Chapter 11 DNA: The Molecule Of Heredity

How Did Scientists Discover That Genes Are Made of DNA? Transformed bacteria revealed the link between genes and DNA In the 1920s, Frederick Griffith worked with two strains of Streptococcus pneumoniae bacteria S-strain caused pneumonia when injected into mice, killing them R-strain did not cause pneumonia when injected When the S-strain was killed and injected into mice, it did not cause disease By the late 1800s early 1900s, scientists had a limited knowledge of genes Heritable information is carried in discrete units called genes Genes are parts of structures called chromosomes Chromosomes are made of deoxyribonucleic acid (DNA) and protein They did not know what genes were made of He was trying to make a vaccine to prevent bacterial pneumonia.

Transformation in Bacteria Fig. 11-1 Mouse remains healthy Mouse contracts pneumonia and dies Living R-strain S-strain Heat-killed Mixture of living R-strain and heat-killed (a) (b) (c) (d) Bacterial strain(s) injected into mouse Results Conclusions R-strain does not cause pneumonia. S-strain causes Heat-killed S- strain does not cause pneumonia. A substance from heat-killed S-strain can transform the harmless R-strain into a deadly S-strain. Griffith made a sample of heat-killed S-strain and mixed it with the R-strain Injection of the combination into mice caused pneumonia and death This was unexpected, since neither element alone caused the disease The results of Griffith’s experiments led to several new deductions about the genetic material Some substance in the heat-killed S-strain changed the living, harmless R-strain bacteria into the deadly S-strain, a process Griffith called transformation The substance that caused this transformation might be the long-sought molecule of heredity

How Did Scientists Discover That Genes Are Made of DNA? The transforming molecule is DNA In 1933, J. L. Alloway showed that transformation occurred just as readily in culture dishes This showed that the mouse in Griffith’s experiments was not involved in the transformation protein-destroying enzymes still induced transformation When DNA-destroying enzymes were added to the samples, transformation did not occur

Molecular Mechanism of Transformation bacterial chromosome DNA fragments are transported into the bacterium Heating S-strain cells killed them but did not completely destroy their DNA When killed S-strain bacteria were mixed with living R-strain bacteria, fragments of DNA from the dead S-strain cells became incorporated into the chromosome of the R-strain bacteria If these fragments of DNA contained the genes needed to cause disease, an R-strain cell would be transformed into an S-strain cell Thus, Avery, MacLeod, and McCarty concluded that genes are made of DNA A DNA fragment is incorporated into the chromosome Fig. 11-2

What Is the Structure of DNA? The secrets of DNA function and, therefore, of heredity itself are found in the three-dimensional structure of the DNA molecule DNA is a polymer of nucleotides Each nucleotide has three components A phosphate group A deoxyribose sugar One nitrogenous base 1. Thymine (T) 2. Cytosine (C) 3. Adenine (A) 4. Guanine (G) sugar phosphate base = adenine

What Is the Structure of DNA? DNA is a double helix From X-ray diffraction patterns, Wilkins and Franklin deduced several qualities of DNA It is long and thin, and has a uniform diameter of 2 nanometers It is helical; that is, twisted like a corkscrew and consisting of repeating subunits

What Is the Structure of DNA? James Watson and Francis Crick combined the X-ray data with bonding theory to deduce DNA structure Double helix - made of two strands of nucleotides Within each DNA strand, the phosphate group of one nucleotide bonds to the sugar of the next nucleotide in the same strand The deoxyribose and phosphate portions make up the sugar-phosphate backbone Covalently bonded together

What Is the Structure of DNA? James Watson and Francis Crick combined the X-ray data with bonding theory to deduce DNA structure The nucleotide bases protrude from this backbone All the nucleotides within a single DNA strand are oriented in the same direction and thus have an unbonded sugar at one end and an unbonded phosphate at the other end

The Watson-Crick Model of DNA Structure free phosphate hydrogen bonds sugar nucleotide base (cytosine) free sugar (a) Hydrogen bonds hold complementary basepairs together in DNA (b) Two DNA strands form a double helix (c) Four turns of a DNA double helix Two DNA strands form a double helix Hydrogen bonds hold complementary base pairs together in DNA The two strands are antiparallel; that is, they are oriented in opposite directions one strand starts with a free sugar and ends with a free phosphate, the other stand starts with a free phosphate and ends with a free sugar Fig. 11-5

What Is the Structure of DNA? Hydrogen bonds between complementary bases hold two DNA strands together in a double helix Because of their structures and the way they face each other, adenine (A) bonds only with thymine (T) and guanine (G) bonds only with cytosine (C) Law of complementary base pairs Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG Adenine and guanine are large molecules; thymine and cytosine are relatively smaller Because base pairing always places a large molecule with a small one, the diameter of the double helix remains constant Complementary base pairing explains Chargaff’s rule that for a given molecule of DNA, adenine equals thymine and guanine equals cytosine

Review Describe a nucleotide. What makes a nucleotide different from the next? Describe the three-dimensional structure of DNA.

