Chapter 12

DNA Structure and Replication

DNA Structure

• DNA is a polymer of nucleic acids.

– DNA consist of chemical units or monomers called nucleotides.

• The sugar in DNA is deoxyribose. Thus, the full name for DNA is deoxyribonucleic acid.

DNA Structure

• There are four kinds of nitrogenous bases

– adenine (A)

– thymine (T)

– cytosine (C)

– guanine (G)

DNA Structure

• DNA bases pair in a complementary fashion:

– adenine (A) pairs with thymine (T)

– cytosine (C) pairs with guanine (G).

Chargaff’s Rule

DNA Structure

– Nucleotides are joined by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a sugar-phosphate backbone.

Watson and Crick’s Discovery of the Double Helix

• James Watson and Francis Crick determined that DNA is a double helix.

• Watson and Crick used X-ray crystallography data to reveal the basic shape of DNA.

• Rosalind Franklin produced the X-ray image of DNA that Watson and Crick used.

• The model of DNA is like a rope ladder twisted into a spiral.

– The ropes at the sides represent the sugar-phosphate backbones.

– Each wooden rung represents a pair of bases connected by hydrogen bonds.

Watson and Crick’s Discovery of the Double Helix

DNA and Chromosomes

• In prokaryotic cells, DNA is located in the cytoplasm.

• Most prokaryotes have a single DNA molecule containing nearly all of the cell’s genetic information.

• Eukaryotic DNA is located in the nucleus as distinct pieces of genetic information called chromosomes.

Typically, eukaryotes have 1000 times the amount of DNA as prokaryotes.

• The number of chromosomes varies widely from one species to the next.

DNA and Chromosomes

DNA in the Nucleus

Figure 8.2

Number of chromosomes in somatic cells

Indian muntjac deer

Species

Opossum

Koala

Human

Mouse

Giraffe

Buffalo

Dog

Red viscacha rat

Duck-billed platypus

102

78

60

54

46

40

30

22

16

6

What exactly is a chromosome?

– threadlike structure found in the nucleus of eukaryotic cells

– Made up of DNA and proteins (histones) called chromatin

– contains the genetic information that is passed down from one generation to the next.

DNA Replication

• Before a cell divides, a complete copy of the DNA must be made to pass from one generation to the next in a process called replication.

• Watson and Crick’s model for DNA suggested that DNA replicates by a template mechanism because each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing.

• DNA replication in eukaryotes

– begins at specific sites on a double helix (called replication forks)

– proceeds in both directions.

DNA Replication

During DNA replication, the DNA molecule separates into two

strands, then produces two new complementary strands following the rules of base pairing. Each strand of the

double helix of DNA serves as a template for

the new strand.

• DNA helicase

– enzymes that unzip the double strand of DNA

• DNA polymerase

– an enzyme that joins individual nucleotides to produce a DNA molecule.

– are involved “proofreading” each new DNA strand

How Replication Occurs

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In prokaryotic cells, DNA is found in the

A. cytoplasm.

B. nucleus.

C. ribosome.

D. cell membrane.

The first step in DNA replication is

A. producing two new strands.

B. separating the strands.

C. producing DNA polymerase.

D. correctly pairing bases.

A DNA molecule separates, and the sequence GCGAATTCG occurs in one strand. What is the base sequence on the other strand?

A. GCGAATTCG

B. CGCTTAAGC

C. TATCCGGAT

D. GATGGCCAG

In addition to carrying out the replication of DNA, the enzyme DNA polymerase also functions to

A. unzip the DNA molecule.

B. regulate the time copying occurs in the cell cycle.

C. “proofread” the new copies to minimize the number of mistakes.

D. wrap the new strands onto histone proteins.