DNA: The Molecular Basis of Inheritance

Building a Structural Model of DNA After most biologists became convinced that DNA was the genetic material of life, the next challenge was to determine its structure. Rosalind Franklin produced a picture of the DNA molecule by using a technique called X-ray crystallography Franklin produced a picture of the DNA molecule using this technique

Franklin’s X-ray diffraction photograph of DNA LE 16-6 Rosalind Franklin Franklin’s X-ray diffraction photograph of DNA

Based on the images, two other scientists named Watson and Crick were able to determine that DNA molecules took a double helix shape.

LE 16-7 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm 5 end Key features of DNA structure Partial chemical structure Space-filling model

Sugar–phosphate backbone Nitrogenous bases 5 end Thymine (T) Adenine (A) Cytosine (C) Phosphate DNA nucleotide Sugar (deoxyribose) 3 end Guanine (G)

Watson and Crick built models of a double helix to match to the X-rays and chemistry of DNA The side strands, or “backbones” of the DNA molecule are made of a sugar (deoxyribose) paired with a phosphate. The deoxyribose backbones are joined together by a series of molecules called nitrogenous bases.

Nitrogenous Bases There are two types of nitrogenous bases: Purines Much wider Include adenine and guanine Pyramidines Much narrower Include cytosine and thymine

How do the four bases combine to form DNA? Purine + purine: too wide LE 16-UN298 How do the four bases combine to form DNA? Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width matches data from X-rays

Watson and Crick reasoned that the pairing was more specific – Adenine paired only with Thymine Guanine paired only with Cytosine

Base Pairing to a Template Strand DNA is a double-helix molecule made of two intertwining strands. The two strands of DNA are complementary, meaning each has a set of bases that corresponds with the other. In DNA replication, the molecule is be separated into its two strands. Two new strands can be made from these templates, duplicating the molecule.

The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. The first step in replication is separation of the two DNA strands.

LE 16-9_3 The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. The first step in replication is separation of the two DNA strands. Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

LE 16-9_4 The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. The first step in replication is separation of the two DNA strands. Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. The nucleotides are connected to form the sugar-phosphate back- bones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

Origins of Replication Replication begins at special sites called origins of replication. The two DNA strands are separated, opening up a replication “bubble” Each chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin, until the entire molecule is copied

LE 16-12 Parental (template) strand 0.25 µm Origin of replication Daughter (new) strand Bubble Replication fork Two daughter DNA molecules In eukaryotes, DNA replication begins at may sites along the giant DNA molecule of each chromosome. In this micrograph, three replication bubbles are visible along the DNA of a cultured Chinese hamster cell (TEM).

Elongating the DNA Strand Enzymes called DNA polymerases catalyze the elongation of new DNA. The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells.

Proofreading and Repairing DNA DNA polymerases also proofread newly made DNA, replacing any incorrect nucleotides. Two types of repair: In mismatch repair, the enzymes replace incorrect bases with the correct ones. In nucleotide excision repair, enzymes cut out and replace entire stretches of DNA that are damaged.

Replicating the Ends of DNA Molecules DNA polymerase has one significant limitation. The enzyme has no way to complete one of the ends. Every time the DNA is copied, it becomes a little shorter. Cells will divide countless times over the lifespan of an organism. How can DNA be protected, given this limitation?

Eukaryotic chromosomal DNA molecules have at their ends repeating nucleotide sequences called telomeres. Telomeres are DNA, but do not actually encode for any traits. Telomeres do not prevent the shortening of DNA molecules, but they postpone it.

Eventually, the telomeres are worn down and essential genes begin to be lost from the chromosomes. This is one of the hypothesized causes of aging. An enzyme called telomerase catalyzes the lengthening of telomeres in stem cells. This enzyme cannot be produced indefinitely due to an increasing risk of the cell growing uncontrollably (cancer)