Download 2.Molecular basis of heredity. Realization of hereditary information

Molecular basis of
heredity. Realization
of hereditary
information.
Lecturer: ass. Nedoshytko Kh.Yu.
Lecture
questions
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Proteins are the main living things.
Types of proteins .
Lipoproteins and Glycoproteins.
Nucleotides and Nucleic Acids.
Protein synthesis.
- Transcription
- Translation
Protein functions
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enzymes are only one class of
proteins.
proteins also form the scaffolding,
or structure, of a good deal of
tissue;
active in transporting molecules
from one site to another;
they allow muscles to contract and
cells to move;
some hormones are made from
them
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Proteins are prime examples of the buildingblock type of molecule.
The monomers in this case are called
amino acids.
String an arbitrary number of them together
in a chain—and you have a polypeptide;
when the polypeptide folds up in a specific
three-dimensional manner, you have a
protein.
As a practical matter, proteins are likely to
be made of hundreds of amino acids strung
together and folded up
Shape is critical to functioning of all proteins
Types of proteins
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Enzymes -Quicken chemical reactions (surcease
brocks sugar to glucose and fructose)
Hormones - chemical messengers (growth
hormone)
Transport –move other molecules (hemoglobin)
Contractive –movement (myosin and actin -allow
muscles to contract)
Protective - healing, defense against invader
(fibrinogen: stops bleeding antibodies: kill bacterial
invaders)
Structural –mechanical support (keratin: hair
collagen: cartiilage)
Storage - stores nutrients (ovalbumin: egg white:
used as nutrient for embryos)
Toxins - defense, predations (bacterial diphteria
toxin)
Communication – cell signaling (glicoprotein:
receptors on cell surface)
Simple proteins
Consist from only polypeptide chains
Large proteins
Consist from polypeptide chains and
nonprotein components
Lipoproteins -are a combination of lipids and proteins; they are
active in transporting fat molecules throughout the body via the
bloodstream. They amount to a capsule of protein surrounding a
globule of fat.
Large proteins
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Glycoproteins are
combinations of
proteins and
carbohydrates.
Most receptors are
usually glycoproteins,
meaning mostly
protein with a sidechain made of
carbohydrate.
Large proteins
• Chromosomes- combination of
nucleic acids and proteins
Fibrous (insoluble in water) –
structural: hair, cartilage
Globular (soluble in water)
–chemical: enzymes, antibodies, receptors.
Nucleic acids
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there are 2 types of nucleic acids: DNA
(deoxyribonucleic acid) and RNA
(ribonucleic acid).
DNA Provides Information for the
Structure of Proteins
Structure
DNA
RNA
Sugar
Deoxyribose
Ribose
Bases
Adenine, guanine,
thymine, cytosine
Adenine, guanine,
uracil, cytosine
Strands
Double-stranded with
base pairing
Single-stranded
Helix
Yes
No
DNA Basics:
Located in nucleus, packaged into
chromosomes
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Organelles (mitochondria,
chloroplasts) have their own
chromosomes
2. DNA is double stranded, present in the
form of a double helix
3. Bases make up complementary
base pairs
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Nucleic acids
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The nucleic acids are
polymers of smaller units
called nucleotides.
A nucleotide consist of:
1) five-carbon sugar
(deoxyribose in DNA and
ribose in RNA);
2) a phosphate group
(PO4);
3) one of five types
nitrogen-containing
compounds called
nitrogenous bases.
Nucleic acids
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The nitrogenous bases are:
Purines, which are larger –
Adenine (A), Guanine (G);
Pyramidines, which are
smaller – Thymine (T), Cytosine
(C), Uracil (U).
Nucleic acids
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The sugar and phosphate
components of the nucleotides
link up to form the outer
"rails" of the DNA molecule,
while the bases point toward
the molecule's interior.
Two chains of nucleotides are
linked, via hydrogen bonds, to
form DNA's double helix.
Two of these chains then link
together—as if a ladder, split
down the middle, were
coming together—forming the
most famous molecule in all of
biology, the DNA double helix.
Nucleic acids
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A gene is generally a collection of DNA
nucleotides that contains the instructions
for an individual protein.
A player in this series of steps protein
synthesis is another nucleic acid,
ribonucleic acid or RNA, which is
involved in ferrying the DNA-encoded
instructions to the sites in the cell where
proteins are put together.
RNA –a polymer of ribose-containing
nucleotides, with forms single-strand.
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Messenger RNA carries the genetic code to the cytoplasm to
direct protein synthesis.
1. This single-stranded molecule (hundreds to thousands of
nucleotides).
2. mRNA contains codons that are complementary to the DNA
codons from which it was transcribed
Transfer RNA is folded into a cloverleaf shape and contains
about 80 nucleotides.
1. Each tRNA combines with a specific amino acid that has
been activated by an enzyme.
2. One end of the tRNA molecule possesses an anticodon, a
triplet of nucleotides that recognizes the complementary
codon in mRNA.
Ribosomal RNA associates with many different proteins
(including enzymes) to form ribosomes.
1. rRNA associates with mRNA and tRNA during protein
synthesis.
2. rRNA synthesis takes place in the nucleolus and is
catalyzed by RNA polymerase.
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Most Genes
encode proteins
DNA  mRNA 
protein
DNA is copied into
RNA via
transcription
The Genetic
Code is universal
DNA replication
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1. DNA helicase
(enzyme) unwinds the
DNA. The junction
between the unwound part
and the open part is called
a replication fork.
2. DNA polymerase adds
the complementary
nucleotides and binds the
sugars and phosphates.
DNA polymerase travels
from the 3' to the 5' end.
DNA replication
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3. DNA polymerase adds
complementary
nucleotides on the other
side of the ladder. Traveling
in the opposite direction.
