Class Notes (811,705)
Canada (494,883)
Biology (6,711)
Biology 1002B (1,340)
Tom Haffie (863)

chapter 14.docx

9 Pages
Unlock Document

Biology 1002B
Tom Haffie

Addition:  mRNA is the nucleus, ,  tRNA or the transfer RNA is produced in the cytoplasm and it helps in the addition of aminoacids to the polypeptide chain. tRNA molecules are transcribed (in eukaryotic cells) by RNA polymerase III,unlike messenger RNA which is transcribed by RNA polymerase II. Molecules of tRNA are formed in the nucleus and migrate into the cytoplasm.  snRNA or small nuclear RNA is produced in the nuleus and it is responsible for the splicing mechanism for the mRNA to chop off the introns from it.  rRNA is present within the ribosome and so called ribosomal RNA. it helps in the recognition. It is made in nucleolus Genes Specify either Protein or RNA Products  The "one gene–one enzyme" hypothesis is wrong, and the “one gene–one polypeptide” hypothesis isn't right either.  "one gene–one enzyme" presumes that all gene products are enzymes. This is obviously false, since genes can code for structural proteins, transport proteins, and other proteins that don't have any enzyme action.  "one gene–one polypeptide” presumes that: a) all gene products are polypeptides. This is false. Some gene products are RNA, such as tRNA or rRNA. b) one gene specifies only one final gene product. This is false in eukaryotes. Differential splicing of mRNA can lead to a number of polypeptides being encoded in the same "gene." Kinds of RNA. Some of genes: Protein coding genes that encode mRNAs to be translated Others: Non protein coding genes that encode RNAS that are never translated, such as Ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA) prokaryotes vs eukaryotes tRNA, rRNA etc in prokaryotes have similar promotes, thus they all get transcribed by the same RNA polymerase complex in eukaryotes, RNA polymerase II transcribes protein-coding gene i.e mRNa. RNA polymerase I and III transcribe non protein coding genes such as tRNA, rNA, snRNA Messenger RNA (mRNA) Messenger RNA contains genetic information. It is a copy of a portion of the DNA. It carries genetic information from the gene (DNA) out of the nucleus, into the cytoplasm of the cell where it is translated to produce protein. Ribosomal RNA (rRNA) This type of RNA is a structural component of the ribosomes. It does not contain a genetic message. Transfer RNA (tRNA) Transfer RNA functions to transport amino acids to the ribosomes during protein synthesis. Small nuclear RNA (snRNA) These strands of RNA are complexed with protein producing small nuclear ribonucleoproteins (snRNP). One function, described later in this chapter, is the modification of the RNA transcript.  The defect causing alkaptonuria is the result of an alteration of a gene that encodes an enzyme that metabolizes a key chemical. The altered gene causes a defect in the function of the enzyme, which leads to the phenotype of the disease.  Beadle and Tatum- ONE GENE_ONE ENZYME HYPOTHESIS  They chose Neurospora, a haploid fungus, that can grow on a minimal medium (MM) – simple nutritional needs.  But when this fungus get exposed to X-rays causing mutations. This mutation causes the fungus to grow unless MM is supplemented with additional nutrients such as amino acid or vit. This mutant is called auxotoph  So, if a mutant needs arginine to grow, that means that it has a defect in a gene for an enzyme involved in the synthesis of arginine.  Based on the figure “Growth on MM+”.  If the defect is on argH+ gene then the mutant can grow in arginine.  If the defect is on argG+ gene then the mutant can grow in arginisoccinate and arginine  if the defect is on arg F+ gene then the mutant can grow in citrulline, arginisoccinate and arginie  if the defect is on argE+ gene then the mutant can grow in ornithine, citrulline, arginisoccinate and arginie  The pathway from Gene to Polypeptide involves Transcription and Translation  The processes of transcription and translation are similar in prokrayotes and eukaryotes. One key difference is that whereas prokaryotes can transcribe and translate in a given gene simulatenously, eukaryotes transcribe and process mRNA in the nucleus before exporting it to the cytoplasm for translation. The genetic Code is Written in Three Letter Words Using a Four Letter Alphabet  DNA : A, T, G, C RNA: A, U, G, C 20 amino acids Breaking the Genetic Code”  The three-letter codons in DNA are first transcribed into complementary three-letter RNA codons.  The template strand for a given gene is always read 3' to 5'  (artifical mRNAs of codon length-three nucleotides-could bind to ribosomes in a test tube and cause a single tRNA, with its linked amino acid, to bind to the ribosome. tRNAs are special class of RNA molecules that bring amino acids to the ribosome for assembly into the polypeptide chain.  The start codon -AUG  The stop codons – UAA, UAG, UGA  Many redundancy in nucleic acid code. Ex: CCU, CCC, CCA, and CCG = proline  There is only one correct reading frame for each mRNA. Thus, the code can be read correctly only by starting at the right place-at the first base of the first three-letter codon at the beginning of a coded message (the start codon) – and reading three nucleotides at a time.  The code is also universal. The same for all living things with a few exceptions. The universality of the nucleic acid code indicates that it was present in early life and it remains unchanged through billions years. ---------------------------------------------------------------------------------------------------------------------------------- CHAPTER 14 1. Liver cells, bones cells, etc are actually the same set of genes that was created in the original single-celled zygote at fertilization. They have different function and structure though? True! a. That is because of the presence or absence of the product of gene expression. And this is not because of the presence or absence of genes themselves. 