Introduction to Genetics as relevant to this course.ppt

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1、Introduction to Genetics as relevant to this course,(Ack: Roche Genetics CD-ROM, Mishras notes at NYU, ),Background (1/18),Genome, Chromosome, Genes made up of DNAs Genetics research (largely over last 100yrs, accelerated in last 30 yrs) Has led to important advances in medical science. Nucleus of a

2、 cell : contains chromosomes (made up of DNA); and proteins. DNA (Deoxy Ribo Nucleic acid) Is the genetic material that is inherited. Contains the information needed by living cells to specify their structure, function, activity and interaction with other cells and environment. A DNA molecule can be

3、 thought of as a very long sequence of nucleotides or bases.,DNA structure (2/18),The Nobel Prize in Physiology or Medicine 1962 - Crick, Watson and Wilkins for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material Made

4、 up of 4 different building blocks (so called nucleotide bases), each an almost planar nitrogenic organic compound Adenine (A) Thymine (T) Guanine (G) Cytosine (C) Base pairs (A - T, C - G),DNA Structure cont. (3/18),Base pairs (A - T,C - G) are attached to a sugar phosphate backbone to form one of

5、2 strands of a DNA molecule. Phosphate (PO4) -3) Deoxyribose Two strands are bonded together by the base pairs (A T, C G). Results in mirror image or complementary strands, each is twisted (or helical), and when bonded they form a double helix. Direction of each strand (5 meaning beginning or 3 mean

6、ing end of the strand) 5 and 3 refer to position of bases in relation to the sugar molecule in the DNA backbone. Are important reference points to navigate the genome. 2 complementary strands are oriented in opposite direction to each other.,DNA Structure,Genome Size,DNA hybridization (DL 3/18),Hybr

7、idization between complementary DNA sequences to form a double stranded DNA molecule. One of the most important DNA technology Applications of Hybridization PCR (Polymerase Chain Reaction) Enzymetically generating millions of copies of a tiny amount of a particular nucleic acid sequence. Northern bl

8、ots analysis Possible to study (in a semi-qualitative manner) the level of transcription of a particular gene. DNA Microarrays Can interrogate the level of transcription of several thousand of different genes in one sample in one experiment.,PCR (Polymerase Chain Reaction) (DL 3/18),PCR allows selec

9、ted amplification of a DNA sequence. Only a tiny amount of DNA is necessary to obtain a PCR product (a drop of blood or less is enough). Complementary DNA primers need to be designed. For this the DNA sequence flanking the target sequence needs to be known in advance. Primers are short synthetic DNA

10、 sequences of about 20 bases (so called oligonucleotides) that can specifically hybridize to a unique complementary DNA sequence. The approach Genomic DNA (the template), Primers (the starters), deoxynucleotides (building blocks), a special DNA polymerase that is resistant to heat (the motor of the

11、reaction) are mixed together in one reaction tube. Reaction takes place in a thermocycler (an apparatus that allows one to precisely heat and cool the reaction). DNA is heated to almost boiling temperature which separates the 2 strands (whole process is called heat denaturation) Cooling of the mixtu

12、re allows the primer to bind to their complementary sequence of the genomic DNA. Once the primers bind the DNA polymerase uses them as the start site to generate a copy of each strand of the targeted gene fragment building 2 new double stranded molecules. Doing it (denaturation followed by cooling)

13、30 times, results in 230 = 109 (1 billion) copies.,Northern Blot analysis,The complete RNA content from a sample is separated according to size by electrophoresis. Usually done in a sheet of agarose (similar to gelatine) In response to electric current, larger molecules move slower, and smaller move

14、 faster, thus separating different RNA molecules by size. Then RNAs are transferred from gel to a filter membrane (blot) Blot is then exposed to a solution containing a nucleic acid (probe) complement to the sequence whose presence in the blot one wants to interrogate. The probe may be cRNA or cDNA

15、with detectable marking (radioactive isotope or a fluoroscent tag) If the targeted sequence is present in the blot then the probe hybridizes and sticks to the blot at the location where the targeted sequence is located. After washing off of excess probe - a signal is detectable and its specificity c

16、an be checked based on the expected size of the RNA that will correlate with how far it has migrated during electrophoresis. With this method It is possible to study in a semiqualitative manner the level of transcription of a particular gene. Comparison of the results from different samples (e.g. di

17、fferent organs etc.) provides information about the transcriptional regulation of the gene.,DNA Structure cont. (4/18),The order of nucleotide bases along a DNA strand is known as the sequence. The genetic information is encoded in the precise order of the base pairs. DL GenBank database http:/www.n

18、cbi.nlm.nih.gov/Entrez/ Human genome project http:/www.genome.gov/page.cfm?pageID=10001694 DNA sequencing Is the process designed to precisely determine the sequence of bases in the DNA.,Cells, DNA and genome (5,6,8/18),During cell division (Mitosis) the entire DNA of the cell is copied 2 strands se

