Ch. 14.1 -14.5

1. Ch. 14.1 -14.5 Dr. Marchette http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/csRicin.html

2. Section 14.1How is RNA transcribed from DNA? • Key Concepts • Life depends on enzymes and other proteins. • All proteins consist of polypeptide chains. • The chains are sequences of amino acids that corresponds to genes—sequences of nucleotide bases in DNA. • The path leading from genes to proteins has two steps. Transcription and translation.

3. Cont. • During transcription, the two stands of DNA double helix are unwound in a gene region. • Exposed chains of one strand become the template for assembling a single strand of RNA. • Only one type of RNA transcript encodes the message that gets translated into protein. • It is called messenger RNA (mRNA).

4. cont. • In transcription, the first step in protein synthesis, a sequence of nucleotide bases is exposed in a unwound region of a DNA strand. • That sequence is the template upon which a single stand of RNA is assembled from adenine, cytosine, quanine and uracil subunits.

5. DNA A-T C-G RNA A-U C-G Nitrogen Bases http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/dna_rna_compared.html

6. Three Classes of RNAs • Messenger RNA (mRNA) • Carries protein-building instruction • Ribosomal RNA (rRNA) • Major component of ribosomes • Transfer RNA (tRNA) • Delivers amino acids to ribosomes

7. A Nucleotide Subunit of RNA uracil (base) phosphate group sugar (ribose)

8. Steps from DNA to Proteins Same two steps produce all proteins: • Transcription- DNA is transcribed to form mRNA • Occurs in the nucleus • mRNA moves into cytoplasm

9. Transcription & DNA Replication • Like DNA replication • Nucleotides added in 5’ to 3’ direction • Unlike DNA replication • Only small stretch is template • RNA polymerase catalyzes nucleotide addition • Product is a single strand of RNA

10. Promoter • A base sequence in the DNA that signals the start of a gene • For transcription to occur, RNA polymerase must first bind to a promoter

11. Gene Transcription DNA to be transcribed unwinds transcribed DNA winds up again mRNA transcript RNA polymerase

12. Adding Nucleotides 5’ 3’ growing RNA transcript 5’ 3’ direction of transcription

13. http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/rna_modifications.htmlhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/rna_modifications.html snipped out snipped out Transcript Modification unit of transcription in a DNA strand 3’ 5’ exon intron exon intron exon transcription into pre-mRNA poly-A tail cap 5’ 3’ 5’ 3’ mature mRNA transcript

14. Base Pairing during Transcription DNA G C A U G C A T RNA C G T A C G T A DNA DNA base pairing in DNA replication base pairing in transcription

15. http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/transcription_details_v2.htmlhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/transcription_details_v2.html

16. tRNA Structure codon in mRNA anticodon amino-acid attachment site amino acid OH

17. Ribosomes tunnel small ribosomal subunit large ribosomal subunit intact ribosome

18. Summary • In gene, transcription, a sequence of exposed bases on one of the two strands of the DNA molecule serves as a template for synthesizing a complementary strand of mRNA. • RNA polymerase assemble the RNA from four kinds of ribonucelotides that differ I their bases: A,U,C,G. • Before leaving the nucleus, each new mRNA transcript, or pre-mRNA, undergoes modification into final form.

19. Section 14.2The Genetic Code • Key Concepts • The nucleotide sequence in DNA is read three bases at a time. • Sixty four base triplets correspond to specific amino acids and represent the genetic code. • The code words have been highly conserved through time. • Only in a few eukaryotes, prokaryotes, and prokaryote derived organelles have slight variation on the code.

20. Cont. • The correspondence between genes and proteins is encoded in protein-building “words” in mRNA transcripts. • Three nucleotide bases make up each three letter code.

21. Genetic Code • Set of 64 base triplets • Codons • 61 specify amino acids • 3 stop translation http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/genetic_code.html

22. Summary • The gentic code is a set of 64 different codons, which are nucleotide bases in mRNA that are “read” in sets of three. • Different codons (base triplets) specify different amino acids.

23. Section 14.3The Other RNA’s • Key Concepts • During translation, amino acids are bonded together in a polypeptide chain in a sequence specified by the base triplets in mRNA. • Transfer RNA delivers amino acids one at a time to the ribosome. • An RNA component of ribosome catalyzes the chain reaction.

