Chapter 13. From DNA to Protein

Transcription

1 Chapter 13 From DNA to Protein

2 Proteins All proteins consist of polypeptide chains A linear sequence of amino acids Each chain corresponds to the nucleotide base sequenceof a gene

3 The Path From Genes to Proteins 1. Transcription Enzymes use the base sequence of a gene as a template to make a strand of RNA 2. Translation Information in the RNA strand is decoded (translated) into a sequence of amino acids transcription translation DNA RNA Protein

4 Prokaryotes and Eukaryotes In prokaryotic cells (no nucleus) Transcription and translation occur in cytoplasm In eukaryotic cells Genes are transcribed in the nucleus Resulting mrna is translated in the cytoplasm

5 Key Concepts: INTRODUCTION Life depends on enzymes and other proteins All proteins consist of polypeptide chains Chains are sequences of amino acids that correspond to sequences of nucleotide bases in DNA called genes The path leading from genes to proteins has two steps: transcription and translation

6 Transcription: DNA to RNA Two DNAstrands unwind in a specific region RNA polymeraseassembles a strand of RNA Covalently bonds RNA nucleotides (adenine, guanine, cytosine, uracil) according to the nucleotide sequence of the exposed gene

7 Three Types of RNA Messenger RNA (mrna) Carries protein-building codes from DNA to ribosomes Ribosomal RNA (rrna) Forms ribosomes (where polypeptide chains are assembled) Transfer RNA (trna) Delivers amino acids to ribosomes

8 RNA and DNA Compared

9 RNA Base Pairing

10 phosphate group base (uracil) sugar (ribose)

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13 gene region newly forming RNA transcript RNA polymerase, the enzyme that catalyzes transcription DNA template winding up DNA template unwinding Fig. 13.3, p.198

14 Fig. 13.3, p.198

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16 RNA Modification: Alternative Splicing Before mrna leaves the nucleus: Intronsare removed Some exonsare removed along with introns; remaining exons are spliced together in different combinations Poly-A tailis added to 3 end of new mrna

17 The Poly-A Tail The longer its poly-a tail, the more time an mrna transcript (and its protein-building message) will remain intact in the cytoplasm

18 unit of transcription in DNA strand exon intron exon intron exon transcription into pre-mrna cap poly-a tail 5' 3' snipped out snipped out mature mrna transcript

19 Key Concepts: TRANSCRIPTION During transcription, the two strands of the DNA double helix are unwound in a gene region Exposed bases of one strand become the template for assembling a single strand of RNA (a transcript) Messenger RNA is the only type of RNA that carries DNA s protein-building instructions

20 The Genetic Code Messenger RNA(mRNA) carries DNA s protein-building information to ribosomes for translation mrna s genetic message is written in codons Sets of three nucleotides along mrna strand

21 Codons Codons specify different amino acids A few codon signals stop translation Sixty-four codons constitute a highly conserved genetic code

22 From DNA to Polypeptide

23 Variation in Genetic Code Variant codons occur among prokaryotes, prokaryote-derived organelles (such as mitochondria), and some ancient lineages of single-celled eukaryotes

24 Key Concepts: CODE WORDS IN THE TRANSCRIPTS The nucleotide sequence in RNA is read three bases at a time Sixty-four base triplets that correspond to specific amino acids represent the genetic code, which has been highly conserved over time

25 trna and rrna Function in Translation Transfer RNA (trna) Anticodon binds to mrna codon Also binds amino acid specified by codon Different trnas carry different amino acids trnas deliver free amino acids to ribosomes during protein synthesis

26 trna

27 rrna Ribosomal RNA (rrna)and proteins make up the two subunits of ribosomes

28 SUMMARY: Protein Synthesis

29 Three Stages of Translation mrna-transcript information directs synthesis of a polypeptide chain during translation Translation proceeds in three stages Initiation Elongation Termination

30 Initiation One initiator trna, two ribosomal subunits, and one mrna come together as an initiation complex

31 Initiation

32 Elongation trnas deliver amino acids to the ribosome in the order specified by mrna codons Ribosomal rrna catalyzes the formation of a peptide bond between amino acids

33 Elongation

34 Elongation

35 Elongation

36 Termination Translation ends when RNA polymerase encounters a STOP codon in mrna New polypeptide chain and mrna are released Ribosome subunits separate from each other

37 Termination

38 Key Concepts: TRANSLATION During translation, amino acids become bonded together into a polypeptide chain in a sequence specified by base triplets in messenger RNA Transfer RNAs deliver amino acids one at a time to ribosomes Ribosomal RNA catalyzes the formation of peptide bonds between the amino acids

39 Mutated Genes and Their Protein Products Mutationsare permanent, small-scale changes in the base sequence of a gene Common mutations include insertions, deletions, and base-pair substitutions

40 Common Gene Mutations

41 Transposable Elements Another type of insertion mutation is caused by the activity of transposable elements Segments of DNA that can insert themselves anywhere in a chromosome

42 Some Causes of Mutations Natural and synthetic chemicals Cigarette smoke Environmental agents Ionizing radiation Nonionizing radiation

43 Key Concepts: MUTATIONS IN THE CODE WORDS Mutations in genes may result in changes in protein structure, protein function, or both The changes may lead to variation in traits among individuals

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