19、parate, complementary strands are generated. Two duplicate DNA sequences are produced. Genome: an organisms total DNA content Diploid cells: cells that carry 2 genome copies Haploid cells: have a single copy of the genome Reproductive germs cells (gametes), ie., egg & sperm cells Human genome consis

20、ts of 22 autosomal chromosomes (same in males and females) 2 sex chromosomes X and Y (males XY, females XX),Structure of Chromosomes (7/18),Center is called centromere. Two ends called Telomere. Center separates two arms Short arm p Long arm q.,Structure of genes 9-11/18,Genes are those parts of the

21、 genome that contain the information necessary for the building of proteins. (size:100-several million base pairs) Exon (coding sequence), Intron (non-coding sequence), regulatory region (at the two ends for regulating how actively protein is to be synthesized from them) Eukaryotes (organisms whose

22、cell have nucleus) have genes segmented into exons and introns Introns can occur between individual codons or within a single codon. Promoter (a regulatory element in the 5 end) Consists of several short sequences which are consensus binding sites for a number of proteins called transcription factor

23、s.,(DL 10/18),Prokaryotes (do not have nucleus) genes are not segmented to exons and introns. Eukaryotes (normally segemented to exons and introns) Except mitochondrial genes & a few nuclear genes. During gene expression exons and introns are transcribed to form a pre-mRNA RNA splicing - removes int

24、rons and exons and produces mature mRNA molecule that codes for a polypeptide. Exons sequences that are represented in the mature mRNA May or may not code for a protein Eg. Exons at the 3 or 5 end of mRNA may not be translated to proteins,Some Genes (from Mishras slides),Some human gene locations (F

25、rom Mishras slides),Gene expresion (12/18),Gene expression (Transcription and Translation) from genes to making proteins the 2 step process Transcription: genetic information in DNA is copied into messenger RNA (mRNA) Translation: mRNA is used as a template to synthesize a protein.,Central Dogma,Due

26、 to Francis Crick 1958 states that these information flows are all unidirectional: “The central dogma states that once information has passed into protein it cannot get out again. The transfer of information from nucleic acid to nucleic acid, or from nucleic acid to protein, may be possible, but tra

27、nsfer from protein to protein, or from protein to nucleic acid is impossible. Information means here the precise determination of sequence, either of bases in the nucleic acid or of amino acid residues in the protein.”,Transcription (13/18),RNA (Ribonucleic acid) Similar to DNA (except for a chemica

28、l modification of the sugar backbone) Instead of T contains U (Uracil) which binds with A. Is not double stranded but single stranded RNA molecules tend to fold back on themselves to make helical twisted and rigid segments. RNA is synthesized By unwinding the DNA double helix separating the 2 strand

29、s. Using one of the strands as a template along which to build the RNA molecule Accomplished by Enzyme RNA polymerase (binds to promoter and copies or transcribes the gene in its full length) Resulting molecule is called Pre-mRNA Single stranded pre-mRNA is then procesed. Splicing (mediated by splic

30、eosome consisting of RNA and proteins) removes the introns. Ends modified (Capping modifies 5 end and Polyadenylation adds adenines at the 3 end) to enhance stability,Translation (14/18),mRNA is used as a template to synthesize a protein. Translation takes place outside the nucleus in the cytoplasm

31、within organelles called endoplasmic reticulum. Except for the 5 & 3 end of the mRNA (which are non-coding) the rest of the molecule codes for 1 protein Proteins: made up of aminoacids 20 different aminoacids used to build proteins in humans Each encoded by one or more sets of 3 nucleotides (called

32、triplets or codons) Initial codon is always AUG (coding for methionine) Translation is terminated by one of 3 stop codons. Translation process is carried out by ribosomes which scan the mRNA, & build the polypetide chain from aminoacids supplied by transport RNAs (tRNA). Starts at a particular locat

33、ion of the mRNA called the translator start sequence (usually AUG) tRNA (transfer RNA) are made up of a group of small RNA molecules each with specificity for a particular amino acid. tRNAs carry the aminoacids to the ribosomes, the site of protein synthesis, where they are attached to a growing pol

34、ypetide. Translation stops when one of UAA, UAG or UGA is encountered,Post-translational modification (DL),The polypetide chain that results from mRNA translation is often subject to chemical modifications. Eg. Glycosylation, phosphorylation, hydrooxylation Addition of lipid groups (eg. Fatty acyl o

35、r prenyl groups) Addition of co-factors (e.g. a heme molecule) Or proteolytic cleavage The type of modification a protein undergoes depends on its function and sub-cellular location.,Genetic Code (15/18),The combination of nucleotides that build the different codons represents the genetic code. Codo

