How Genes Work. Chapter 9. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display

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1 How Genes Work Chapter 9

2 DNA or Protein? Mendel s work left a key question unanswered: What is a gene? The work of Sutton and Morgan established that genes reside on chromosomes But chromosomes contain proteins and DNA So which one is the hereditary material Several experiments ultimately revealed the nature of the genetic material.dna

3 9.3 Discovering the Structure of DNA DNA is made up of nucleotides Each nucleotide has a central sugar, a phosphate group and an organic base The bases are of two main types Purines Large bases Adenine (A) and Guanine (G) Pyrimidines Small bases Cytosine (C) and Thymine (T)

4 Fig. 9.3 The four nucleotide subunits that make up DNA Nitrogenous base 5-C sugar

5 Fig. 9.4 The DNA double helix Erwin Chargaff made key DNA observations that became known as Chargaff s rule Purines = Pyrimidines A = T and C = G The two possible basepairs

6 In 1953, James Watson and Francis Crick deduced that DNA was a double helix They came to their conclusion using Tinkertoy models and the research of Chargaff and Franklin Fig. 9.4 James Watson (1928- ) Francis Crick ( )

7 9.4 How the DNA Molecule Replicates The two DNA strands are held together by weak hydrogen bonds between complementary base pairs A and T C and G If the sequence on one strand is The other s sequence must be ATACGCAT TATGCGTA Each chain is a complementary mirror image of the other So either can be used as template to reconstruct the other

8 There are 3 possible methods for DNA replication Fig. 9.5 Daughter DNAs contain one old and one new strand Original DNA molecule is preserved Old and new DNA are dispersed in daughter molecules

9 How DNA Copies Itself The enzyme helicase first unwinds the double helix The enzyme primase puts down a short piece of RNA termed the primer DNA polymerase reads along each naked single strand adding the complementary nucleotide Fig. 9.8

10 Transcription & Translation Gene expression is the use of information in DNA to direct the production of proteins The path of genetic information is often called the central dogma Fig DNA RNA Protein

11 transcription translation

12 9.5 Transcription The transcriber is RNA polymerase Fig It binds to one DNA strand at a site called the promoter It then moves along the DNA pairing complementary nucleotides It disengages at a stop signal

13 Transcription & Translation A cell uses three kinds of RNA to make proteins Messenger RNA (mrna) Transfer RNA (trna) Ribosomal RNA (rrna)

14 9.6 Translation Translation converts the order of the nucleotides of a gene into the order of amino acids in a protein The rules that govern translation are called the genetic code mrnas are the blueprint copies of nuclear genes mrnas are read by a ribosome in threenucleotide units, termed codons Each three-nucleotide sequence codes for an amino acid or stop signal

15 Fig What happened to Thymine (T)?

16 Making the Protein mrna binds to the small ribosomal subunit The large subunit joins the complex, forming the complete ribosome mrna threads through the ribosome producing the polypeptide Fig. 9.16

17 Transfer RNA trnas bring amino acids to the ribosome They have two business ends Anticodon which is complementary to the codon on mrna 3 OH end to which the amino acid attaches Fig Hydrogen bonding causes hairpin loops 3-D shape

18 Fig How translation works The process continues until a stop codon enters the A site The ribosome complex falls apart and the protein is released

19 transcription translation

20 9.7 Architecture of the Gene In eukaryotes, genes are fragmented They are composed of Exons Sequences that code for amino acids Introns Sequences that don t Eukaryotic cells transcribe the entire gene, producing a primary RNA transcript This transcript is then heavily processed to produce the mature mrna transcript This leaves the nucleus for the cytoplasm

21 Fig Processing eukaryotic mrna Protect from degradation and facilitate translation Different combinations of exons can generate different polypeptides via alternative splicing

22 7. Phosphorylation or other chemical modifications can alter the activity of a protein after it is translated. 6. The polypeptide chain grows until the protetin is completed. Completed polypeptide Amino acid trna 5 Ribosome moves toward 3 end Fig How protein synthesis works in eukaryotes RNA polymerase Cytoplasm DNA 3 1. In the cell nucleus, RNA polymerase transcribes RNA from DNA 5 5. trnas bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and trnas exit the ribosome from the E site. 5 3 Primary RNA transcript 5 3 Exons Cap Poly-A tail Poly-A Introns tail mrna 3 2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mrna Nuclear membrane Nuclear pore Ribosome 3 Small ribosomal subunit mrna 5 4. trna molecules become attached to specific amino acids with the help of activating enzymes. Amino acids are brought to the ribosome in the order dictated by the mrna. Cap Large ribosomal subunit 3. mrna is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mrna

23 9.9 Mutation The genetic material can be altered in two ways Recombination Change in the positioning of the genetic material Mutation Change in the content of the genetic material Bithorax mutant Fig. 9.22

24 9.9 Mutation Mutation and recombination provide the raw material for evolution Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives The rate of evolution is ultimately limited by the rate at which these alternatives are generated Mutations in germ-line tissues can be inherited Mutations in somatic tissues are not inherited They can be passed from one cell to all its descendants

25 Kinds of Mutation Mutations are caused in one of two ways Errors in DNA replication Mispairing of bases by DNA polymerase Mutagens Agents that damage DNA

26 Kinds of Mutation The sequence of DNA can be altered in one of two main ways Point mutations Alteration of one or a few bases Base substitutions, insertion or deletion Frame-shift mutations Insertions or deletions that throw off the reading frame

27 Fig. 9.23

28 Kinds of Mutation The position of genes can be altered in one of two main ways Transposition Movement of genes from one part of the genome to another Occurs in both eukaryotes and prokaryotes Chromosomal rearrangements Changes in position and/or number of large segments of chromosomes in eukaryotes

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31 Mutation, Smoking and Lung Cancer Agents that cause cancer are called carcinogens These are typically mutagens The hypothesis that chemicals cause cancer was first advanced in the 18 th century Many investigations since then have determined that chemicals can cause cancer in both animals and humans For example, tars and other chemicals in cigarette smoke can cause cancer of the lung