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Gene Expression: Transcription and Translation

Gene Expression: Transcription and Translation . DNA Terminology. Chromatin: DNA wrapped around histone proteins located in the nucleus of every cell when a cell is not dividing.

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Gene Expression: Transcription and Translation

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  1. Gene Expression: Transcription and Translation

  2. DNA Terminology • Chromatin: DNA wrapped around histone proteins located in the nucleus of every cell when a cell is not dividing. • Chromosome: A supercoiled DNA molecule that appears during cell division (mitosis or meiosis). Humans have a total of 46 chromosomes or 46 DNA molecules. • Gene: A small portion of a DNA molecule that contains the codes to make a protein, rRNA, or tRNA molecule. A single molecule of DNA is made up of thousands of genes. • Genome: the entire collection of genes that a particular organism or species has in its cells. • http://hypertextbook.com/facts/1998/StevenChen.shtml

  3. Genes Gene 2 Gene 3 Gene 1 What is the basic unit of nucleic acids?

  4. Genome Individual 1 Individual 2

  5. Gene Expression • Gene Expression looks at how the purpose of a particular gene unfolds from start to finish. Gene Expression involves two major stages: 1. Transcription and 2. Translation. • Most genes codes for proteins. • Review Question: What are the different and important functions that proteins have? • How many genes are in the human genome? • What is the Human Genome Project? • http://www.genome.gov/10001772

  6. The Central Dogma https://courses.cit.cornell.edu/biomi290/z.OldWebSite/TRANSCON.HTML

  7. Central Dogma Overview • DNA carries the genetic code and transcribes a mRNA copy of the code • The mRNA copy is translated by ribosomes to make a protein DNARNAProtein Transcription Location: Nucleus Translation Location: Cytoplasm at a Ribosome

  8. The Big Picture • The process of converting the information contained in a DNA segment into proteins begins with the synthesis of mRNA molecules containing anywhere from several hundred to several thousand nucleotides, depending on the size of the protein to be made. • Each of the thousands of proteins in a given species is synthesized from a different mRNA strand that had been transcribed from a specific gene on DNA. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html#

  9. Codons • The “words” of the DNA “language” are triplets of 3 nitrogen bases adjacent to each other (in a row) and are called codons • The codons in DNA are transcribed into codons of mRNA which are translated into amino acids

  10. Why do we need mRNA if DNA holds all the genetic instructions for the proteins the cell is supposed to produce? Mutations • DNA must be protected…If DNA is damaged in any way, then the coding sequence is changed and a mutation could result which could greatly affect a cell or even the whole organism! • Side Note: Not all mutations are harmful. Some can actually be beneficial. Some can be also be neutral.

  11. Transcription

  12. Transcription • If a protein is required by a cell, that gene is activated (or turned on like a light switch) • The gene makes an RNA copy of itself in the form of a messenger RNA molecule (mRNA) • The enzyme RNA polymerase runs along an open DNA molecule and copies only one side (or strand) of a gene. RNA polymerase basically puts together complimentary RNA nucleotides as it reads the DNA nucleotides • This occurs in the nucleus of the cell. • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter15/stages_of_transcription.html • http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html • http://bcs.whfreeman.com/thelifewire/content/chp12/1202001.html

  13. Transcription - Initiation RNA Polymerase 5’ 3’ G C A C T A A A A T RNA Polymerase attaches to the promoter region of the open DNA template strand. The promoter region contains a TATA box which has the core DNA sequence of 5'-TATAAA-3'

  14. Transcription – Elongation RNA Polymerase 5’ 3’ G C C G C U A T A A A A T G RNA Polymerase ‘reads’ the DNA nucleotides in a 3’ → 5’ direction, and adds complementary RNA nucleotides in a 5’→ 3’ direction. During elongation, the pre-mRNA strand is growing as RNA nucleotides are added. Note that Uracil is used in place of Thymine! 3’ 5’

  15. Transcription – Termination RNA Polymerase RNA Polymerase reaches a termination sequence along the DNA template strand and detaches along with the newly synthesized pre-mRNA strand. Transcription is now complete! The pre-mRNA will now undergo some processing before leaving the nucleus. C G U G T A A A T C A C G A

  16. mRNA Processing

  17. Eukaryotic mRNA Processing • A 5’ cap is added to the pre-mRNA strand • A poly-A tail is added to the 3’ end. • Splicing: Introns (non-coding regions) are removed and exons are joined. The mRNA is now ready to leave the nucleus and translation can begin! Exon Intron AAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAA Helpful Animations http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/bio30.swf::How%20Spliceosomes%20Process%20RNA http://faculty.salisbury.edu/~flerickson/animations/mrna%20processing%20animation.swf

  18. Transcription and mRNA Processing Practice Transcribe this gene: The promoter region is blue,the exons are gray, the introns are purple.Template strand is indicated by the stars. 5’AATCGCTTTATAA T G GACGT G T A C A G C CCCGGA G C T C CCA AC G T G ATGT3’ *3’ TTAGCGAAATATT A C CTGCA C A T G T C G GGGCCT C G A G GG T TG C A C TACA5’* 5’ A U G GG U A C A G C A G C U C CC A C G U G A 3’

  19. The mRNA Codon • The mRNA codons consist of 4 nucleotides: AUGC (U replaces T in DNA) • How many possible combinations are there? • 43 = 64 possible combinations • How many amino acids are there?