36、n = 3 nucleotides; 4 kinds of nucleotides. So 4X4X4 = 64 possible codons. But 20 amminoacids + start & stop. So several codons can specify the same aminoacid. (genetic code is degenerate) Start codon (AUG) and Stop codons (UAA, UAG, UGA). Open reading frame (ORF) the sequence of nucleotides between

37、and including the start and stop codons. The Nobel Prize in Physiology or Medicine 1968 Holley, Khorana and Nirenberg for their interpretation of the genetic code and its function in protein synthesis http:/www.nobel.se/medicine/laureates/1968/,Amino Acids with Codes (From Mishras slide),A Ala alani

38、ne GC(U+A+C+G) C Cys cysteine UG(U+C) D Asp aspertic acid GA(U+C) E Glu glutamic acid GA(G+A) F Phe phenylanine UU(U+C) G Gly glycine GG(U+A+C+G) H His histine CA(U+C) I Ile isoleucine AU(U+A+C) K Lys lysine AA(A+G) L Leu leucine (C+U)U(A+G) + CU(U+C) M Met methionine AUG N Asn asparginine AA(U+C) P

39、 Pro proline CC(U+A+C+G) Q Gln glutamine CA(A+G) R Arg arginine (A+C)G(A+G)+CG(U+C) S Ser serine (AG+UC)(U+C)+UC(A+G) T Thr threonine AC(U+A+C+G) V Val valine GU(U+A+C+G) W Trp tryptophan UGG Y Tyr tyrosine UA(U+C),Biological Function of Proteins,Enzyme catalysis: DNA polymerases, lactate dehydrogen

40、ase, trypsin Transport: hemoglobin, membrane transporters, serum albumin Storage: ovalbumin, egg-white protein, ferritin Motion: myosin, actin, tubulin, flagellar proteins Structural and mechanical support: collagen, elastin, keratin, viral coat proteins Defense: antibodies, complement factors, bloo

41、d clotting factors, protease inhibitors Signal transduction: receptors, ion channels, rhodopsin, G proteins, signalling cascade proteins Control of growth, differentiation and metabolism: repressor proteins, growth factors, cytokines, bone morphogenic proteins, peptide hormones, cell adhesion protei

42、ns Toxins: snake venoms, cholera toxin,Differential Gene Expression 17/18,All cells in the body (that contain a nucleus) carry the full set of genetic information, but only express about 20% of the genes at any particular time. Gene expression is selective Different proteins are expressed in differe

43、nt cells according to the function of the cell. Gene expression is tightly controlled and regulated. The differential expression of genes ensures that cells develop correctly and can differentiate into and function as specialized cell types. For eg. Neurons, muscle cell, or fibroblast.,cDNA and gene

44、 expression (DL),Goal: Identify all possible genes expressed in one tissue or cell line. (Use cDNA libraries) cDNA libraries are prepared from mRNA isolated from the cells or tissue being studied. cDNA are DNA molecules that are complementary to the mRNA sequences in a sample. cDNA is synthesized by

45、 the enzyme reverse transcriptase (RT), that uses the mRNA as a tenplate. RT is a viral enzyme used by viruses whose genome is made of RNA, not DNA. A cDNA library represents the collection of all genes expressed in a particular cell or tissue type. DNA sequence mRNA sequence cDNA sequence (much sma

46、ller as while generating mRNAs the introns are eliminated) Hence very useful when trying to isolate a particular gene to study the protein it codes.,NEXT SEC.,Gene Cloning 1/11,First step in identifying genes and their function is to isolate it from the rest of genome and produce a large quantity of

47、 it (called cloning a gene). Cloning a DNA fragment using bacteria DNA fragment is isolated from the entire genome using restriction enzyme. These enzymes can cut the DNA (in a staggered fashion or straight through) at specific sites defined by a short sequence. Typically they recognize specific DNA

48、 sequences of 4, 6, or 8 bases These enzymes are found in bacterias, where their role is to protect the bacteria from foreign DNA by digesting them into smaller pieces This fragment is inserted into a vector (like a mini-chromosome) using DNA ligase and the recombinant product is introduced into bac

49、teria (this process is called transformation) Cloning vectors are DNA fragments that are able to replicate within a cell and allow the addition of exogenous DNA. They are derived from plasmids, viruses, phages or chromosomes. Vectors are classified according to: the type of host cell they can replic

50、ate in, or the size of the exogenous DNA they are able to carry. The bacteria now makes new copies with every cell division.,DNA Sequencing (DL 1/11),It is the process designed to precisely determine the sequence of bases in the DNA. Involves enzymetically copying the DNA in the presence of compounds that terminate this copying process in a base specific manner, resulting in a mixture of DNA copies that differ in size by one base. Different technologies are used to resolve the mixture and detect the different fragments.,