Download Organization of Genes Differs in Prokaryotic and Eukaryotic DNA

Organization of Genes Differs in Prokaryotic and Eukaryotic DNA
Chapter 10 p.110-114
Arrangement of information in DNA----- requirements for RNA
Common arrangement of protein-coding genes in prokaryotes= e.g Operon
----------------- operates as a unit from a single promoter.
Transcription of operon produces a continuous strand of mRNA:
carries message for a related series of proteins
Genes in prokaryotic packed with very few noncoding gaps---- DNA
transcribed
into colinear mRNA-----translated into protein.
Economic clustering of genes devoted to a single metabolic function In Prok.
does not occur in eukaryotes
Yeasts= metabolically similar to bacteria.
Eukaryotic genes in multicellular devoted to a single pathway physically
separated in DNA
Exons and Introns (Introns rare in Bacteria and
Archea,uncommon in unicellular eukaryotes e.g baker’s yeast
Introns present in DNA of viruses infect eukaryotic cells (Introns
first discovered in viruses)
Genes located on different chromosomes, transcribed from its own
promoter producing one mRNA, translated to a single polypeptide
Eukaryotic Precursor mRNAs Are Processed
to Form Functional mRNAs
Transcription & translation occur concurrently in prokaryotes but not in
Eukaryotic, Why?
Primary transcripts = precursor mRNAs (pre-mRNAs)
undergo modifications at both ends &RNA processing, to yield a functional
mRNA
------- mRNA exported to cytoplasm
5_ end of a nascent RNA chain emerges from surface of RNA polymerase
II
---------- acted on by enzymes that synthesize the 5_ cap=7-methylguanylate
connected to terminal nucleotide of RNA by an unusual 5_,5_
triphosphate linkage
The cap function?
-Protection from enzymatic action
-Export to cytoplasm
-bound by a protein factor required to begin translation
Processing at 3_ end of a pre-mRNA involves cleavage by an endonuclease----free 3_-hydroxyl group to adenylic acid residues added one at a time by poly(A)
polymerase -----poly(A) tail ( 100–250 bases) shorter in yeasts invertebrates
than in vertebrates
----poly A addition doesnt not require a template
.
Final step in processing of mRNA is RNA splicing:
e.g Globin gene
mRNAs produced by RNA processing retain noncoding regions:
5_ and 3_ untranslated regions (UTRs).
5_ UTR hundred or more nucleotides / 3_ UTR several kilobases in length
.
Prokaryotic mRNAs usually have 5_ and 3_ UTRs shorter than eukaryotic
mRNAs
Alternative RNA Splicing Increases the Number
of Proteins Expressed from a Single Eukaryotic Gene
Proteins in Euk. have a multidomain tertiary structure
Repeated protein domains encoded by one exon/ small number of exons
that code for identical or nearly identical amino acid sequences
Multiple introns in eukaryotic genes permits expression of multiple, related
proteins (isoforms/different forms) from a single gene by alternative splicing.
e.g Fibronectin (multidomain extracellular adhesive protein)
Fibroblasts produce fibronectin mRNAs contain exons EIIIA&EIIIB
------------ encode a.a bind tightly to proteins in fibroblast plasma membrane.
Alternative splicing of fibronectin primary transcript in hepatocytes, lack EIIIA &
EIIIB exons-----does not adhere tightly to fibroblasts ----- circulate in blood.
formation of blood clots , domains of hepatocyte fibronectin binds to fibrin,
one of the principal constituents of clots
fibronectin interacts with integrins on the membranes---- activated platelets
----- expanding clot by addition of platelets.
---------------------------------------------------------------------------Sequencing of genomic DNA revealed 60% of all human genes expressed as
alternatively spliced mRNAs
▲ FIGURE 4-14 Overview of RNA processing to
produce functional mRNA in eukaryotes.
▲ FIGURE 4-13 Structure of the 5 methylated
cap of eukaryotic mRNA.
▲ FIGURE 4-15 Cell type–specific splicing of fibronectin
pre-mRNA in fibroblasts and hepatocytes. The ≈75-kb
fibronectin gene (top) contains multiple exons. The EIIIB and
EIIIA exons (green) encode binding domains for specific proteins
on the surface of fibroblasts.
Whereas exons are spliced out of fibronectin mRNA in hepatocytes.
MOLECULAR STRUCTURE OF GENES AND
CHROMOSOMES
p.405-408
P408-424
By beginning of 21st century, completed sequencing, entire genomes
of viruses, bacteria & budding yeast S. cerevisiae, D. melanogaster, and
humans
- sequencing data revealed large portion of genomes of higher eukaryotes
-----more 95% human chromosomal DNA non-coding
-regions similar but not identical( application: e.g DNA
“fingerprint”)
-Some repetitious DNA sequences not found in constant positions
in the same species--------Mobile” DNA elements (in prokaryotic
&eukaryotic)
Mobile DNA-----can cause mutations when move to new sites in
genome.
------have no function in life cycle, probably played role in
evolution.
In higher eukaryotes, DNA regions encoding proteins— genes
nonfunctional DNA=noncoding introns common
within genes
Sequencing of same protein-coding gene (Exons)in a variety of
eukaryotic species Shown relatively similar sequences
sequence variation, including total loss, occurs among introns(less
functional significance)
Objectives:
- a molecular definition of genes
- complexities that arise in higher organisms from
processing of mRNA precursors into alternatively spliced
mRNAs.
