Post on 25-Jan-2021
transcript
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
© 2013 Pearson Education, Inc.
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.