gene activity 1 ch. 12-b outline – gene activity
TRANSCRIPT
Gene ActivityGene Activity 1
Ch. 12-b Outline – Gene ActivityCh. 12-b Outline – Gene Activity
Gene ActivityGene Activity 2
Function of Genes
Archibald Garrod:
● First to propose a link between genes and proteins and metabolic diseases.
● Inherited disorders could be caused by lack of a particular enzyme in a metabolic
pathway.
Gene ActivityGene Activity 3
Genes Specify Enzymes
Beadle and Tatum:
Experiments on fungus Neurospora crassa
Induced mutations with X-rays. Spores were no longer able to grow on minimal media.
Figured out which enzymes were lacking & found that each mutant strain only had 1 defective gene
Proposed that each gene specifies the synthesis of one enzyme
One-gene-one-enzyme hypothesis
4Beadle & Tatum Experiment
Gene ActivityGene Activity 5
Genes Specify a Polypeptide
A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide
Suggests that genetic mutations cause changes in the primary structure of a protein
Examples of genetic mutations that cause disease:
1. Sickle cell disease : a change in DNA causes hemoglobin to have one different amino acid which changes the function of the entire protein. (This was discovered by Linus Pauling and Harvey Itano in 1949)
6Sickle-Cell Disease in Humans
Gene ActivityGene Activity 7Protein Synthesis:From DNA to RNA to Protein
The mechanism of gene expression
DNA in genes specify information, but information is not structure and function
Genetic info is expressed into structure & function through protein synthesis
The expression of genetic info into structure & function:
DNA in gene controls the sequence of nucleotides in an RNA molecule
RNA controls the primary structure of a protein
Gene ActivityGene Activity 8
Types of RNA (see transparency here)
RNA is a polymer of RNA nucleotides
RNA nucleotides are of four types:
Uracil, Adenine, Cytosine, and Guanine
Uracil (U) replaces thymine (T) of DNA
Types of RNA Messenger (mRNA) - Takes genetic message from DNA in nucleus to ribosomes in cytoplasm
Ribosomal (rRNA) - Makes up ribosomes which read the message in mRNA
Transfer (tRNA) - Transfers appropriate amino acid to ribosome when “instructed”
9Structure of RNA
10Three Types of RNA
Gene ActivityGene Activity 11Steps in Gene Expression:
There are two steps in gene expression:
1. Transcription
● DNA serves as a template for RNA formation
2. Translation
● mRNA transcript directs the creation of a sequence of amino acids in a polypeptide
12Overview of Gene Expression
Gene ActivityGene Activity 13
The Genetic Code
Each of the 20 amino acids found in proteins is specified by one or more codons Genetic alphabet has only four “letters”: U,A,C,G
Codons in the genetic code are all three bases long
There are 64 possible arrangements of four symbols taken three at a timeOften referred to as triplets
Genetic language has 64 “words” that code for 20 amino acids
Gene ActivityGene Activity 14
Finding the Genetic Code
Nirenberg & Matthei (1961)1. They found that cell enzymes could be used
to construct synthetic RNA.2. Then found that these RNA could be
translated in a test tube into polypeptides3. The first synthetic RNA was made of only U.
● This resulted in a polypeptide made up only of phenylalanine.
4. They repeated this with all possible triplet combinations until they constructed the genetic code table.
15The Genetic Code (mRNA)
CAG
Gene ActivityGene Activity 16
Genetic Code is Universal
All organisms use the same genetic code- Slight differences in mitochondria and chloroplasts DNA, however.
Degenerate (redundant) There are 64 codons available for 20 amino acids
Most amino acids encoded by two or more codons
Contains start and stop signals Makes it possible to transfer genes between different organisms.
Gene ActivityGene Activity 17Steps in Gene Expression:First = Transcription
All types of RNA are created by transcription
DNA unzips and exposes unpaired bases
One side serves as template for mRNA formation. This strand is also called the sense strand.
Loose RNA nucleotides bind to template DNA bases using the C=G & A=U rule
This occurs with the help of RNA polymerase.
● The strand of DNA that is not transcribed is called the noncoding strand or nonsense strand.
Gene ActivityGene Activity 18Steps in Gene Expression:Transcription (cont’d)
Transcription begins when RNA polymerase attaches to a region of DNA known as the promoter.
RNA polymerase joins the nucleotides together in the 5’ to 3’ direction.
- It only adds a nucleotide to the 3’ end of the RNA being formed.
Elongation of mRNA continues until RNA polymerase comes to a DNA stop sequence.
Gene ActivityGene Activity 19Steps in Gene Expression:Transcription (cont’d)
When entire gene is transcribed into mRNA, result is an mRNA transcript of the gene
The base sequence in the mRNA is complementary to the base sequence in DNA
Many RNA polymerase molecules can be working at the same time. Thus, a cell can produce thousands of copies of the same mRNA within a short period of time.
