Presentation on theme: "Gene-Proteins-Mutations"— Presentation transcript:
1Gene-Proteins-Mutations A tutorial exploring how genetic mutations affect proteins structure and function
2Before you begin…Read the opening case study (Meeting the Affected Family) on the first page of lab - Discovering the Genetics and Molecular Biology of Sickle Cell Anemia .
3As you explore this tutorial, think about the question posed in Part I of the Sickle Cell Anemia-Malaria lab case study:Meeting the Affected Family:Why do you think that Dr. Dufall wants a blood sample from Emily and Chaka, if neither of them has sickle cell anemia? In other words, what could looking at their hemoglobin tell us about this genetic condition?
4Genes code for polypeptides! Remember that the hemoglobin protein is comprised of 4 polypeptide chains (2 alpha-globin chains and 2 beta-globin chains).One gene codes for the beta globin polypeptides!One gene codes for the alpha globin polypeptides!
5How do cells use the information encoded in genes to make proteins… CytoplasmNucleusMakes a mRNA copy (or transcript) of the gene.mRNA leaves the nucleus and combines with a ribosome in the cytoplasm.The ends of the mRNA transcript are modified to facilitate transport from the nucleus. Regions of the mRNA transcript that don’t provide instructions for making a polypeptide are removed (introns). The remaining mRNA sequences (exons) are spliced back together.The ribosome “reads” the mRNA sequence and use the information to build a polypeptide (a chain of amino acids).
6Transcription…in a little more detail! First, watch an animation of transcription (click on picture above) to get a sense of how the whole process proceeds. Return to this show and continue after viewing the video.
7Let’s first examine the structure of RNA more closely…
8DNA RNA DNA RNA Double stranded helix Single stranded Nucleotides are: C,G,A,TRibonucleotides are: C,G,A,UA DNA nucleotide:A RNA ribonucleotide:
9During transcription of mRNA notice that: DNAmRNADuring transcription of mRNA notice that:Adenine (DNA) pairs with Uracil (RNA)Guanine (DNA) pairs with Cytosine (RNA)Cytosine (DNA) pairs with Guanine (RNA)Thymine (DNA) pairs with Adenine (RNA)
10Initiating transcription… DNARNA polymerase begins to pry open the DNA and transcribes the gene using one of the strands as a template. Other transcription factors facilitate the process and regulate the expression of the gene.New ribonucleotides are added according to nitrogenous base pairing rules (U=A; G=C).
11mRNA ProcessingAn interesting process happens during processing. A complex of proteins and a kind of RNA (snRNA) grabs the mRNA forming loops…like this!mRNAThese complexes (called splicosomes) chop out whole sections of mRNA called intervening sequences (aka introns) and splices together the expressed sequences (aka exons).From The Cartoon Guide to Genetics by Larry Gonick
12Spliceosomes…Sequences of snRNA associated with proteins that recognize introns by binding to specific sequences of RNA, “chop” out the intron and splice the exons together!
13mRNA processing…Introns – intervening sequences that are not expressed.Exons – expressed sequences..these are translated into a polypeptide.“5’ Caps” and “Poly-A Tails” are ribonucleotides that are added to facilitate export of mRNA from the nucleus and to prevent the degradation of mRNA in the cytoplasm.
14Translating the genetic code! Making polypeptides from the instructions now encoded in mRNA!
15Click on the picture above to watch an animation of the process of protein translation to get a sense of how this process occurs.
16An example: The insulin polypeptide! Remember that polypeptides are composed of specific sequences of amino acids!The DNA sequence for a gene determines the amino acid sequence.Each circle represents and amino acid whose name is abbreviated with 3 letters. “Ala” in this case stands for the amino acid Alanine.An example: The insulin polypeptide!A hormone that regulate blood sugar levels in humans!
17Here are the 20 amino acids that comprise all polypeptides! Notice that it is the “R-groups” (highlighted in white) that differ between each amino acid.Also notice that amino acids have different chemical characteristics (non-polar, polar, and electrically charged (- and +))- charge+ charge
18The mRNA transcript is read three “letters” (aka ribonucleotides) at a time. The triplet is called a codon.Codons act as the code that tells the ribosome the order in which to link amino acids during translation.
19Table of mRNA codons and the amino acids for which they code! The genetic code!Notice that the codons are somewhat “redundant”. That is, several different codons can code for the same amino acid.Also notice that 3 codons serve as “STOP” codons. These signal the end of the gene!
20This can alter the structure and therefore function of the protein! A mutation can alter the DNA sequence for a gene which can cause a change in the mRNA sequence which can result in a change in the amino acid sequence of the polypeptide.This can alter the structure and therefore function of the protein!
21Types of mutations…Normal mRNA from the geneA base-pair substitution, which replaces one DNA nucleotide for another can have different effects on the polypeptide depending on the nature of the mutation.Just so you know…These changes occurred as mutations in the DNA, and resulted in a change in the mRNA, depicted in the figure.
22Types of mutations… Thought Question… Normal mRNA from the geneFrameshift mutations…shift the reading frame during translation resulting in many changes in the amino acid sequence downstream of the mutation!Thought Question…What effect do you think each of the mutations discussed so far would have on the structure (size & shape) and function of the polypeptide?
23Back to the case studyMeeting the Affected Family:Why do you think that Dr. Dufall wants a blood sample from Emily and Chaka, if neither of them has sickle cell anemia? In other words, what could looking at their hemoglobin tell us about this genetic condition?Be sure to complete the homework for the Discovering the Genetics and Molecular Biology lab in your lab manual prior to coming to lab.