Chapter 12. DNA Structure and Replication

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1 Chapter 12 DNA Structure and Replication

2 DNA Structure DNA is a polymer of nucleic acids. DNA consist of chemical units or monomers called nucleotides.

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

4 DNA Structure There are four kinds of nitrogenous bases adenine (A) thymine (T) cytosine (C) guanine (G)

5 Chargaff s Rule DNA bases pair in a complementary fashion: adenine (A) pairs with thymine (T) cytosine (C) pairs with guanine (G).

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

7 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.

8 Watson and Crick s Discovery of the Double Helix The model of DNA is like a rope ladder twisted into a spiral. The ropes at the sides represent the sugarphosphate backbones. Each wooden rung represents a pair of bases connected by hydrogen bonds.

9 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.

10 DNA and Chromosomes 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.

11 DNA in the Nucleus

12 Figure 8.2 Species Number of chromosomes in somatic cells Indian muntjac deer Koala Opossum Giraffe Mouse Human Duck-billed platypus Buffalo Dog Red viscacha rat

13 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.

14 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.

15 DNA Replication DNA replication in eukaryotes begins at specific sites on a double helix (called replication forks) proceeds in both directions.

16 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.

17 How Replication Occurs 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

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20 In prokaryotic cells, DNA is found in the A. cytoplasm. B. nucleus. C. ribosome. D. cell membrane.

21 The first step in DNA replication is A. producing two new strands. B. separating the strands. C. producing DNA polymerase. D. correctly pairing bases.

22 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

23 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.