Transcript DNA - Doctor Jade Main

DNA
Molecular Biology
of the Gene
Genes
• biological blueprints
• give us attributes & traits
• every nucleus, in every cell
carries genetic blueprint
• every cell has all
information needed to
make a complete you
• genes are located on
chromosomes
• humans have 46
• each containing 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
• differences maybe as
small as one base
substitution in one
gene
Genes & DNA
• genes are made of DNA
– deoxyribonucleic acid
• macromolecule
• made of 4 different
nucleotides
• paired in a precise manner
• order of nucleotides is
genetic code
• each 3 combinations of
nucleotides = one amino acid
• DNA gives instructions to
make proteins
• smallest chromosome-Y has
50 million nucleotides
• largest has 250 million
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)
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 and 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
• replication
• uses template
mechanism
Replication
• to replicate
• 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 that 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
• protein production is
directed by DNA
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 itAACCDDGGGACAC
• 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
• made of monomers or
nucleotides
– ribonucleotides
• same basic components as
DNA
• single strand
• 5 C sugar-ribose
• phosphate groups
• nitrogenous bases
– same as in DNA with 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
• bases are added at
50/second
• 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 eukaryotes RNA is
processed before
moving to cytoplasm
for translation
• post-transcriptional
modifications
• capping-tailing
• splicing
• ligation
Capping-Tailing
• nucleotides are added to either end of
RNA
• a “G” nucleotide might be added to
one end
• A nucleotides might be added to other
• additions make RNA more stable
• ends protect molecule from attack by
enzymes
• helps ribosomes recognize mRNA
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 are common to all
organisms
• sequence of nucleotides in
mRNA dictates sequence
of amino acids in
polypeptide
• sequence of bases in a
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 that
message into another
• tRNA (transfer RNA) is
interpreter
• pick s appropriate amino
acid & recognizes
appropriate codon in mRNA
• converts 3 letter code of
nucleic acids into amino
acidsproteins
tRNA
•
•
•
•
•
•
•
•
•
•
•
structure allows it to match correct amino
acids to mRNA sequence
tRNA is composed of one strand of RNA
chain twists & folds on itself 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 is site where amino acid can
attach
enzyme recognizes both tRNA and 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 is 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
called P site
• other tRNA binding site-A site is
vacant
• P site holds tRNA containing
growing peptide chain
• A site holds tRNA carrying next
amino acid to be added to chain
Elongation
• amino acids are added one
by one to first amino acid
• each addition is composed
of 3 steps
• First
• anticodon of incoming
tRNA carrying an amino
acid pairs with mRNA
codon in A site of
ribosome
Elongation
• next peptide bond
forms between carboxyl
group of one amino acid
& amino group of next
• to do this polypeptide
leaves tRNA in P site &
attaches to amino acid
on tRNA in A site
• attached by a peptide
bond
• ribosome catalyzes bond
formation
Elongation
• last stage-translocation
• P site tRNA leaves ribosome
• ribosome moves or translocates tRNA in the A site with its
attached polypeptide to P site
• 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peptid
e released from
last tRNA
• Ribosome splits
back into its
separate subunits
Polysomes
• single mRNA has many ribosomes
traveling along it
• Polysomes
• in various stages of synthesizing
polypeptide
Mutations
• any change in
nucleotide sequence of
DNA
• production of mutations
is mutagenesis
• some are 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 will change
reading frame for tRNA
• Frame-shift mutations
have dramatic effects
• all nucleotides
downstream from
insertion or deletion will
be regrouped into
different codons
• result is usually
nonfunctional protein