dna molecular biology of the gene. genes biological blueprints give attributes & traits every...
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DNA
Molecular Biology of the Gene
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Genes• biological blueprints• give attributes & traits• every nucleus, in every
cell carries genetic blueprint
• every cell has all information needed to make a complete you
• located on chromosomes• humans have 46 • each contain thousands of
genes
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Genes• share genes with all living organisms
• 98% match chimpanzees
• 99.9% match all other humans
• differences exist at particular sites
• causes each of us to be unique
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Genes & DNA• genes are made of DNA
– deoxyribonucleic acid• macromolecule • made of 4 different
nucleotides• paired in precise manner• order of nucleotides-
genetic code• DNA gives instructions to
make proteins• each 3 combinations of
nucleotides = one amino acid
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DNA• nucleic acid• macromolecule composed of smaller
subunits –nucleotides• contains• carbon sugar-deoxyribose• nitrogenous base• 1-3 PO4 groups• contains 4 different nucleotides• each with different nitrogenous base• bases are found in 2 major groups• Purines
– double ring structures– adenine (A)– guanine (G)
• Pyrimidines– single ring structures– thymine (T)– cytosine (C)– uracil (U)
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DNA NUCLEOTIDES
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Sugar-Phosphate Backbone• bases are linked via
dehydration synthesis into phosphodiester bonds
• phosphate of one nucleotide covalently bonds to sugar of next
• forms sugar-PO4 backbone
• nitrogenous bases are arranged as appendages along backbone
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Sugar-Phosphate Backbone
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DNA• structure determined by
Watson & Crick-1953• discovered DNA is double
stranded helix• composed of two strands• wrapped around each other in
helical formation• core -bases of one DNA
strand bonded to bases in other strand
• if think of DNA molecule as ladder– sugar-phosphate backbone
would be sides of ladder– paired bases would be
rungs
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DNA• base pairing is
specific• A-T • G-C• amount of A =
amount of T• one strand is
complementary to the other
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Replication • cells divide & reproduce
daily• giving rise to 2 daughter
cells• with same genetic
makeup• before cell can divide,
DNA must duplicate• called-replication• uses template
mechanism
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Replication• to begin• strands of DNA must
separate• double helix unwound
by helicase– breaks H bonds
between base pairs
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REPLICATION• unwinding takes place in a replication
bubble
• new strand of DNA is formed in both directions on both strands of DNA in bubble
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Replication• proceeds in both directions• DNA strand has 3’ end & 5’ end• at one end carbon 3 of sugar is
attached to –OH group• at other end carbon 5 is attached
to a phosphate group• DNA polymerase
– enzyme – binds single nucleotides into
new strand of DNA– works only in 3' to 5' direction
• consequently DNA synthesis only occurs in 5' to 3' direction
• means one daughter strand can be made as continuous strand– leading strand
• other is made in short pieces• linked together with DNA ligase
– lagging strand
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REPLICATION• each strand of
DNA is used as template to make new, complementary strand
• semi-conservative replication
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REPLICATION• at completion of
process 2 DNA molecules have been formed each identical to original
• one strand of each of new DNA molecules is strand of original DNA
• other strand is complementary strand made during replication
• semi conservative replication
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PHENOTYPIC EXPRESSION • small sections of
chromosomes are genes
• genetic makeup is genotype
• expression of genes into specific traits is phenotype
– result of proteins
• one geneone protein
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Expression of Genotype• protein production is
dictated by DNA
• information about specific proteins is transferred to another nucleic acid-RNA
• RNA is translated into a protein
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Genetic Code• DNAmRNAproteins• Proteins are long strands of
amino acids held by peptide bonds
• each has unique amino acid sequence
• language of DNA is chemical• must be translated into
different chemical language-that of polypeptides
• DNA language is written in linear sequence of nucleotide bases that comprise it-AACCGTTGGACAC
• specific sequence of bases makes up a gene glu lys ser ala met phe leu glu
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Expression of Genotype• transfer of
information from DNA to RNA and then to proteins takes place in two processes
• Transcription• Translation
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Transcription• DNA directs
ribonucleic acid synthesis
• transfers genetic information from DNA to RNA
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RNA• nucleotides – ribonucleotides
• same basic components as DNA
• single strand• 5 C sugar-ribose• phosphate groups• nitrogenous bases
– same as DNA– one exception
• RNA has Uracil (U) instead of T
• base pairing rules are same• Uracil is substituted for
thymine• U-A not T-A
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Types of RNA• Messenger
– mRNA
• Ribosomal – rRNA
• Transfer– tRNA
• all involved in translation
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Transcription• DNAmRNA
• nucleic acid language of DNA is rewritten as sequence of RNA bases
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Transcription• process of
transferring genetic information from DNA to RNA
• similar to DNA replication
• DNA is used as template to make RNA
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Transcription• stands of DNA must separate• only one serves as template• nucleotides take their places
one at a time along template using same base pairing rules as replication except A-U
• 3 stages• Initiation• Elongation• Termination
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Initiation• RNA polymerase
attaches to promoter– specific nucleotide
sequence• RNA synthesis begins• RNA polymerase decides
which strand to use as template