24. Cont. • The codons in mRNA transcript are the words in protein building messages. • Without translators, words that originated from DNA mean nothing: it take the other two classes of RNA to synthesize the proteins.

25. Steps from DNA to Proteins cont. 2) Translation- mRNA with tRNA and rRNA at a ribosome is translated to form polypeptide chains of amino acids, which fold to form proteins, occurs in cytoplasm

26. Summary • Only mRNA carries DNA’s protein building instructions from the nucleus to the cytoplasm. • Transfer RNA (tRNA) deliver amino acids to ribosome. • Their anticodons base pair with codons in the order specified by mRNA. • Polypeptide chains are built on ribosomes, each consisting of a large and a small subunit made of tRNA and proteins.

27. Section 14.4The three Stages of Translation • Key Concepts • An mRNA transcript encodes DNA’s information about a protein enters an intact ribosome. • There, its codons are translated into polypeptide chain – a protein primary structure. • Translation of the protein building message proceeds through three continuous stages called: initiation, elongation and termination.

28. Three Stages of Translation Initiation Elongation Termination http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/translation_v2.html

30. http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/ribosome.htmlhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/ribosome.html Initiation • Initiator tRNA binds to small ribosomal subunit • Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon • Large ribosomal subunit joins complex

31. Binding Sites binding site for mRNA A (second binding site for tRNA) P (first binding site for tRNA)

32. Elongation • mRNA passes through ribosomal subunits • tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA • Peptide bonds form between the amino acids and the polypeptide chain grows

33. Elongation

34. Termination • Stop codon into place • No tRNA with anticodon • Release factors bind to the ribosome • mRNA and polypeptide are released mRNA new polypeptide chain http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/protein_synthesis.html

35. Summary • Translation is initiated when a small ribosomal subunit and an initiator tRNA arrive at a mRNA transcript's start codon, and a large ribosomal subunit binds to them. • tRNA delivers amino acids to a ribosome in the order dictated by the linear sequence of mRNA codons. • A polypeptide chain lengthens as peptide bonds from between amino acids. • Translation ends when a stop codon triggers events that cause the polypeptide chain and the mRNA to detach from the ribosome.

36. Section 14.5Mutated Genes & Their protein Products • Key Concepts • Gene mutations introduce changes in protein structure, protein function, or both. • The changes may lead to small variation in the shared traits that characterize individuals of a population. • When a cell taps its genetic code, it is making proteins with precise structural and functional roles that keep it alive.

37. Cont. • If a gene changes, the mRNA transcribed from it may change a specify an altered protein. • If the protein has a critical role, the out come will be a dead or abnormal cell.

38. Gene Mutations Base-Pair Substitutions Insertions Deletions

39. http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/base_pair_sub.htmlhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/base_pair_sub.html Base-Pair Substitution a base substitution within the triplet (red) original base triplet in a DNA strand During replication, proofreading enzymes make a substitution possible outcomes: or original, unmutated sequence a gene mutation

40. Frameshift Mutations • Insertion • Extra base added into gene region • Deletion • Base removed from gene region • Both shift the reading frame • Result in many wrong amino acids

41. http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/frameshift.htmlhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/frameshift.html Frameshift Mutation mRNA parental DNA arginine glycine tyrosine tryptophan asparagine amino acids altered mRNA DNA with base insertion altered amino- acid sequence arginine glycine leucine leucine glutamate

42. Transposing • DNA segments that move spontaneously about the genome • When they insert into a gene region, they usually inactivate that gene

43. Mutation Rates • Each gene has a characteristic mutation rate • Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation • Only mutations that arise in germ cells can be passed on to next generation

44. Mutagens • Ionizing radiation (X rays) • Nonionizing radiation (UV) • Natural and synthetic chemicals

45. Summary • A gene mutation is permanent in one or more bases in the nucleotide sequence of DNA. • The most common types are base pair substitution, deletion, and insertion . • Exposure to harmful radiation and chemicals in the environment can cause mutations in DNA. • A protein specified by a mutated gene may be harmful, neutral, or beneficial effect on the individual’s capacity to function in the environment.

46. Ionizing Radiation Fig. 14-12, p.227

47. Fig. 14-14, p.229