2. Regulation of Gene Expression in Prokaryotic Cells a. Prokaryotes undergo rapid and reversible alterations in biochemical pathways that allow them to adapt quickly to changes in their environment. b. Turn off and turn on gene expression (lactose and tryptophan) cause a versatile and responsive control system to allow the organism to make the most efficient use of particular array of nutrients and energy sources c. The Operon is a Unit of Transcription i. An operon is a cluster of prokaryotic genes and the DNA sequences involved in their regulation ii. Promoter is (TATA box) a region where the RNA polymerase begins transcription which happens after the promoter iii. Operator is a short segment that is a binding sequence for a regulatory protein iv. Repressor bounds to theDNA increases the likelihood that genes will be transcribed d. The lac Operon for Lactose Metabolism is Transcribed when an inducer inactivates a repressor i. The promoter binds RNA polymerase and the operator binds the activated Lac repressor ii. Lactose absent from medium: structural genes expressed at very low levels 1. Active Lac repressor expressed from LacI gene binds to operator 2. RNA polymerase blocked from binding to operator iii. Lactose present in medium: structural genes expressed at high levels 1. Lactose enters the cell and the low level of beta-galactosidase molecules already present convert some of the lactose to the allolactose=inducer 2. Allolactose binds to the active Lac repressor -> inactive Lac repressor by altering its shape. Now Lac repressor cannot bind to the operator 3. No more a barrier. RNA polymerase can bind to the promoter. 4. Transcription occurs 5. Translation produces the three enzyme (lacZ, Y, A) e. Transcription of the lac Operon is also controlled by a Positive Regulatory system i. Jacob and Monod-lactose metabolism genes= negative gene regulation ii. Now researcher found that it can be a positive gene regulation iii. Note: it is easier to used glucose than lactose to produce energy 1. Lactose present and glucose low/absent: structural genes expressed at very high levels a. Lactose converted to the inducer allolactose. It binds to the Lac repressor which inactivates repressor. So it cannot block the operator b. When glucose is absent, cAMP concentration tends to be high, causing the level of activated CAP high as well. c. CAP is activated by binding with cAMP to the promoter on the CAP site -> CAP and cAMP complex. d. RNA polymerase binds efficiently to the promoter e. Transcription the lac operon gene occurs f. Translation produces high amount of enzymes 2. Lactose present and glucose present: structural genes expressed at low levels a. Lactose converted to the inducer allolactose. It binds to the Lac repressor which inactivates repressor. So it cannot block the operator b. When glucose present, cAMP concentration tends to be low. CAP cant bind to promoter on the CAP site which causes CAP cannot be activated as well as cAMP. (happen together) c. RNA polymerase is unable to bind to the promoter of RNA polymerase binding site. d. Transcription occurs at a low level. e. Since there is no lac repressor present there is a chance that transcription can occurs, thus the level of transcription is higher than when lactose is absent iv. Whether gene expression is under negative or positive control depends on the impact of the respective DNA-binding proteins, not the impact of the available substrates sus as glucose or lactose. f. Transcription of the trp Operon Genes for Tryptophan Biosynthesis is Repressed When Tryptophan Activates a Repressor – negative gene regulation – turn off gene expression when it binds DNA i. Tryptophan acts as a corepressor, a regulatory molecule that combines with a repressor to activate it and thus shut off the operon ii. When tryptophan is absent, it must be made by the cell and the trp operon genes are expressed 1. No tryptophan ->The Trp repressor is inactive and cannot bind to the operator YAY!!!!! 2. RNA polymerase is able to bind to the promoter and transcribe the structural genes onto a single mRNA molecule iii. When tryptophan is present, there is no need for the cell to make it, so the trp operon is shut off 1. Trytophan entering the cell acts as a corepressor y binding to the inactive Trp repressor and activating it 2. The active Trp repressor binds to the operator. OH NOO!!!!!!!!!! 3. RNA polymerase unable to bind to the promoter 4. Transcription blocked 3. Regulation of Transcription in Eukaryotes a. In Eukaryotes, Regulation of Gene Expression Occurs at Several Levels i. Prok operon – simple, while Euk operon – complex ii. Why complex? 1. The nuclear DNA is organized with histones into chromatin 2. Euk produce large numbers and different types of cells 3. Euk nuclear envelope separates the processes of transcription and translation unlike prok. In pro, translation can start on mRNA that is still being made 4. Result: Gene expression in euk is regulated at more levels :transcriptional regulation (most important!), posttranscriptional regulation, translational regulation and posttranslational regulation a. Transcriptional regulation i. Regulation of Transcription initiation ii. Chromatin remodelling to make genes accessible for transcription b. Posttranscriptional regulation (intron splicing) i. Variation in pre-mRNA processing ii. Removal of masking proteins iii. Variations in rate of mRNA breakdown iv. RNA interference c. Translational Regulation i. Variation in rate of initiation of protein synthesis d. Posttranslational regulation i. Variation in rate of protein processing
More Less

Related notes for Biology 1002B

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.