  20. The Genetic Code l

  21. Genetic CodeA different format

  22. The Genetic Code • The specific sequence of 3 nucleotide bases in a row indicates how a protein is to be constructed • An mRNA sequence of: UUU-UUG-GUA-CCC Means that the protein amino acid sequence will be: Phenylalanine-Leucine-Valine-Proline

  23. Types of RNA • There are 3 types of RNA involved with transcription and translation. • They are all made up of RNA nucleotides; however the stand of nucleotides is folded into a different shape, which ultimately determines its function. • A nucleotide = 1 phosphate + 1 ribose sugar + 1 nitrogen base (no T!) • mRNA: Messenger RNA is the ‘recipe’ for making a protein. • rRNA: Ribosomal RNA makes up the subunits of ribosomes. The function of the ribosome is to ‘read’ the mRNA. • tRNA: Transfer RNA holds one particular amino acid. It brings or transfers that amino acid over to the ribosome.

  24. tRNA • The top of the tRNA molecule holds a specific amino acid. • The bottom of the tRNA molecule contains the anticodon. Amino acid is held here. The anticodon does not code for the amino acid! The mRNA contains the amino acid codes! The anticodon is simply the spot that aligns itself with the mRNA strand.

  25. The Ribosome • The ribosome is made up of two subunits of RNA nucleotides (a large and small subunit). • The ribosomal subunits are synthesized in the nucleolus of the nucleus. • The ribosomal subunits come together in the cytoplasm when a mRNA molecule needs to be ‘read’. • The ribosomes are either freely suspended in the cytoplasm or embedded in the membranes of the Rough Endoplasmic Reticulum. The ribosomes of the Rough ER are synthesizing proteins to be sent in vesicles out of the cell. • The ribosome ‘reads’ the mRNA strand in a 5’ → 3’ direction.

  26. Ribosomal Anatomy Small subunit 3’ 5’ Large subunit

  27. Transcription and Translation Overview • Pre-mRNA is produced in the nucleus by the enzyme RNA polymerase reading the DNA template strand. • mRNA leaves the nucleus and travels to the cytoplasm where it attaches to a ribosome (either freely suspended or embedded in the Rough ER) • The ribosome reads the mRNA codons and tRNA brings over the appropriate amino acid. • Amino acids are joined together by peptide bonds to make a protein. Transcription – mRNA Synthesis Translation – Protein Synthesis

  28. Translation

  29. Translation - Initiation Small ribosomal subunit A U G C C G A A C U G A 5’ Large ribosomal subunit The large and small ribosomal units come together over the mRNA strand http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html#

  30. Translation - Initiation A U G C C G A A C U G A 5’ 3’ P Site A Site The start codon AUG is ‘read’ at the P site.

  31. Translation - Initiation 5’ 3’ A U G C C G A A C U G A P Site A Site The tRNA carrying the amino acid, methionine, aligns its anticodon with the mRNA codon U A C MET

  32. Translation – Elongation 5’ 3’ A U G C C G A A C U G A P Site A Site The next mRNA codon is ‘read’ at the A site. U A C MET

  33. Translation – Elongation 5’ 3’ A U G C C G A A C U G A P Site A Site The next tRNA molecule brings over the appropriate amino acid. U A C G G C MET ???

  34. Translation – Elongation 5’ 3’ A U G C C G A A C U G A P Site A Site The amino acids are joined together by peptide bonds. The ribosome shifts down a space of one codon. The A site reads the next mRNA codon. G G C ??? MET

  35. Translation – Elongation 5’ 3’ A U G C C G A A C U G A P Site A Site The A site of the ribosome accepts the next tRNA molecule carrying the appropriate amino acid. U A C G G C ??? MET ???

  36. Translation – Termination 5’ 3’ A U G C C G A A C U G A P Site A Site The A site of the ribosome ‘reads’ a stop codon. No amino acid is brought over. The protein then gets transported to where it is needed. The ribosomal subunits, mRNA strand, and tRNA molecules disassociate from one another U A C ??? ??? MET

  37. Review Transcription Translation Location: Cytoplasm Function: Production of polypeptide (protein) chains. Description: The ribosome translates the mRNA coding sequence while the tRNA molecules bring over the appropriate amino acids. Amino acids are linked together by peptide bonds to form a protein. • Location: Nucleus • Function: Production of pre-mRNA strands • Description: A copy of a gene on a DNA molecule is made. The enzyme RNA polymerase puts RNA nucleotides together to form a mRNA molecule

  38. Mutations • Mutations, simply put, are alterations in the original base sequences of DNA. • Mutations can occur through: • Mistakes that occur during DNA replication • Exposure to chemicals or radiation that cause the DNA molecules to break down (mutagens). • Types of mutations • Point or Substitution Mutations: a change in a single base. • Silent mutation: the change results in no change in the amino acid (thanks to degeneracy in genetic code) • Missense: the change results in a single different amino acid • Nonsense: the change results in a stop codon • Continuation or readthrough: was supposed to be a stop codon, but now no longer is.

  39. Mutations – cont. http://evolution.berkeley.edu/evolibrary/article/0_0_0/mutations_03 http://evolution.berkeley.edu/evolibrary/article/0_0_0/sicklecase_01 http://commons.wikimedia.org/wiki/File:Types-of-mutation.png http://commons.wikimedia.org/wiki/File:Types_of_mutations_01.png

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