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Nuclear Processes
Nuclear Processes
• Why are we similar to our parents and our relatives - yet not identical?
• What material actually gets passed down from parent to offspring?
Introduction
Head
Tail
Tail fiber
DNA
300,
000×
Hershey and Chase Used Phages
Result: no 35S inside pellet (bacteria);
all 35S in supernatant
Experiment I: 35S - labeled protein capsule
Result: All 32P inside pellet (bacteria);
no 32P in supernatant
Experiment 2: 32P - labeled DNA
Molecules of Inheritance
• But what do the polymerized nucleic acids look like and how do they store information?
BUT...
Strange Findings...
X-ray Diffraction Image of DNA Photo 51 by Rosalind Franklin
James Watson and Francis Crick 1953
The Nobel Prize in Physiology or Medicine 1962 was awarded jointly to Francis Harry Compton Crick, James Dewey Watson and Maurice Hugh Frederick Wilkins (L to R) "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".
• held together by hydrogen bonds between the bases:
• A-T and G-C
• The information is stored as base-pair sequences
• Strings of base pairs that code for a product are called genes
DNA is a double-stranded helix
Base-Pairing is the Key
• Human genome = 3 billion base pairs, probably about 50,000 genes
How Many Bases and How Many Genes?
• Involves the 'unzipping' of a section of DNA and the matching of new bases with the parent strand
!
• Requires DNA polymerase, helicase and ATP
DNA Replication
A T
C G
G C
A T
T A
A T
C G
G C
A T
T A
A T
C G
G C
A T
T A
A T
C G
G C
A
T
A T
C G
AC
T
A
Parental moleculeof DNA
Both parental strands serve as templates
Two identical daughtermolecules of DNA
G C
A T
G C
A T
C G
A
GA
C
GC
GC
G
T
AG
C
T
AT
A
A
TT
A
CG
C
G
CG
T
A
G
C
T
A
T
A
A
T
T
A
T
C
T
• Genes code for one thing… PROTEINS
• DNA structure ! polypeptide structure.
!
• How?
• How many bases are needed to encode all 20 amino acids?
Gene Expression
• DNA structure ! polypeptide structure.
• Codons are translated into amino acid sequences
• But HOW?
Gene Expression
Central Dogma of Molecular Biology: DNA ! RNA ! Protein
• Transcription = copying a message into a new medium.
!
• Major players: promoter, RNA polymerase
1. DNA ! RNA is called TRANSCRIPTION
• Translation = rewording a message into a new language.
• Translation requires a. ribosomes
b. transfer RNA molecules which carry amino acids
c. enzymes
d. energy
2. RNA ! protein is called TRANSLATION
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Anticodon
Transfer RNA Molecule
Amino acid attachment site
Anticodon
Transfer RNA
tRNA molecules
mRNA Small subunit
Growing polypeptide
Large subunit
Ribosome, tRNA, mRNA, Protein Interaction
Transcription and Translation
WHICH Genes Get Expressed? And When?
• Embryonic stem cells are todipotent = able to differentiate into any type of cell depending on signals it receives during development.
• ALL YOUR CELLS have ALL THE SAME DNA, but differential expression makes the cells very different from one another.
Control of Differential Gene Expression
• Differentiated cells can be "re-programmed"
• Ex: Cloning
• Ex: Regeneration
Control of Differential Gene Expression
Remove nucleusfrom egg cell
Add somatic cell from adult donor
Grow in culture to produce an early embryo (blastocyst)
Implant blastocyst insurrogate mother
Remove embryonic stemcells from blastocyst and
grow in culture
Induce stem cells toform specialized cells(therapeutic cloning)
Clone of donor is born (reproductive cloning)
Donorcell
Nucleus fromdonor cell
Cloning
Adult stemcells in bone
marrow
Culturedembryonicstem cells
Different cultureconditions
Heart muscle cells
Different types ofdifferentiated cells
Nerve cells
Blood cells
Mechanisms of Differential Gene Expression
• Induction: Enhancer sequences
• Make it more likely for a gene to be transcribed into mRNA
!
• Repression: Silencer sequences
• Block transcription by sitting on promoters
Transcription Factors
Enhancers Promoter
Gene
DNA
Activatorproteins
Other proteins
Transcriptionfactors
RNA polymerase
Bendingof DNA
Transcription
• Caps and Tails. G caps and A tails are added to the mRNA.
• Reduces rate of enzymatic breakdown of the mRNA
• Helps ribosomal recognition of the mRNA molecule
mRNA Processing
• RNA contains coding regions (exons) and noncoding regions (introns)
• Enzymes splice introns out before the mRNA leaves the nucleus.
• Alternative splicing of the same gene means that different mRNA molecules can be made from the same transcript!
mRNA Processing: Differential Splicing
• mRNA half-life
• Inhibitory molecules
• Bind to mRNA and prevent translation (reversible)
Translational Regulation
• Zymogens
• Many translated products are released in an inactive form
• Zymogens are then activated by the presence of other molecules.
• Ex: pepsinogen + HCl ! pepsin
Post-Translational Regulation
• Any change in the nucleotide sequence is called a mutation.
• Spontaneous or due to mutagen exposure
• Results?
• Examples:
• Base substitution, base insertion, base deletion
• Reading frame (triplet grouping) of the genetic message may be lost if a base is deleted or inserted – this is called a “frame-shift” mutation
Mutations
• Point Mutation
• The cat was fat
• The bat was fat
• Insertion
• The hat was old
• The hba twa sol
Examples of Mutations
C T T C A T
Normal hemoglobin
Mutant hemoglobin DNA
G A A G U A
Sickle-cell hemoglobin
Normal hemoglobin DNA
Glu Val
mRNA mRNA
Point Mutation
Normal gene
mRNA
Base substitution
Base deletion Missing
Met Lys Phe Gly Ala
Met Lys Phe Ser Ala
Met Lys Leu Ala His
A U G A A G U U U G G C G C A
A U G A A G U U U A G C G C A
A U G A A G U U G G C G C A U
U
Protein
Mutations
C U C
What kind of mutation is this? What is the result?
C U
What kind of mutation is this? What is the result?
Questions…
• Why do recessive alleles persist over time?
• Why do lethal mutations persist over time?
• Why do some heterozygotes have an advantage?
