THE STRUCTURE OF THE GENETIC MATERIAL © 2012 Pearson Education, Inc. 1

SCIENTIFIC DISCOVERY: DNA is a double-stranded helix In 1953, James D. Watson and Francis Crick deduced the secondary structure of DNA, using X-ray crystallography data of DNA from the work of Rosalind Franklin and Maurice Wilkins and Chargaff’s observation that in DNA the amount of adenine was equal to the amount of thymine and the amount of guanine was equal to that of cytosine. © 2012 Pearson Education, Inc. 2

SCIENTIFIC DISCOVERY: DNA is a double-stranded helix Watson and Crick reported that DNA consisted of two polynucleotide strands wrapped into a double helix. The sugar-phosphate backbone is on the outside. The nitrogenous bases are perpendicular to the backbone in the interior. Specific pairs of bases give the helix a uniform shape. A pairs with T, with two hydrogen bonds G pairs with C, with three hydrogen bonds © 2012 Pearson Education, Inc. 3

DNA and RNA are polymers of nucleotides DNA and RNA = nucleic acids composed of long chains of nucleotides A nucleotide = nitrogenous base, five-carbon sugar, and phosphate group Nucleotides are joined to one another by a sugar-phosphate backbone. DNA is antiparallel © 2012 Pearson Education, Inc. 4

Partial chemical structure Figure 10.3D_2 Hydrogen bond G C T A A T Figure 10.3D_2 Three representations of DNA (part 2) C G Partial chemical structure 5

Each DNA nucleotide has a different nitrogen-containing base: adenine (A), cytosine (C), thymine (T), guanine (G). Nitrogenous base (can be A, G, C, or T) Thymine (T) Phosphate group Figure 10.2A_3 The structure of a DNA polynucleotide (part 3) Sugar (deoxyribose) DNA nucleotide 6

Thymine (T) Cytosine (C) Adenine (A) Guanine (G) Pyrimidines Purines Figure 10.2B The nitrogenous bases of DNA 7

RNA contains the nitrogenous base uracil (U) instead of thymine Figure 10.2C An RNA nucleotide Sugar (ribose) 8

DNA REPLICATION © 2012 Pearson Education, Inc. 9

DNA replication depends on specific base pairing In their description of the structure of DNA, Watson and Crick noted that the structure of DNA suggests a possible copying mechanism. DNA replication follows a semiconservative model where, The two DNA strands separate. Each strand is used as a template to produce a complementary strand, using specific base pairing. Semiconservative replication means that each new DNA helix has one old strand with one new strand. © 2012 Pearson Education, Inc. 10

Daughter DNA molecules A T G C A T Parental DNA molecule A T T A C G C G T Daughter strand A C G C G C G Parental strand T C G A Figure 10.4B The untwisting and replication of DNA C G A T T A A T C G G C A T T A T A A T G C Daughter DNA molecules 11

DNA replication proceeds in two directions at many sites simultaneously DNA replication begins at the origins of replication where DNA unwinds at the origin to produce a replication bubble Replication proceeds in both directions from the origin Replication ends when products from the bubbles merge with each other © 2012 Pearson Education, Inc. 12

Two daughter DNA molecules Figure 10.5A Parental DNA molecule Origin of replication Parental strand Daughter strand “Bubble” Figure 10.5A Multiple bubbles in replicating DNA Two daughter DNA molecules 13

DNA replication proceeds in two directions at many sites simultaneously Both daughter strands are synthesized in the 5’ to 3’ direction = new nucleotides are only added to the 3’ end of the growing strand One daughter strand is synthesized in one continuous piece, toward replication fork=leading strand The other daughter strand is synthesized in (Okazaki) fragments =lagging strand Consequence of the antiparallel nature of DNA © 2012 Pearson Education, Inc. 14

DNA replication proceeds in two directions at many sites simultaneously Two key proteins are involved in DNA replication. DNA polymerase adds nucleotides to a growing chain and proofreads and corrects improper base pairings. DNA ligase joins Okazaki fragments into a continuous chain. Helicase-is a helix breaker; it unwinds the DNA double helix by breaking the H-bonds. http://www.youtube.com/watch?v=zdDkiRw1PdU © 2012 Pearson Education, Inc. 15

Leading Strand Lagging Strand 3 DNA polymerase molecule 5 Parental DNA 5 3 Replication fork Lagging Strand 3 5 Figure 10.5C How daughter DNA strands are synthesized 5 3 DNA ligase Overall direction of replication 16

