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CHAPTER 12 DNA & RNA

CHAPTER 12 DNA & RNA. 12-1 DNA A . Griffith and Transformation 1 . Griffith’s Experiment Fig 12-2 pg 288 1928 Frederick Griffith - showed that genetic information could be transformed from one bacterium to another

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CHAPTER 12 DNA & RNA

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  1. CHAPTER 12 DNA & RNA 12-1 DNA A. Griffith and Transformation 1. Griffith’s Experiment Fig 12-2 pg 288 1928 Frederick Griffith - showed that genetic information could be transformed from one bacterium to another - heat-killed bacteria passed their disease killing ability to a harmless strain

  2. Griffith’s Experiment

  3. 2. Transformation - this process was called transformation where one strain of bacteria (harmless) had been changed into a disease-causing strain His hypothesis: When the heat-killed and the harmless bacteria were mixed, some factor was transferred from the heat-killed cells to the live cells and this factor might contain a gene with information to cause this change

  4. B. Avery and DNA 1944 Oswald Avery -repeated Griffith’s work and discovered that DNA is the nucleic acid that stores and transmits genetic information from one generation to the next. -DNA was the transforming factor

  5. Avery’s Experiment

  6. C. The Hershey-Chase Experiment 1. Bacteriophages - in 1952 a virus was being studied called a bacteriophage which infects and kills bacteria - they are made up of a DNA or RNA core with a protein coat - they attach to the surface of bacteria, inject their DNA into it, and force the bacteria to make new viruses which kill the bacteria and eventually burst out of the bacteria to infect other cells

  7. 2. Radioactive markers - Hershey and Chase designed their experiment to try to find out which part of the bacteriophage infected the bacteria; the DNA or the protein coat - Thus, they would learn whether genes were made of DNA or protein - Using radioactive markers, they concluded that the genetic material of the bacteriophage was DNA See Fig. 12-4

  8. D. The structure of DNA - long molecules made up of units called nucleotides - each nucleotide is made up of 3 parts: 1. 5-carbon sugar called deoxyribose 2. phosphate group 3. nitrogen-containing base - there are four kinds of nitrogen-containing bases: adenine (A) purines 2 rings guanine (G) cytosine (C) pyrimidines 1 ring thymine (T)

  9. Structure of DNA

  10. 1. Chargraff’s rules (late 1940s, early 1950s) - showed the percentage of guanine and cytosine in DNA were almost equal - the same is true for adenine and thymine - all organisms obeyed the rules, but no one knew why

  11. 2. X-ray evidence (early 1950s) - Involved the Rosalind Franklin molecule - She studied DNA using x-ray diffraction - The x-shaped pattern showed that the strands of DNA were twisted around each other in a shape known as a helix - The angle suggested there were two strands in the structure and that the nitrogenous bases are near the center of the molecule

  12. 3. The Double Helix - discovered by Watson and Crick in 1953 - they made cardboard and wire models of DNA structure based on information provided by x-ray diffraction -DNA is a double helix in which two strands are wound around each other and looks like a twisted ladder - The sides are the sugar and phosphate group - The steps are the nitrogenous bases held together by hydrogen bonds and that bonding occurs only between certain base pairs; this explained Chargraff’s rules

  13. Watson and Crick

  14. 12-2 Chromosomes and DNA Replication A. DNA and Chromosomes Prokaryotes -lack nuclei and many organelles - contain a single, circular DNA molecule located in the cytoplasm (Fig 12-8)

  15. eukaryotes - more complicated - contain 1000 times more DNA than prokaryotes - DNA is located in the nucleus in the form of a number of chromosomes which varies widely with different species

  16. 1. DNA length - very long and folded into a very small space Ex. E. coli chromosome contains 4,639,221 base pairs and is 1.6 mm in length 2. Chromosome structure (eukaryotic) - human DNA is packed even more tightly - more than 1 meter of DNA made up of 30 millions base pairs - contains both DNA and protein to form a substance called chromatin - chromatin consists of DNA coiled around proteins called histones; the DNA and histone molecules form a beadlike structure called a nucleosome - fibers made of nucleosomes coil up tightly and become visible - nucleosomes are able to fold large lengths of DNA into the tiny cell nucleus - nucleosomes may regulate how genes are “read” to make proteins

  17. Chromosome Structure

  18. B. DNA Replication -each strand of DNA has all the information needed to reconstruct the other half by means of base pairing and are said to be complimentary 1. Duplicating DNA - the process by which DNA duplicates itself before a cell divides is called replication - replication ensures that each new cell has a complete set of chromosomes

  19. prokaryotes - DNA replication begins at a certain point and proceeds in two directions until the entire chromosome is replicated

  20. eukaryotes - DNA replication occurs at hundreds of places, proceeding in both directions until each chromosome is copied -Replication forks are the sites where separation and replication occurs