• strand used- antisense strand
• other stand-sense strand
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Elongation• RNA strand grows longer• RNA strand peels away
from template allowing separated DNA strands to come back together
• RNA strand formed is directly complementary to its DNA template
• each time C is found in antisense strand of DNA template a G is paired with it
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Termination• RNA polymerase
reaches special sequence of bases in template-terminator
• ends transcription• RNA polymerase
detaches
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Post-transcriptional Modifications
• in prokaryotic cells RNA can function immediately
• in eukaryotic cells RNA is processed before moving to cytoplasm for translation
• post-transcriptional modifications
• capping-tailing• splicing-ligating• ligation
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Capping-Tailing• nucleotides are added to either end of
RNA
• “G” nucleotide(s) might be added to one end; “A” nucleotides might be added to other
• additions make RNA more stable
• protects molecule from attack by enzymes
• helps ribosomes recognize mRNAA
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Splicing & Ligation• precursor mRNA contains exons &
introns• exons
– segments containing information for formation of proteins
• Introns– internal non-coding regions
• before mRNA can leave nucleus-introns must be removed from strand
• Introns are spliced out• exons are ligated (or attached)
together• RNA can now move to cytoplasm
through nuclear membrane pores
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Translation• conversion of nucleic acid
language into protein language
• proteins are macromolecules-polymers of amino acids
• 20-common to all organisms
• sequence of nucleotides in mRNA dictates sequence of amino acids in polypeptide
• sequence of bases in molecule of DNA is genetic code
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GENETIC CODE• DNA & RNA are made of 4 different
nucleotides• there are 20 amino acids• if each nucleotide coded for one
amino acidcould only be 4 amino acids
• if each 2 coded for onecould be 16 amino acids
• smallest number of bases that can code for 20 amino acids is 3
• particular triplet of nucleotides in mRNA is a codon– specific for a particular amino
acid• 64 possible triplet codes• code is redundant
– more than one codon for each amino acid
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Codons• 61 code for amino
acids• some have regulatory
purposes– start & stop
translation• AUG-start codon
– codes for MET-methionine
• UAA, UAG, UGA- stop codons– tell ribosomes to end
polypeptide synthesis
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Genetic Code• highly conserved
• same in all organisms
• genes can be transcribed & translated even if transferred from one species into another
• opened door for genetic recombinant technology & genetic engineering
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Translation• amino acids are not able to
recognize codons of mRNA• requires an interpreter
– intermediate that can understand language of one form & translate message into another
• tRNA (transfer RNA) is interpreter
• picks appropriate amino acid & recognizes appropriate codon in mRNA
• converts 3 letter code of nucleic acids into amino acidsproteins
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tRNA• composed of one strand of RNA• chain twists & folds making some
double stranded areas• one end-special triplet of bases-
anticodon• contains complementary sequence of
bases to sequence of bases in mRNA • recognizes bases in mRNA by
applying standard base pairing rules• other end-site where amino acid can
attach• enzyme recognizes both tRNA & its
amino acid partner• there are at least 32 different tRNA in
eukaryotic cells• anticodons are redundant• there is at least one anticodon for
each amino acid
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Translation• ribosomes coordinate
process of translation• ribosomes are formed
from 2 subunits each made of proteins & rRNA (ribosomal RNA)
• completely assembled ribosome has binding site for mRNA on its small subunit & two binding sites for tRNA on its large subunit
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Translation Stages
• Initiation• Elongation• Termination
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Initiation• mRNA molecule binds to small
ribosomal subunit• special initiator tRNA binds to specific
codon-AUG– start codon
• Anticodon-UAC• start codon also carries amino acid
methionine• Next
– large ribosomal subunit binds to small one creating functional ribosome
• initiator tRNA fits into one of two tRNA binding sites on ribosome-P site
• other tRNA binding site-A site is vacant
• P site holds tRNA containing growing peptide chain
• A site holds tRNA carrying amino acidsext amino acid to be added to chain
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Elongation• amino acids are added
one by one to first amino acid
• ribosome moves along mRNA in the 5'-to-3'direction
• tRNA (anticodon) corresponding to second codon binds to A site, carries amino acid
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Elongation• peptide bond forms
between carboxyl group of one amino acid & amino group of next
• after peptide bond forms-ribosome shifts, or translocates, causing tRNA to occupy the P site
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Elongation• movement brings next mRNA codon to
be translated into A site • process begins again• elongation continues until stop codon
is reached
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Termination• UAA, UAG & UGA
are stop codons• when one of these
sequences is detetectedpeptide released from last tRNA
• ribosome splits back into its separate subunits
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Mutations• any change in
nucleotide sequence of DNA
• production of mutations-mutagenesis
• some-spontaneous• some due to mutagens• radiation, chemicals &
viruses• two categories
– base substitutions– insertions & deletions
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Base Substitutions• Point mutation
– replacement of one nucleotide for another
• may go unnoticed• may cause significant
issues• hemophilia• sickle cell anemia• Huntingtons Chorea• Tay Sachs disease
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Insertion & Deletion• mRNA is read as a series of triplet codons during translation
• adding or deleting one base changes reading frame for tRNA
• Frame-shift mutations– dramatic effects– all nucleotides downstream
from insertion or deletion will be regrouped into different codons
– result is usually nonfunctional protein