DNA replication proceeds in two directions at many sites simultaneously DNA polymerases and DNA ligase also repair DNA damaged by harmful radiation and toxic chemicals. DNA replication ensures that all somatic cells in a multicellular organism carry the same genetic information. © 2012 Pearson Education, Inc. 17

THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN © 2012 Pearson Education, Inc. 18

The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits DNA specifies traits by dictating protein synthesis. The molecular chain of command is from DNA to RNA to protein Transcription is the synthesis of RNA under the direction of DNA. Translation is the synthesis of proteins under the direction of RNA. © 2012 Pearson Education, Inc. 19

DNA Transcription RNA NUCLEUS CYTOPLASM Translation Protein Figure 10.6A_s3 DNA Transcription RNA NUCLEUS CYTOPLASM Translation Figure 10.6A_s3 The flow of genetic information in a eukaryotic cell (step 3) Protein 20

The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits The connections between genes and proteins In the 1940’s Beadle and Tatum suggested a one gene–one enzyme hypothesis based on studies of inherited metabolic diseases Their hypothesis is still accepted but with important changes: The hypothesis includes all proteins not just enzymes, and that one gene specifies a polypeptide © 2012 Pearson Education, Inc. 21

Genetic information written in codons is translated into amino acid sequences The genetic code is written in DNA as a series of three base sequences called triplets During transcription the genetic code in DNA is copied into a complementary three base sequences in RNA called codons Translation involves switching from the nucleotide “language” to the amino acid “language.” Each amino acid is specified by a codon. 64 codons are possible. © 2012 Pearson Education, Inc. 22

DNA Transcription RNA Codon Translation Polypeptide Amino acid A A A C Figure 10.7_1 DNA A A A C C G G C A A A A Transcription U U U G G C C G U U U U RNA Codon Translation Figure 10.7_1 Transcription and translation of codons (partial) Polypeptide Amino acid 23

The genetic code dictates how codons are translated into amino acids Characteristics of the genetic code Three nucleotides (a codon) specify one amino acid. AUG = start codon that codes for methionine; signals the start of translation 3 “stop” codons signal the end of translation. © 2012 Pearson Education, Inc. 24

The genetic code dictates how codons are translated into amino acids The genetic code is redundant, more than one codon for the same amino acid Unambiguous, in that any codon for one amino acid does not code for any other amino acid, nearly universal, the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals © 2012 Pearson Education, Inc. 25

Second base First base Third base Figure 10.8A Dictionary of the genetic code (RNA codons) 26

T A C T T C A A A A T C A T G A A G T T T T A G A U G A A G U U U U A Figure 10.8B_s3 Strand to be transcribed T A C T T C A A A A T C DNA A T G A A G T T T T A G Transcription RNA A U G A A G U U U U A G Figure 10.8B_s3 Deciphering the genetic information in DNA (step 3) Start codon Stop codon Translation Polypeptide Met Lys Phe 27

Transcription produces genetic messages in the form of RNA Overview of transcription RNA polymerase oversees transcription by unwinding DNA, and linking RNA nucleotides together to synthesize an RNA molecule The promoter is a nucleotide sequence in DNA that signals the start of transcription © 2012 Pearson Education, Inc. 28

Transcription produces genetic messages in the form of RNA Steps of Transcription Initiation RNA polymerase attaches to promoter. Elongation RNA grows longer. As RNA peels away, DNA strands rejoin. Termination RNA polymerase reaches a sequence of bases in the DNA template called a terminator, signaling the end of the gene. RNA pol detaches from RNA and the gene being transcribed © 2012 Pearson Education, Inc. 29

RNA polymerase Terminator DNA DNA of gene Promoter DNA 1 Initiation Figure 10.9B_1 The transcription of a gene (part 1) 30

2 Elongation Area shown in Figure 10.9A Growing RNA Figure 10.9B_2 The transcription of a gene (part 2) Growing RNA 31

Growing RNA 3 Termination Completed RNA RNA polymerase Figure 10.9B_3 The transcription of a gene (part 3) Completed RNA RNA polymerase 32

Eukaryotic RNA is processed before leaving the nucleus as mRNA Messenger RNA (mRNA) The RNA formed from transcription, carrying the genetic code is called mRNA mRNA carries the message from DNA (nucleus) to ribosomes (cytoplasm) In prokaryotes transcription and translation occur in the same place In eukaryotes, mRNA must exit nucleus via nuclear pores to enter cytoplasm. Eukaryotic mRNA has introns or interrupting sequences that separate exons, the coding regions © 2012 Pearson Education, Inc. 33

