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Cell Division, Genetics, Molecular Biology

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1 Cell Division, Genetics, Molecular Biology
20.2 Gene Expression Transcription/Translation Protein Synthesis

2 Amino Acids Frederick Sanger 1952: - proteins consist of sequence of molecules called amino acids - specific sequence of amino acids determines chemical properties of each protein - proteins produced by cell determine structure, function & development of cell Proteins responsible for inherited traits Sequence of base pairs in DNA makes up genetic code of organism Genetic code determines how amino acids strung together (how proteins are made)

3 Genetic Code In a gene: each set of 3 bases is a CODON
Genetic code always interpreted in terms of mRNA codon rather than original DNA sequence Each mRNA codon codes for a specific amino acid Only 20 amino acids found in proteins, depend on combination of bases in codon Start and stop codons initiate or terminate protein synthesis

4 Ex) A U G C U U A A A G C C U G A

5 Gene Expression Way information in a gene is converted into a specific trait through production of a polypeptide (protein) - products of all genes = polypeptides RNA is utilized to convert genes into proteins - messenger RNA (mRNA) - transfer RNA (tRNA) - ribosomal RNA (rRNA) 2 stages of gene expression: transcription and translation Transcription: genetic information converted from DNA sequence into mRNA, carries information from nucleus to cytoplasm Translation: genetic information from mRNA used to synthesize polypeptide chains of specific amino acids

6 Gene Expression Central Dogma: 2 step process of transferring genetic information from DNA to RNA, then from RNA to protein

7 Transcription Overview
DNA sequence copied (transcribed) into the sequence of a single stranded mRNA molecule THREE PROCESSES: 1. Initiation: RNA polymerase binds to DNA at specific site near beginning of gene 2. Elongation: RNA polymerase uses DNA as template to build mRNA molecule 3. Termination: RNA polymerase passes the end of gene and stops mRNA then released from template strand Carried through nuclear pores, into cytoplasm of cell

8 Transcription – Initiation
Only one strand of the double-stranded DNA is transcribed for each gene - template strand: one that is transcribed - either strand can serve as the template strand for different genes RNA polymerase binds to segment of DNA) - opens double helix - binds in front of gene to be transcribed

9 Transcription - Elongation
DNA strand to be transcribed = template RNA polymerase moves along template strand of DNA and begins building mRNA in 5’ to 3’ direction Promoter not transcribed mRNA strand complementary to DNA template strand EXCEPT thymine is replaced with URACIL

10 Transcription - Termination
RNA polymerase reaches end of gene Termination sequence: RNA polymerase recognizes it as end Transcription stops, mRNA disconnects from DNA template strand RNA polymerase free to bind to another promoter region - transcribe another gene mRNA free and diffuses out of nucleus into cytoplasm

11 Review Transcription

12 Translation Overview mRNA now in the cytoplasm needs to be translated - puts amino acids together according to genetic information and creates proteins Ribosomes: site of translation - composed of 2 subunits: clamp together around mRNA, moves along mRNA THREE PROCESSES: 1. Initiation 2. Elongation 3. Termination

13 Translation - Initiation
Occurs when ribosome recognizes specific sequence on mRNA – binds to it Ribosome moves along mRNA in 5’ to 3’ direction - adds amino acids to polypeptide chain once it reads a codon Must start reading in correct spot on mRNA - START codon (AUG) - ensures ribosome translates code using reading frame of mRNA molecule - results in correct sequence of amino acids Transfer RNA (tRNA) carries amino acids back to ribosome structure of tRNA contains an ANTICODON - complementary to codon of mRNA - tells tRNA which amino acid to bind to - amino acid binds to opposite end - tRNA delivers amino acid to ribosome

14 tRNA Structure

15 Translation - Elongation
Start codon recognized by ribosome - codes for methionine 2 sites for tRNA to attach - A (aminoacyl) - P (peptidyl) tRNA with anticodon complementary to start codon enters P site Next tRNA carrying required amino acid enters the A site - peptide bond formed between both amino acids Ribosome shifts over one codon so that SECOND tRNA is now in P site Released first tRNA from P site and allowed THIRD tRNA to enter empty A site - like a tickertape! Released tRNA’s recycled back into cytoplasm


17 Translation - Termination
Ribosome reaches one of THREE STOP CODONS - UGA, UAG, UAA Do not code for an amino acid, no corresponding tRNA’s Protein release factor recognizes that ribosome has stalled and helps release polypeptide chain from ribosome TERMINATION. Translation Transcribe & Translate!

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