  21. 2. How replication occurs - it is carried out by enzymes that “unzip” the DNA after the hydrogen bonds between the base pairs are broken and the two stands unwind - each strand is a template (model) for attaching complimentary bases - the principle enzyme involved is DNA polymerase because it polymerizes individual nucleotides to produce DNA - it also “proofreads” the new strand to help eliminate mistakes

  22. 12-3 RNA and Protein Synthesis The first step in decoding the genetic message to make proteins is to copy a nucleotide sequence from DNA to RNA A. The structure of RNA - RNA consists of a long chain of nucleotides - Each nucleotide consists of a 5-carbon sugar, a phosphate group, and a nitrogenous base - There are 3 main differences between RNA and DNA: 1. The RNA sugar is ribose, not deoxyribose 2. RNA is single stranded 3. RNA contains uracil in place of thymine

  23. RNA

  24. B. Types of RNA 1. messenger RNA (mRNA) - RNA molecules that carry copies of instructions from DNA 2. ribosomal RNA (rRNA) - the major part of a ribosome - proteins are assembled on ribosomes which are made up of several proteins and rRNA 3. transfer RNA (tRNA) - transfers each amino acid to the ribosome as specified by coded messages in the mRNA

  25. C. Transcription - the process by which RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complimentary sequence of RNA - requires an enzymes known as RNA polymerase - during transcription, RNA polymerase binds to DNA and separates the DNA strands - RNA polymerase then uses the DNA as a template (model) from which RNA is made - The enzyme only binds to regions of DNA known as promoters which have specific base sequences - Promoters signal where the enzyme should bind to make RNA - Similar signals in DNA cause the transcription process to end when the RNA strand is completed

  26. D. RNA Editing - often, large pieces are removed from the RNA molecule that are transcribed before they become functional - these pieces (introns) are cut out while the RNA is still in the nucleus - what remains, (exons), are spliced back together, and a cap and tail are added to form the final mRNA

  27. E. The Genetic Code - proteins are made by joining amino acids into long chains called polypeptides - each polypeptide contains a combination of any or all the 20 different amino acids - the properties of proteins are determined by the order in which the different amino acids are joined - the instructions in mRNA for assembling the protein is called the genetic code - RNA consists of 4 bases (U,A,G,C) - Each “word” in the instructions consists of 3 letters; each letter representing a base - The genetic code is read 3 letters at a time (represents 3 bases)

  28. - Each 3-letter “word” in mRNA is known as a codon - A codon consists of 3 consecutive nucleotides that specify a single amino acid Ex. mRNA sequence UCG-CAC-CCU Codon Amino acid codons represent: serine-histidine-glycine - one codon (AUG) may serve as a “start” codon for protein synthesis - there are also 3 “stop” codons that do not code for an amino acid but act like a period at the end of a sentence and signifies the end of the polypeptide

  29. Genetic Code

  30. F. Translation - the decoding of a mRNA message into a protein is known as translation - the ribosome reads the instructions (See Fig. 12-18) STEPS: 1. mRNA is made from a DNA model in the nucleus 2. mRNA leaves the nucleus and enters the cytoplasm and attaches to a ribosome - each codon on mRNA is read as it moves through the ribosome - each tRNA has an anticodon which is complimentary to the mRNA codon and it carried the appropriate amino acid to the ribosome which gets attached to the growing polypeptide by a peptide bond

  31. 3. The ribosome joins the arriving amino acid and breaks the bond between the amino acid and its tRNA. The tRNA leaves to find another amino acid. The ribosome then moves to another codon 4. The polypeptide chain grows until the ribosome reaches a “stop” codon where it releases the newly formed polypeptide and the mRNA molecule, completing the process of translation

  32. G. The Roles of RNA and DNA The cell uses DNA to make RNA which leaves the nucleus and goes to protein-building sites in the cytoplasm (ribosomes)

  33. 12-4 Mutations - cells sometimes make mistakes when copying their own DNA - these mistakes are called mutations and are changes in the DNA sequence that affect genetic information -gene mutations result from a change in a single gene -chromosomal mutations involve changes in whole chromosomes

  34. A. Gene Mutations - may involve several nucleotides, but the majority involve just one -point mutations involve one nucleotide and occur at a single point in the DNA sequence and may involve substituting one nucleotide for another

  35. - when point mutations involve insertions or deletions of a nucleotide, bigger changes may result and these are called frameshift mutations because they shift how the codes are read -frameshift mutations can alter a protein so that it is unable to perform its normal function

  36. B. Chromosomal Mutations - involve changes in the number or structure of chromosomes Types: 1. deletion - the loss of all or part of a chromosome 2. duplication - a segment of chromosome is repeated; the opposite of deletion

  37. 3. inversion - when a chromosome is oriented in the reverse of its usual direction 4. translocation - when part of one chromosome breaks off and attaches to another

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