Eukaryotic RNA is processed before leaving the nucleus as mRNA Eukaryotic mRNA undergoes processing before leaving the nucleus. RNA splicing removes introns and joins exons to produce a continuous coding sequence. A 5’ cap and a 3’ tail of extra nucleotides are added to the ends of mRNA to Facilitate export of mRNA from nucleus, Protect the mRNA from attack by cellular enzymes To help ribosomes bind to the mRNA. © 2012 Pearson Education, Inc. 34

Transcription Addition of cap and tail Cap Figure 10.10 Exon Intron Exon Intron Exon DNA Transcription Addition of cap and tail Cap RNA transcript with cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence Figure 10.10 The production of eukaryotic mRNA NUCLEUS CYTOPLASM 35

Start of genetic message Figure 10.13A Start of genetic message Cap Figure 10.13A A molecule of eukaryotic mRNA End Tail 36

Transfer RNA molecules serve as interpreters during translation Transfer RNA (tRNA) molecules convert the codons of mRNA into the amino acid sequence of proteins. tRNA’s recognize the codons in mRNA with a three base sequence alled an anticodon Amino acid attachment site Anticodon © 2012 Pearson Education, Inc. 37

Ribosomes build polypeptides Translation occurs on the surface of the ribosome. Ribosomes coordinate the functioning of mRNA , tRNA & therefore synthesis of polypeptides. Ribosomes have two subunits: small and large. Each subunit is composed of ribosomal RNAs and proteins. Ribosomal subunits come together during translation Ribosomes have binding sites for mRNA and tRNAs. © 2012 Pearson Education, Inc. 38

tRNA binding sites Large subunit Small subunit mRNA binding site P site A site Small subunit Figure 10.12B A ribosome with empty binding sites mRNA binding site 39

The next amino acid to be added to the polypeptide Growing polypeptide Figure 10.12C The next amino acid to be added to the polypeptide Growing polypeptide mRNA tRNA Codons Figure 10.12C A ribosome with occupied binding sites 40

An initiation codon marks the start of an mRNA message Initiation occurs in two steps. An mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the start codon. Start codon = AUG and codes for methionine. The first tRNA has the anticodon UAC. A large ribosomal subunit joins the small subunit, allowing the ribosome to function. The first tRNA occupies the P site, which will hold the growing peptide chain. The A site is available to receive the next tRNA. © 2012 Pearson Education, Inc. 41

Large ribosomal subunit Figure 10.13B Met Met Initiator tRNA Large ribosomal subunit P site A site mRNA U A C U A C A U G A U G Start codon Figure 10.13B The initiation of translation Small ribosomal subunit 1 2 42

Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Elongation is the addition of amino acids to the polypeptide chain and each cycle of elongation has three steps. © 2012 Pearson Education, Inc. 43

Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Codon recognition: The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome. Peptide bond formation: The new amino acid is joined to the chain. Translocation: tRNA is released from the P site and the ribosome moves tRNA from the A site into the P site. © 2012 Pearson Education, Inc. 44

Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Termination 1. Ribosome reaches a stop codon 2. Completed polypeptide is freed from the last tRNA, 3. The ribosome splits into separate subunits. © 2012 Pearson Education, Inc. 45

Polypeptide Amino acid Anticodon mRNA Codons Codon recognition Figure 10.14_s4 Polypeptide Amino acid P site A site Anticodon mRNA Codons 1 Codon recognition mRNA movement Stop codon http://www.youtube.com/watch?v=BY_A8HyDnQ4 2 Peptide bond formation New peptide bond 3 Translocation 46

Mutations can change the meaning of genes A mutation is any change in the nucleotide sequence of DNA. © 2012 Pearson Education, Inc. 47

Mutations can change the meaning of genes Mutations within a gene can be divided into two general categories. Base substitutions involve the replacement of one nucleotide with another. Base substitutions may Have no effect at all, producing a silent mutation, A missense mutation still produces an amino acid, just not the correct amino acid A nonsense mutation changes an amino acid codon into a stop codon, usually producing a nonfunctional protein © 2012 Pearson Education, Inc. 48

Missense Mutation

Nonsense Mutation

Mutations can change the meaning of genes Mutations can result in deletions or insertions that may alter the reading frame (triplet grouping) of the mRNA, so that nucleotides are grouped into different codons This leads to significant changes in amino acid sequence downstream of the mutation, and produce a nonfunctional polypeptide. © 2012 Pearson Education, Inc. 51

Mutations can change the meaning of genes Mutations can be caused by spontaneous errors that occur during DNA replication or recombination mutagens, which include high-energy radiation (X-rays) and UV light and chemicals © 2012 Pearson Education, Inc. 53