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Molecular GeneticsIf you learned anything….this is what
you should REALLY know
Important people Frederick Griffith
Discovered transformation
Avery, MacLeod, McCarty Showed that despite removing nearly all protein,
transformation could still occur….proposed DNA
Hershey & Chase Proved DNA as transformation agent Bacteriophage experiment
Structure of DNA Watson, Crick, Wilkins Don’t forget about Franklin Chargaff’s rules
Components of DNA 5-carbon sugar Phosphate group (PO4) Nitrogen containing base
Purines: two ringed structure Adenine Guanine
Pyrimidines: single ringed structure
Thymine Cytosine
Structure of DNA
DNA Replication
Fig. 14.16
Table 14.1
Overview: Flow of Genetic Information Information on DNA is in the form of specific
sequences of nucleotides along the DNA strands The DNA inherited by an organism leads to
specific traits by dictating the synthesis of proteins
DNA Proteins Traits
Defining a gene
Polypeptide 1
Gene polypeptide 2 gene RNA
polypeptide 3
RNA (Ribnucleic acid) Genes provide specific
instructions for making specific proteins
Link between DNA and proteins is RNA
RNA is very similar to DNA chemically A few key differences
RNA DNA
Ribose Deoxyribose
Adenine
Cytosine
Guanine
Uracil
Adenine
Cytosine
Guanine
Thymine
Usually single
stranded
Always double
stranded
Prokaryotic cells Transcription and translation are very
similar in process to eukaryotic cells DNA is not segregated from ribosomes in
the cytoplasm Transcription and translation are coupled Ribosomes attach to the leading end of
mRNA molecule while transcription is still progressing
Eukaryotic cells Transcription occurs in the nucleus
Results in a pre-mRNA (primary transcript) RNA processing yields the finished mRNA
Translation occurs at ribosomes in the cytoplasm
Transcription: a closer look
Beginning and ending Specific sequences along the DNA mark
where transcription begins and ends Promoter: point where RNA polymerase attaches
and initiates transcription Terminator: sequence that signals the end of
transcription Transcription unit: stretch of DNA that is being
transcribed into an RNA molecule
Transcription occurs “downstream”
RNA modification Enzymes in the nucleus modify pre-mRNA
before it exits the nucleus Both ends of the primary transcript are usually altered Interior parts may be cut out and the remaining parts
are spliced together
5` cap: At the 5` end a modified form of guanine is added
Poly-A tail: 50-250 adenine molecules are added to the 3` end
In higher eukaryotes, 90% or more of gene can be introns
No one knows why…yet….
Alternative RNA splicing Gives rise to two or
more different polypeptides depending on which segments are exons Sex differences in fruit flies
may be due to differences in splicing RNA
May explain why we (humans) have relatively few genes
Translation initiation RNA must be able to bind to DNA at the
gene promotor Regulatory proteins
Bind to specific sequences 100’s have been identified Either block transcription or stimulate it
DNA Binding motifs DNA binding domain
Functionally distinct region in the DNA binding motif the specifically bind to DNA in a set location
Helix turn Helix Homeodomain Zinc finger Leucine zipper
Operons Multiple genes Single transcription
unit Often same metabolic
pathway
lac operon
Effector: allolactose
Glucose repression
Prevents repressor from binding Allows repressor binding
trp operon
In Summary
Activators: Specific transcription factors Bind to enhancers at distance sites Increase rates of transcription
Coactivators: Transmit signals from activators proteins to the general
factors
General factors Position RNA polymerase at start of protein coding
sequence
Eukaryotic Chromatin structure
Nucleosomes may block binding of transcription factors
Histone modifications
Post transcription regulation
miRNAs: bind directly to mRNA and preventtranslation
Alternative slicing Different tissues Different timing in cells
Calcitonin CGRP
Different tissues, different functions, same transcription unit
mRNA transport mRNA transcript cannot
move through nuclear pore while splicing enzymes are attached
Transcript must be recognized by nuclear pore receptors Poly A tail
Only 5% of total mRNA transcripts reach cytoplasm
Degradation of mRNA mRNA half life
3 min: prokaryotic mRNA transcripts 10 hours: eukaryotic B-globin transcripts 1 hour: eukaryotic regulatory genes
Targeted for degradation Enables levels of regulatory proteins to be altered
quickly in response to changes
3 stages Initiation Elongation Termination All three phases require protein factors that
aid in the translation process Initiation and elongation require energy
provided by the hydrolysis of GTP Similar to ATP
Initiation Brings together mRNA and a tRNA (carrying the first
amino acid) and the two ribosomal subunits 1st: a small subunit binds with mRNA and a initiator tRNA
(methionine) 2nd: the small subunit moves downstream along the mRNA
until it reaches the start codon (AUG) This established the reading frame for the mRNA
The initiator tRNA hydrogen bonds with the start codon 3rd: initiation factors (proteins) bring in the large subunit so
that the initiator tRNA occupies the P site
Elongation Involves several elongation factors
(proteins) Three step cycle as each amino acid is
added Codon recognition Peptide bond formation translocation
Termination Occurs when one of the three stop
codon reaches the A site A release factor binds to the stop codon
and hydrolyzes the bond between the polypeptide and its tRNA in the P site
The frees the polypeptide and the translocation complex disassembles
Lets go to the video A ribosome requires less than one minute to
translate an average sized mRNA into a polypeptide
During and after synthesis a polypeptide coils and folds to its three dimensional shape Primary structure (order of amino acids) determines
secondary and tertiary structure Chaperone proteins may aid in correct proteins
Posttranslational modifications Proteins may require additional
modifications after translation Addition of sugars, lipids, or phosphate groups to
amino acids Removal of some amino acids Cleavage of whole polypeptide chains Joining of two or more polypeptide chains
Signal peptides
Differences between eukaryotes and prokaryotes Different RNA polymerases Eukaryotic RNA polymerases require transcription
factors Differences in termination Ribosomal structure differences Prokaryotes can transcribe and translate a gene at
the same time In Eukaryotes, extensive RNA processing occurs Eukaryotes have complicated mechanisms for
targeting proteins to the appropriate organelle
Mutations Changes in the genetic material of a cell (or
virus) Large scale mutations in which long segments
of DNA are affected Translocations Duplications Inversions
Chemical change in just one base pair of genes Point mutation