DNA: The Molecule of Heredity Chemical nature of DNA Chromosomes are composed of protein and deoxyribonucleic acid Gene – functional segment of DNA located at a particular place on a chromosome DNA consists of building blocks called nucleotides consisting of: 5-carbon sugar – deoxyribose carbons are designated by numbers (1’ – 5’) a phosphate group a nitrogen-containing base

there four kinds of nucleotides – differ by the type of base: Purines – adenine and guanine (double ring structure Pyrimidines – cytosine and thymine (single ring structure) Four types of nucleotides (bases) can be arranged in any linear order along a strand of DNA each sequence represents a unique set of genetic instructions

The Double Helix – James Watson and Francis Crick’s model proposed that the DNA molecule consists of: two strands, each composed of a series of nucleotides in each single strand, the phosphate group of one nucleotide bonds to the sugar of another nucleotide (phosphate groups link the 3’ C of one sugar to the 5’ C of the next) this forms a “backbone” of alternating sugars and phosphates nitrogenous bases protrude toward the interior from the backbone (attached to 1’ C of the sugar) the two strands of nucleotides are twisted around each other into a double helix (similar to a twisted ladder) strands are held together by hydrogen bonds between the bases adenine – thymine (two hydrogen bonds) guanine – cytosine (three hydrogen bonds) forms complementary base pairs (follows Chargaff’s Rules) strands are antiparallel (two strands run in opposite directions of each other) double helix has a constant width (follows Chargaff’s Rules)

Research – be familiar with the following experiments Research – be familiar with the following experiments! (read about in textbook) 1928 Frederick Griffith (pneumococcus bacteria – transformation)

1944 Avery, MacLeod, and McCarty (repeated Griffith’s experiments and identified “transforming principle”) 1952 Hershey and Chase (bacteriophages) Hershey and Chase experiment video clip

Rosalind Franklin and Maurice Wilkins – used X-ray diffraction to learn about structure of DNA

Erwin Chargaff (Chargaff’s Rules) discovered that individuals of the same species have the same percentage of the four bases (never varies) DNA from similar species have similar base composition the number of adenines always equals the number of thymines (A=T) the number of cytosines equals the number of guanines (C=G) the number of purines always equals the number of pyrimidines

DNA Replication Watson and Crick also proposed a possible mechanism by which DNA could replicate itself – Semiconservative replication their model suggested that, because the nucleotides pair with each other in a complementary fashion, each strand of the DNA molecule could serve as a template, or pattern, for the synthesis of the opposite strand 1957 Meselson and Stahl – performed experiments to support model (read about in text)

Steps in DNA replication: Replication video DNA strands unwind – accomplished by DNA helicase enzymes – move along molecule and open double helix Binding proteins bind to single DNA strands to hold it open

each parental strand is used as a template for the formation of a new daughter strand of DNA daughter strand is formed by connecting nucleotides in an order determined by the nucleotide sequence of the parental strand according to complementary base-pairing

Origins of replication video clip The replication of a DNA molecule begins at special sites, origins of replication enzymes separate the strands, forming a replication “bubble” Replication proceeds in both directions until the entire molecule is copied In eukaryotes, there may be hundreds or thousands of origin sites per chromosome Origins of replication video clip

DNA polymerases catalyze the elongation of new DNA at a replication fork As nucleotides align with complementary bases along the template strand, they are added to the growing end of the new strand by the polymerase The raw nucleotides are nucleoside triphosphates

As each nucleotide is added, the last two phosphate groups are hydrolyzed to form pyrophosphate. The exergonic hydrolysis of pyrophosphate to two inorganic phosphate molecules drives the polymerization of the nucleotide to the new strand.

DNA polymerase III can only add nucleotides to the free 3’ end of a growing DNA strand. A new DNA strand can only elongate in the 5’->3’ direction. This creates a problem at the replication fork because one parental strand is oriented 3’5’ into the fork, while the other antiparallel parental strand is oriented 5’3’ into the fork. At the replication fork, one parental strand (3’ 5’ into the fork), the leading strand, can be used by polymerases as a template for a continuous complimentary strand.

The other parental strand (5’->3’ into the fork), the lagging strand, is copied away from the fork in short segments (Okazaki fragments). Okazaki fragments, each about 100-200 nucleotides, are joined by DNA ligase to form the sugar-phosphate backbone of a single DNA strand.

DNA polymerase III can only add onto an already existing molecule therefore DNA replication begins with an RNA primer (about 5 nucleotides long) primer is synthesized by RNA primase (primosome)– after a few nucleotides have been added, it is replaced by DNA polymerase III (primer later removed and replaced with DNA by DNA polymerase I) Each Okazaki fragment also begins with a primer

Leading strand Video clip Lagging strand Video clip DNA Replication Review Video Clip

The final error rate is only one per billion nucleotides. Enzymes proofread DNA during its replication and repair damage in existing DNA Mistakes during the initial pairing of template nucleotides and complementary nucleotides occurs at a rate of one error per 10,000 base pairs. DNA polymerase proofreads each new nucleotide against the template nucleotide as soon as it is added. If there is an incorrect pairing, the enzyme removes the wrong nucleotide and then resumes synthesis. The final error rate is only one per billion nucleotides.