Transcript DNA - Doctor Jade

DNA
Molecular Biology
of the Gene
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
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
Genes & DNA
• genes are made of DNA
– deoxyribonucleic acid
• macromolecule
• made of 4 different
nucleotides
• paired in precise manner
• order of nucleotidesgenetic code
• DNA gives instructions to
make proteins
• each 3 combinations of
nucleotides = one amino
acid
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)
DNA NUCLEOTIDES
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
Sugar-Phosphate
Backbone
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
DNA
• base pairing is
specific
• A-T
• G-C
• amount of A =
amount of T
• one strand is
complementary
to the other
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
Replication
• to begin
• strands of DNA must
separate
• double helix
unwound by
helicase
– breaks H bonds
between base
pairs
REPLICATION
• unwinding takes place in a replication
bubble
• new strand of DNA is formed in both
directions on both strands of DNA in
bubble
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
REPLICATION
• each strand of
DNA is used as
template to make
new,
complementary
strand
• semi-conservative
replication
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
PHENOTYPIC EXPRESSION
• small sections of
chromosomes are
genes
• genetic makeup is
genotype
• expression of genes
into specific traits is
phenotype
– result of proteins
• one geneone protein
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
Genetic Code
• DNAmRNAproteins
• 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 languagethat of polypeptides
• DNA language is written in
linear sequence of nucleotide
bases that comprise itAACCGTTGGACAC
• specific sequence of bases
makes up a gene
glu lys ser ala met phe leu glu
Expression of Genotype
• transfer of
information from
DNA to RNA and
then to proteins
takes place in two
processes
• Transcription
• Translation
Transcription
• DNA directs
ribonucleic acid
synthesis
• transfers genetic
information from
DNA to RNA
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
Types of RNA
• Messenger
– mRNA
• Ribosomal
– rRNA
• Transfer
– tRNA
• all involved in
translation
Transcription
• DNAmRNA
• nucleic acid
language of
DNA is rewritten
as sequence of
RNA bases
Transcription
• process of
transferring genetic
information from
DNA to RNA
• similar to DNA
replication
• DNA is used as
template to make
RNA
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 AU
• 3 stages
• Initiation
• Elongation
• Termination
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
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
Termination
• RNA polymerase
reaches special
sequence of bases
in templateterminator
• ends transcription
• RNA polymerase
detaches
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
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
Splicing & Ligation
• precursor mRNA contains exons &
introns
• exons
– segments containing information
for formation of proteins
• Introns
– internal non-coding regions
• before mRNA can leave nucleusintrons 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
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
GENETIC CODE
• DNA & RNA are made of 4 different
nucleotides
• there are 20 amino acids
• if each nucleotide coded for one
amino acidcould only be 4 amino
acids
• if each 2 coded for onecould 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
Codons
• 61 code for amino acids
• some have regulatory
purposes
– start & stop
translation
• AUG-start codon
– codes for METmethionine
• UAA, UAG, UGA- stop
codons
– tell ribosomes to end
polypeptide
synthesis
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
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
acidsproteins
tRNA
• composed of one strand of RNA
• chain twists & folds making some
double stranded areas
• one end-special triplet of basesanticodon
• 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
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
Translation Stages
• Initiation
• Elongation
• Termination
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
Elongation
• amino acids are added
one by one to first amino
acid
• ribosome moves along
mRNA in the 5'-to3'direction
• tRNA (anticodon)
corresponding to second
codon binds to A site,
carries amino acid
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
Elongation
• movement brings next mRNA codon to
be translated into A site
• process begins again
• elongation continues until stop codon
is reached
Termination
• UAA, UAG & UGA
are stop codons
• when one of these
sequences is
detetectedpeptide
released from last
tRNA
• ribosome splits back
into its separate
subunits
Mutations
• any change in
nucleotide sequence of
DNA
• production of mutationsmutagenesis
• some-spontaneous
• some due to mutagens
• radiation, chemicals &
viruses
• two categories
– base substitutions
– insertions & deletions
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
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