- special properties of mobile DNA.
- packaging of DNA and proteins into compact complexes,
large-scale structure of chromosomes, and e functional elements
required for chromosome duplication
and segregation.
Molecular Definition of a Gene
A gene= entire nucleic acid sequence necessary for synthesis of a
functional gene product (polypeptide or RNA).
A gene also includes:
- all DNA sequences required for synthesis of RNA
transcript
e.g transcription-control regions= enhancers lie 50 kb or more from
coding region.
- noncoding regions= sequences specify 3_ cleavage &
polyadenylation( poly(A) sites)
- splice sites
10.1
-most genes transcribed into mRNAs, and some transcribed into
tRNAs & rRNAs
=tRNA, rRNA genes??
Most Eukaryotic Genes Produce Monocistronic
mRNAs and Contain Lengthy Introns
-many bacterial mRNAs polycistronic---------- a single mRNA molecule
(e.g., mRNA encoded by trp operon) includes coding region(Cistrones) for
several proteins , function together in a biological process.
bacterial polycistronic mRNA a ribosome binding site located near start
site for each cistrons------- Translation initiation begin at any of these
multiple internal sites ----------------producing multiple proteins
-Most eukaryotic mRNAs monocistronic------- each mRNA molecule encodes
a single protein.
In most eukaryotic mRNAs, 5_-cap directs ribosome binding, and
translation begins at closest AUG start codon---------- translated to
give a single type of polypeptide
Simple and Complex Transcription Units Are
Found in Eukaryotic Genomes
Cluster of genes that form a bacterial operon= a single transcription unit
transcribed from a promoter into a single primary transcript.
Eukaryotic transcription units classified into two types, depending
on fate of primary transcript.
-1-------Primary transcript produced from a simple transcription
unit processed to yield a single type of mRNA---encoding a single
protein.
Mutations in exons, introns and transcription-control regions ---influence expression of protein
2- complex transcription units---- common in multicellular organisms,
----- primary RNA transcript processed more than one way----Leading to formation of mRNAs containing different exons
. Each mRNA monocistronic----translated into a single polypeptide,
Multiple mRNAs can arise from a primary transcript in three ways:
1. exon skipping :Use different splice sites, producing mRNAs with the
same 5_ and 3_ exons but different internal exons.
.
2. Use of alternative poly(A) sites, producing mRNAs that
share same 5_ exons but have different 3_ exons.
3. Use of alternative promoters, producing mRNAs -----have different 5_
exons and common 3_ exons.
A gene expressed selectively in two or more types of cells transcribed
from distinct cell-type-specific promoters
Examples of all three types of alternative RNA processing
occur during sexual differentiation in Drosophila
Differences in RNA splicing of primary fibronectin
transcript in fibroblasts and hepatocytes determines
whether or not the secreted protein includes domains
that adhere to cell surfaces (see Figure 4-15).
The relationship between a mutation and a gene not
straightforward in complex transcription units.
A mutation in control region or in an exon shared by
alternative mRNAs will affect alternative proteins encoded
by complex transcription unit.
Mutations in an exon present in one of alternative mRNAs
will affect only the protein encoded by that mRNA.
Genetic complementation tests :
To determine if two mutations in same or different genes
In complex transcription unit, mutations d and e complement each
other in genetic complementation test, even they occur in same gene-----a chromosome with mutation d can express a normal protein encoded by
mRNA2 and a chromosome with mutation e can express a normal protein
encoded by mRNA1
.!
However, a chromosome with mutation c in an exon common to
both mRNAs not complement either mutation d or e.
In other words, mutation c would be in the same complementation
groups as mutations d and e,
even though d and e themselves would not be in the same
complementation group
FIGURE 10-1 Overview of the
structure of genes and chromosomes.
DNA of higher eukaryotes consists of unique and repeated sequences. ~5%
of human DNA encodes proteins and functional RNAs and regulatory
sequences ;remainder spacer DNA between genes and introns within genes.
~50% in humans, is derived from mobile DNA elements, genetic symbiots
contributed to evolution of contemporary genomes.
FIGURE 10-2 : A simple transcription unit
includes a region that encodes one protein, Introns lie between exons (blue rectangles)
removed during processing of primary transcripts (dashed red lines);
---------not in functional monocistronic mRNA.
Mutations in a transcription-control region (a, b) may reduce or prevent transcription,
------------- reducing or eliminating synthesis of the encoded protein.
A mutation within exon (c) may result in abnormal protein with diminished activity.
A mutation within an intron (d ) ----- introduces a new splice site----- in
abnormally spliced mRNA -------- nonfunctional protein.
(b) Complex transcription units produce primary transcripts processed in alternative ways.
(Top) a primary transcript contains alternative splice sites----- processed into mRNAs with
same 5 and 3 exons but different internal exons. (Middle primary transcript has 2 poly(A)
sites-------processed into mRNAs with alternative 3 exons. (Bottom) alternative promoters (f
or g) active in different cell types, mRNA1, produced in a cell type , f activated, has a
different exon (1A) than mRNA2 , produced in a cell type ,g activated
.Mutations in control regions (a and b) and c within exons shared by alternative mRNAs
affect the proteins encoded by both alternatively processed mRNAs. In contrast, mutations
(designated d and e) within exons unique to one of alternatively processed mRNAs affect
only protein translated from that mRNA. For genes transcribed from different promoters in
different cell types (bottom), mutations in different control regions (f and g) affect expression