4. One side is the leading
strand - it follows the
helicase as it unwinds.
5. The other side is the
lagging strand - its
moving away from the
helicase
DNA replication
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Problem: it reaches the
replication fork, but the
helicase is moving in
the opposite direction.
It stops, and another
polymerase binds
farther down the chain.
This process creates
several fragments,
called Okazaki
Fragments, that are
bound together by
DNA ligase.
DNA replication
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6. During replication, there are many points
along the DNA that are synthesized at the same
time (multiple replication forks). It would take
forever to go from one end to the other, it is
more efficient to open up several points at one
time.
All organisms use the same genetic code
Each set of three nucleotides codes for an amino
acid = “The Genetic Code”
The genetic code
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consists of 64 triplet codons (A, G, C, U) 43 = 64
all codons are used in protein synthesis
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20 amino acids
3 termination (stop) codons: UAA, UAG, UGA
AUG (methionine) is the start codon (also used
internally)
multiple codons for a single amino acid =
degeneracy
Genetic code is unambiguous. Each triplet codon
has only one meaning
5 amino acids are specified by the first two
nucleotides only
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3 additional amino acids (Arg, Leu, and Ser) are
specified by six different codons
Accuracy of Replication
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The mismatched nucleotide causes a
pause in replication, and the mismatched
nucleotide is excised from the daughter
strand.
The errors that slip through nucleotide
selection and proofreading cause a gene
mutation to occur.
Actually it is of benefit for mutations to
occur occasionally because variation is
the raw material for the evolutionary
process.
Point Mutations
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Point mutations involve a change in
a single nucleotide and therefore a
change in a specific codon.
When one base is substituted for
another, the results can be variable.
For example, if UAC is changed to
UAU, there is no noticeable effect,
because both of these codons code
for tyrosine. Therefore, this is called
a silent mutation.
Point Mutations
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If UAC is changed to UAG(stop
codon.
If this substitution occurs early in
the gene, the resulting protein may
be too short and may be unable to
function. Such an effect is called a
nonsense mutation.
Point Mutations
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Finally, if UAC is changed to CAC,
then histidine is incorporated into
the protein instead of tyrosine. This
is a missense mutation.
Point Mutations
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The term reading frame applies to the
sequence of codons because they are
read from some specific starting point as
is this sentence:
THE CAT ATE THE RAT. If the latter C is
deleted from this sentence and the
reading frame is shifted,
THE ATA TET HER AT –something that
doesn't make sence.
That call frameshift mutations.
Gene Expression
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The process by which a gene produces a product,
usually a protein, is called gene expression.
DNA not only serves as a template for its own
replication, it is also a template for RNA formation.
Gene Expression in prokaryotes:
transcription, translation.
Gene Expression in eukaryotic cells:
transcription, processing, translation.
Transcription
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process by which a mRNA copy is made of a portion
of DNA is called transcription
It is the first step
required for gene
expression.
During transcription, a
mRNA molecule is
formed that has a
sequence of bases
complementary to a
portion of one DNA
strand;
A, T, G, or С is present
in the DNA template,
U, A, C, or G is
incorporated into the
mRNA molecule
Transcription
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Transcription begins at a region of DNA called a
promoter.
A promoter is a special sequence of DNA bases
where RNA polymerase attaches and the
transcribing process begins. A promoter is at the
start end of the gene to be transcribed.
Elongation of the mRNA molecule occurs as long
as transcription proceeds. Finally, RNA
polymerase comes to a terminator sequence at
the other end of the gene being transcribed.
The terminator causes RNA polymerase to
stop transcribing the DNA and to release the
mRNA molecule, now called a RNA transcript
RNA Processing
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Special molecular splicing complexes cut out sections of
introns. Then the remaining portions of the mRNA tape
are spliced together.
exons: portions of the mRNA transcript that code for
amino acids.
From this, it follows that introns are noncoding portions of
the original mRNA tape; they do not contain information
for the sequencing of amino acids in a protein.
Translation
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During translation, the
sequence of codons in
mRNA
directs
the
sequence of amino acids
in a protein.
Two other types of RNA are
needed for protein synthesis.
rRNA is contained in the
ribosomes, where the codons
of mRNA are read
tRNA carries amino acids to
the ribosomes so that protein
synthesis сan occur.
The steps of translation.
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mRNA Binds to Ribosome;
Polypeptide Chain Is
Elongated
Termination of the Growing
Chain
The steps in translation are:
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The ribosome binds to mRNA at a specific area.
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The ribosome starts matching tRNA anticodon
sequences to the mRNA codon sequence.
Each time a new tRNA comes into the ribosome, the amino acid
that it was carrying gets added to the elongating polypeptide
chain.
The ribosome continues until it hits a stop sequence,
then it releases the polypeptide and the mRNA.
The polypeptide forms into its native shape and
starts acting as a functional protein in the cell
Regulation of Gene expression
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Function - to produce enzymes which break down lactose
(milk sugar)
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lac operon in E. Coli
when lactose is present, they turn on and produce enzymes
Two components - repressor genes and functional genes
Promoter (P) - aids in RNA polymerase binding
Operator (O) - "on/off" switch - binding site for the
repressor protein
Repressor (lacI) gene
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Repressor gene (lacI) - produces repressor protein w/ two
binding sites, one for the operator and one for lactose
 The repressor protein is under allosteric control - when not
bound to lactose, the repressor protein can bind to the
operator
 When lactose is present, an isomer of lactose, allolactose,
will also be present in small amounts. Allolactose binds to
the allosteric site and changes the conformation of the
repressor protein so that it is no longer capable of binding to
the operator
Operation - If lactose is not present:
the repressor gene produces repressor, which binds
to the operator. This blocks the action of RNA
polymerase, thereby preventing transcription
Thank You for attention!
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