DNA: The Molecule Of Heredity Chapter 11 DNA: The Molecule Of Heredity

Chapter 11 At a Glance 11.1 How Did Scientists Discover That Genes Are Made of DNA? 11.2 What Is the Structure of DNA? 11.3 How Does DNA Encode Information? 11.4 How Does DNA Replication Ensure Genetic Constancy During Cell Division? 11.5 How Do Mutations Occur?

11.1 How Did Scientists Discover That Genes Are Made of DNA? By the late 1800s, 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

11.1 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

11.1 How Did Scientists Discover That Genes Are Made of DNA? Transformed bacteria revealed the link between genes and DNA (continued) 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

11.1 How Did Scientists Discover That Genes Are Made of DNA? Transformed bacteria revealed the link between genes and DNA (continued) 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

Transformation in Bacteria Bacterial strain(s) injected into mouse Results Conclusions (a) Mouse remains healthy R-strain does not cause pneumonia. Living R-strain (b) Mouse contracts pneumonia and dies S-strain causes pneumonia. Living S-strain (c) Mouse remains healthy Heat-killed S- strain does not cause pneumonia. Heat-killed S-strain (d) Mouse contracts pneumonia and dies A substance from heat-killed S-strain can transform the harmless R-strain into a deadly S-strain. Mixture of living R-strain and heat-killed S-strain Fig. 11-1

11.1 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

11.1 How Did Scientists Discover That Genes Are Made of DNA? The transforming molecule is DNA (continued) In the 1940s, Oswald Avery, Colin MacLeod, and Maclyn McCarty isolated DNA from S-strain bacteria, mixed it with live R-strain bacteria, and produced live S-strain bacteria This suggested that the transforming molecule from the S-strain was DNA To rule out the possibility that a protein contaminant was actually causing the transformation, Avery treated samples with protein-destroying enzymes and still induced transformation When DNA-destroying enzymes were added to the samples, transformation did not occur

11.1 How Did Scientists Discover That Genes Are Made of DNA? The transforming molecule is DNA (continued) Griffith’s experiments can be explained if DNA is the transforming agent 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

Molecular Mechanism of Transformation bacterial chromosome DNA fragments are transported into the bacterium A DNA fragment is incorporated into the chromosome Fig. 11-2

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

11.2 What Is the Structure of DNA? DNA is composed of four nucleotides DNA is made of chains of small subunits called nucleotides Each nucleotide has three components A phosphate group A deoxyribose sugar One of four nitrogen-containing bases 1. Thymine (T) 2. Cytosine (C) 3. Adenine (A) 4. Guanine (G)

11.2 What Is the Structure of DNA? DNA is composed of four nucleotides (continued) DNA is a double helix of two nucleotide strands Hydrogen bonds between complementary bases hold two DNA strands together

DNA Nucleotides phosphate base = adenine sugar base =guanine base = thymine base = cytosine base =guanine Fig. 11-3

11.2 What Is the Structure of DNA? DNA is composed of four nucleotides (continued) In the 1940s, biochemist E. Chargaff determined that the amount of A in a DNA molecule equaled the amount of T, and the amount of C equaled the amount of G This finding was called “Chargaff’s rule”

11.2 What Is the Structure of DNA? DNA is a double helix In the 1940s, several other scientists investigated the structure of DNA Rosalind Franklin and Maurice Wilkins studied the structure of DNA crystals using X-ray diffraction

11.2 What Is the Structure of DNA? DNA is a double helix (continued) From X-ray diffraction patterns, they 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

X-ray Diffraction Studies of DNA Fig. 11-4

11.2 What Is the Structure of DNA? DNA is a double helix (continued) James Watson and Francis Crick combined the X-ray data with bonding theory to deduce the structure of DNA DNA is 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

11.2 What Is the Structure of DNA? James Watson and Francis Crick combined the X-ray data with bonding theory to deduce DNA structure (continued) 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 Discovery of DNA Fig. E11-3

11.2 What Is the Structure of DNA? Hydrogen bonds between complementary bases hold two DNA strands together in a double helix The bases protrude inward toward each other from the sugar-phosphate backbone like rungs on a ladder Hydrogen bonds hold the base pairs together The ladder-like structure is twisted into a double helix

11.2 What Is the Structure of DNA? Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) The two strands in a DNA double helix are said to be antiparallel; that is, they are oriented in opposite directions From one end of the DNA molecule, if one strand starts with the free sugar and ends with the free phosphate, the other stand starts with the free phosphate and ends with the free sugar

11.2 What Is the Structure of DNA? Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) 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) Bases that bond with each other are called complementary base pairs Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG

11.2 What Is the Structure of DNA? Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) Complementary base pairing explains Chargaff’s rule that for a given molecule of DNA, adenine equals thymine and guanine equals cytosine Since every adenine, for example, is paired with a thymine, no matter how many adenines are in the DNA molecule, there will be an equal number of thymines

11.2 What Is the Structure of DNA? Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) 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 In 1953, James Watson and Francis Crick consolidated all the historical data about DNA into an accurate model of its structure

The Watson-Crick Model of DNA Structure nucleotide nucleotide free phosphate free sugar phosphate base (cytosine) sugar hydrogen bonds free sugar free phosphate (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 Fig. 11-5

11.3 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

11.3 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”)

11.3 How Does DNA Encode Information? 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

Replication of DNA is a critical event in a cell’s life 11.4 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

11.4 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 fact of complementary base pairing suggests a model for how DNA replicates 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

11.4 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 (continued) DNA replication begins with enzymes, called DNA helicases, that pull apart the parental DNA double helix at the hydrogen bonds between the complementary base pairs A second strand of new DNA is synthesized along each separated strand by DNA polymerases, which pair free nucleotides with their complementary nucleotides on each separated strand

11.4 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 (continued) 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

Author Animation: DNA Replication

Basic Features of DNA Replication free nucleotides The parental DNA is unwound New DNA strands are synthesized with bases complementary to the parental astrands 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

11.4 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 (continued) 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

11.4 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 (continued) The two resulting DNA molecules have one old parental strand and one new strand (semiconservative replication)

Semiconservative Replication of DNA Two identical DNA double helices, each with one parental strand (blue) and one new strand (red) One DNA double helix DNA replication Fig. 11-7

Accurate replication and proofreading produce almost error-free DNA 11.5 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

11.5 How Do Mutations Occur? Accurate replication and proofreading produce almost error-free DNA (continued) 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

11.5 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

11.5 How Do Mutations Occur? Mutations range from changes in single nucleotide pairs to movements of large pieces of chromosomes 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 Insertion mutations occur when one or more new nucleotide pairs are inserted into the DNA double helix Deletion mutations occur when one or more nucleotide pairs are removed from the double helix

Types of mutations (continued) 11.5 How Do Mutations Occur? Types of mutations (continued) An inversion occurs when a piece of DNA is cut out of a chromosome, turned around, and re-inserted into the gap A translocation occurs when a chunk of DNA (often very large) is removed from one chromosome and attached to another

Mutations Fig. 11-8a

Mutations Fig. 11-8b

Mutations Fig. 11-8c

Mutations Fig. 11-8d

Mutations Fig. 11-8e

Mutations may have varying effects on function 11.5 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