molecular genetics if you learned anything….this is what you should really know

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