How Does DNA Encode Information? How can a molecule with only four simple parts be the carrier of genetic information? The key lies in the sequence, not the number, of subunits Within a DNA strand, the four types of bases can be arranged in any linear order, and this sequence is what encodes genetic information

How Does DNA Encode Information? The genetic code is analogous to languages, where small sets of letters combine in various ways to make up many different words English has 26 letters Hawaiian has 12 letters The binary language of computers uses only two “letters” (0 and 1, or “on” and “off”) The sequence of only four nucleotides can produce many different combinations A 10-nucleotide sequence can code for more than 1 million different combinations of the four bases Must be in the right order to make sense.

How Does DNA Replication Ensure Genetic Constancy During Cell Division? Replication of DNA is a critical event in a cell’s life All cells come from pre-existing cells Cells reproduce by dividing in half Each of two daughter cells gets an exact copy of the parent cell’s genetic information Duplication of the parent cell DNA is called DNA replication mitosis

How Does DNA Replication Ensure Genetic Constancy During Cell Division? DNA replication produces two DNA double helices, each with one original strand and one new strand The ingredients for DNA replication are threefold: The parental DNA strands Free nucleotides A variety of enzymes that unwind the parental DNA double helix and synthesize new DNA strands When replication is complete, each parental strand and the daughter strand that was copied from it by base pairing wind together into a new DNA double helix, thus creating two copies of the original double helix

How Does DNA Replication Ensure Genetic Constancy During Cell Division? DNA replication produces two DNA double helices, each with one original strand and one new strand Base pairing is the foundation of DNA replication An adenine on one strand pairs with a thymine on the other strand; a cytosine pairs with guanine If a parental strand reads T-A-G, for example, the new strand reads A-T-C The two resulting DNA molecules have one old parental strand and one new strand (semiconservative replication) The parental and two new strands should all be identical to each other.

Semiconservative Replication of DNA free nucleotides The parental DNA is unwound by DNA Helicase New DNA strands are synthesized with bases complementary to the parental strands using DNA polymerase Each new double helix is composed of one parental strand (blue) and one new strand (red) Parental DNA double helix 1 2 4 3 Fig. 11-6

How Do Mutations Occur? Accurate replication and proofreading produce almost error-free DNA Infrequent changes in the sequence of bases in DNA result in defective genes called mutations Mutations range from changes in single nucleotides to movements of large pieces of chromosomes Mutations may have varying effects on function

How Do Mutations Occur? Mutations may have varying effects on function Mutations are often harmful, and an organism inheriting them may quickly die Some mutations may have no functional effect Some mutations may be beneficial and provide an advantage to the organism in certain environments These advantageous mutations may be favored by natural selection and are the basis for evolution of life on Earth

11.5 How Do Mutations Occur? Accurate replication and proofreading produce almost error-free DNA During replication, DNA polymerase mismatches nucleotides once every 1,000 to 100,000 base pairs, yet completed DNA strands contain only about one mistake in every 100 million to 1 billion base pairs In humans, this amounts to less than one error per chromosome per replication This reduction in errors is accomplished by DNA repair enzymes, which “proofread” each new daughter strand and replace mismatched nucleotides Proofreading occurs both during and after replication 1. …because of hydrogen bonds making replication highly accurate.

How Do Mutations Occur? Mistakes do happen DNA is altered or damaged in a number of ways Mistakes are made during normal DNA replication Certain chemicals (some components of cigarette smoke, for example) increase DNA errors during and after replication Ultraviolet radiation or X-rays also contribute to incorrect base pairing 2. Free radicals, some mold toxins Most changes in DNA are repaired by enzymes

Mutations Fig. 11-8a Point mutations—also called nucleotide substitutions—involve changes to individual nucleotides in the DNA sequence One type of point mutation occurs when a repair enzyme finds a mismatch but mistakenly cuts out the correct base and puts in the complement of the erroneous base

Mutations Insertion mutations occur when one or more new nucleotide pairs are inserted into the DNA double helix Fig. 11-8b

Mutations Deletion mutations occur when one or more nucleotide pairs are removed from the double helix Fig. 11-8c

Mutations An inversion occurs when a piece of DNA is cut out of a chromosome, turned around, and re-inserted into the gap Fig. 11-8d

Mutations Fig. 11-8e A translocation occurs when a chunk of DNA (often very large) is removed from one chromosome and attached to another

Review How does DNA encode hereditary information? Why is DNA replication considered “semi-conservative”? How do mutations occur? Why are mutations rare? Why are mutations usually harmful?