20Transcription
21RNA Polymerase
Gene ActivityGene Activity 22
Processing Messenger RNA
In eukaryotes the primary mRNA transcript is modified before it leaves the nucleusRNA splicing occurs:
Primary transcript consists of:Some segments that will not be expressed (introns)
Segments that will be expressed (exons) Performed by spliceosome complexes in nucleoplasmIntrons are excised (cut out)Remaining exons are spliced back together
Gene ActivityGene Activity 23
Processing Messenger RNA (cont’d) Modifications to ends of primary transcript:
Cap of modified guanine on 5′ end Poly-A tail of 150+ adenines on 3′ end
Result is mature mRNA transcript
24
mRNA Processing
in Eukaryotes
Gene ActivityGene Activity 25
Functions of Introns
Eukaryotes have many introns while prokaryotes don’t.In humans up to 95% of genes are introns.
Genome has only about 25,000 coding genes
Possible functions of introns: Exons might combine in various combinations
Would allow different mRNAs to result from one segment of DNA
Introns might regulate gene expression
Gene ActivityGene Activity 26Steps in Gene Expression:Second = Translation
● Takes place in cytoplasm.
● Codons in mRNA direct the sequence of amino acids in a polypeptide. One “language” is translated into another.
● Involves all three types of RNA
● tRNA
- Single-stranded RNA
- Transfers amino acids to the ribosomes.
- There is at least one tRNA for each of the 20 amino acids found in proteins.
Gene ActivityGene Activity 27Steps in Gene Expression:Role of tRNA
tRNA molecules have two binding sites: An amino acid binds to the 3’ end of the tRNA
- This is helped along by an enzyme called aminoacyl-tRNA synthetase.
The opposite end of the tRNA contains an anticodon:
- This is a group of 3 bases that are complementary to a specific codon of mRNA
All tRNA molecules with a specific anticodon will always bind with the same amino acid
28Structure of tRNA
Gene ActivityGene Activity 29
Role of Ribosomes
Ribosomal RNA (rRNA):Produced from a DNA template in the nucleolus
Combined with proteins into large and small ribosomal subunits
A completed ribosome has three binding sites to facilitate pairing between tRNA and mRNAThe E (for exit) site
The P (for peptide) site, and
The A (for amino acid) site
30Ribosomal Binding Sites
31Ribosomal Structure and Function
Gene ActivityGene Activity 32Steps in Gene Expression:Second = Translation
An mRNA transcript migrates to rough endoplasmic reticulum
Associates with the rRNA of a ribosome
The ribosome “reads” the information in the transcript
Ribosome directs various species of tRNA to bring in their specific amino acid “fares”
tRNA specified is determined by the code being translated in the codons of mRNA transcript
Gene ActivityGene Activity 33Steps in Translation:#1 - Initiation
Components necessary for initiation are:Small & large ribosomal subunitsmRNA transcript Initiator tRNA Initiation factors (special proteins that bring the above together)
Initiator tRNA:Always has the UAC anticodonAlways carries the amino acid methionineCapable of binding to the P site
Gene ActivityGene Activity 34Steps in Translation:#1 - Initiation
● Small ribosomal subunit attaches to mRNA transcript
● Initiator tRNA (UAC) attaches to P site
- Beginning of transcript always has the START codon (AUG). So first tRNA always carries the amino acid methione.
● Large ribosomal subunit joins the small subunit
35
Steps in Translation:
#1 - Initiation
Gene ActivityGene Activity 36Steps in Translation:#2 - Elongation
“Elongation” refers to the growth in length of the polypeptide
● tRNA molecules bring their amino acid fares to the ribosome
They must have the anticodon complementary to the mRNA codon
being read
Joins the ribosome at it’s A site
Methionine of initiator is connected to amino acid of 2nd tRNA by peptide bond
Gene ActivityGene Activity 37Steps in Translation:#2 – Elongation (cont’d)
Second tRNA moves over to the P site (translocation)
Spent initiator moves to E site and exits
Ribosome reads the next codon in the mRNA
Joins the ribosome at it’s A site
Dipeptide on 2nd amino acid is connected to amino acid of 3rd tRNA by peptide bond
- A ribozyme and energy is needed to bring about this transfer.
38Steps in Translation:# 2 Elongation
Gene ActivityGene Activity 39Steps in Translation:#3 – Termination
Final step in protein synthesis:● Ribosome reads the STOP codon at the end of
the mRNA. These are the following codons:UAA, UAG, or UGAThese do not code for an amino acid
Polypeptide is released from last tRNA by release factor
Ribosome releases mRNA and dissociates into subunits
mRNA read by another ribosome
40Steps in Translation:
#3 - Termination
41Summary of Gene Expression
(Eukaryotes)
42DNA Replication Video I
http://www.courses.fas.harvard.edu/~biotext/animations/replication1.html
DNA Replication Video II
http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf
Protein Synthesis Video
http://highered.mcgraw-hill.com/olc/dl/120077/micro06.swf