein

synthesisDetermines an organism’s

traits

Structure of DNA DNA is a polymer made of repeating

subunits called nucleotides. Nucleotides:

Phosphate group Simple sugar A nitrogenous base:

Purines: Pyrimidines:Guanine (G) Cytosine (C)Adenine (A) Thymine (T)

Watson and Crick DNA is made of two chains of nucleotides

held together by nitrogenous bases Nitrogenous bases form hydrogen bonds

Complementary base pairs: A bonds with T C bonds with G

The shape of DNA is called a double helix Two strands twisted together

Nucleotide sequences

Nucleotide arrange to form unique genetic sequences

The more similar sequence the closer relation between organisms

Used to determine evolutionary relationships

P

P

C

C

CC

CO

1

2

3

4

5

Nucleotides composed of Pentose sugar Phosphate Nitrogenous base

(A,T,C,G) Phosphates bond to the 3’

and 5’ carbons,… Nitrogenous bases bond to

the 1’ carbon.

Nucleotide Structure Review (Ch. 12)

Nucleotides – a simpler diagram• Nitrogen bases• 5-carbon sugar• Phosphate group(s)

A

Adenine

Deoxyribose

Deoxyribose gives its name to DNA, ribose (shown on right) to RNA

Ribose, C5H10O5 Oxygens and hydrogens

have been omitted... The carbons are numbered

clockwise from the oxygen atom:

Deoxyribose = a Sugar

C

C

CC

CO

1

2

3

4

5

Nitrogenous Bases

Pyrimidines = single-ring structure Thymine (T) & Cytosine (C)

Purines = double-ring structure Adenine (A) & Guanine (G)

What about Uracil

Uracil - Similar in structure to thymine.

Is nitrogenous base for RNA (ribonucleic acid)

Nitrogenous Base Pairing Purines are always paired with pyrimidines and

vice versa. Each base has side groups that H bond with

one another: A is complementary to T (2 H bonds) C is complementary to G (3 H bonds)

These findings explain Chargaff’s earlier findings concerning equal amounts of Adenine and Thymine in DNA samples.

TissueADENINE GUANINE CYTOSINETHYMINE

A G CT

THYMUSHUMANSHEEPPIG

30.929.330.9

19.921.419.9

29.428.329.4

19.821.019.8

SPLEENHUMANSHEEPPIG

29.228.029.6

21.022.320.4

29.428.629.2

20.421.120.8

LIVERHUMANSHEEPPIG

30.329.329.4

19.520.720.5

30.329.229.7

19.920.820.5

Chargaff measured the amount of each of the nucleotides in various tissues and organs...

Pairing of Nucleotides: Chargaff’s Data

Note the amount of adenine and thymine in each sample…

Similarly, the amount of guanine and cytosine...

Two nucleotides: adenine and thymine

Two hydrogen bonds between them...

Adenine & Thymine

glycosidiclinkage to

deoxyribose

Adenine

Thymine

glycosidiclinkage to

deoxyribose

Cytosine and guanine are also connected by three hydrogen bonds

Cytosine & Guanine

Guanine

Cytosine

glycosidiclinkage to

deoxyribose

glycosidiclinkage to

deoxyribose

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/BasePairing.gif

DNA Structure

DNA structure similar to that of a ladder:

Sugar phosphate backbone = sides of ladder

Nitrogenous bases = rungs of ladder.

P

C

C

CC

CO

1

2

3

4

5

1

2

3

4

5

P

C

C

CC

CO

P

P

C

C

CC

CO

1

2

3

4

5

Note that the complementary strands are upside with respect to each other (antiparallel).

DNA is a polymer, made of

nucleotide monomers

DNA forms a double helix,

two molecules spiraled

DNA replicates itself with the

help of polymerase enzymes

DNA Replication

Nucleotides

Phosph.Group

NUCLEOTIDES in DNA & RNANitrogen

Base5- C

SugarAdenineGuanineCytosineThymine

Uracil

Phos-phate

Deoxy-ribose

Ribose

DNA Replication

A portion of the double helix is unwound by the enzyme helicase.

DNA polymerase binds to one strand of the DNA and begins moving along 3' to 5' direction forming a new double helixForms the leading strand

DNA Replication DNA synthesis can only occur in the 5'

to 3' direction. A second type of DNA polymerase binds

to the other template strand as the double helix opens Discontinuous segments (Okazaki

fragments) DNA ligase I then stitches these together

new strand Forms the lagging strand

DNA Replication Results:

Chromosomes copied Formation of two DNA molecules New DNA strand is identical to

the original DNA strand DNA replicates in interphase prior

to mitosis and meiosis