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Flashcards in Introns and Exons Deck (35):

Characteristics of Prokaryotic cells: (7)

• No membrane bound organelles
• Simple genome organization
• Genes usually do not have introns
• Between 500 and 4000 genes
– M. genitalium has 477 genes
• Great degree of diversity
• Unicellular
• Circular DNA


Characteristics of Eukaryotic Cells (9)

• All organelles are membrane-bound (e.g. nucleus)
• Complex genome organization
• Large genome size
• Between 6,000 and 30,000 genes
• Genes have introns (non-coding regions)
• Large amount of regulatory DNA - to control gene expression
• High degree of genetic redundancy
• Unicellular or multicellular
• Linear DNA



the large-scale study of proteins that
are produced in a cell using the genome - particularly their structures and functions.



analysis of transcriptomes (gene expression patterns)
transcriptomes are dynamic – patterns vary with cell type, developmental stage etc.


Central dogma of molecular biology

DNA to RNA to Protein

You cannot go Protein to RNA to DNA though you can go RNA to DNA.


Process of gene expression

1) transcription - RNA is transcribed from DNA template
2) RNA processing - RNA transcript is spliced and modified to produced mRNA which moves from the nucleus to the cytoplasm
3) Amino acid activation - each amino acid attaches to its proper tRNA via specific enzymes and ATP
4) Translation - tRNA add the amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time.


Steps in transcription

1) initiation - RNa polymerase binds to promoter (starting point) and initiates RNA at the start point of the template strand
2) Elongation - polymerase moves downstream from 5' to 3' creating the RNa strand
3) Termination - upon transcribing the terminator at the end of the sequence the RNA polymerase detaches from the DNA and the RNA transcript is released.


Difference in transcription between prokaryotes and eukaryotes

• In prokaryotes – the RNA transcript of a protein-coding gene is immediately useable as mRNA
• However, in eukaryotic cells it must first
undergo processing


RNA processing

1) a cap (modified guanosine triphosphate) is added to the 5' end
2) Introns are excised and the exons spliced together
3) 50 to 250 adenine nucleotides are added to the 3' (called a Ploy (A) tail)



The complex combination of DNA, RNA and protein that makes up eukaryotic chromosomes



a lightly packed form of chromatin - that is rich in gene concentration,
and is often (but not always) under active transcription



a tightly packed form of DNA - little or no gene expression in heterochromatic regions of the genome


Role of exons in protein domains

Different exons encode
separate domains of
the protein product.
Each domain, an
independantly folding
part of the protein
performs a different
function in the cell.


What the pressence of exons and introns means:

The presence of exons
and introns means that
new proteins can
evolve by exon
shuffling among genes
and therefore the
potential to form new
proteins with novel


Control of gene expression

• The control of gene expression enables
individuals to adjust their metabolism to changes in their external and internal
environment and to carry out each of the functions of the cell only when needed.
• The enzymes that transcribe DNA must locate
the right genes at the right time – otherwise serious imbalances and diseases may occur



clustered genes (functionally related)


repressor protein

binds to operator and blocks


derepression of transcription

molecules bind to
repressor and lower DNA binding affinity


activation possible

e.g. CAP (catabolite-activator
protein) protein binds to promoter and stimulates
RNA polymerase binding


2 levels of metabolic control

a) Repress expression of the genes for all the enzymes needed for the pathway,
(b) inhibit the activity of the first enzyme in the pathway (feedback inhibition)


trp operon

• Involved in the production of the amino acid
tryptophan (5 genes)
• If E.coli is deprived of tryptophan it activates a
metabolic pathway to make its own from another
• If the environment has lots of tryptophan then the
bacterial cell stops producing it (saving energy)


lac operon

- Bacteria can use lactose (sugar in milk) as an energy source or as a source
of carbon to make other molecules
- The lac transcription unit is under the control of a single operator and promoter.
- A regulatory gene (encoded by lacI) is located outside the operon.


Examples of negative control of genes

• The lac operon is an inducible operon – it is
normally off, but when an inducer molecule is
present, the operon turns on
• The trp operon is a repressible operon – it is
normally on but when a repressor molecule is
present the operon turns off


Methods of gene expression control in eukaryotes

1) Transcriptional regulation in
eukaryotes – the nuclear envelopeoffers post transcriptional control


Why eukaryotes respond to environmental changes slowly

Eukaryotic DNA sequestered in nucleus
– Environmental signals must be transmitted from cell surface to
– Elaborate intracellular signaling systems to control transcription
– Also intercellular signaling important in transcriptional
regulation of multicellular organisms


Transcription factors

The protein which mediates the inititation of transcription by RNA polymerase.


Example of Eukaryotic promoters

usually include a TATA box (T and A repeats – usually about 25 bp long)


Transcription initiation complex

Additional transcription factors which join the polymerase on the DNA


What types of regulation use special transcription factors and where do they bind on the DNA?

STFs bind to DNA sequences called Enchancers and often interact with basal transcription factors and RNA polymerase II


Histone Acetylation:

attachment of acetyl groups to certain amino acids of histone proteins. When histones are acetylated they grip the DNA less tightly, therefore easier access of genes in that region


DNA Methylation

Attachment of methyl groups to DNA bases after the DNA is synthesized -essential for long-term inactivation of genes that occurs during cellular differentiation


Alternative RNA splicing:

Different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which are treated as introns


Regulation of mRNA degradation

The life span of mRNA molecules in the cytoplasm is an important factor in determining the pattern of protein synthesis in a cell


Prokaryotic mRNA life span

degraded by enzymes after a few minutes –quick response to environmental changes


eukaryotic mRNA life span

hours, days or even weeks.
E.g. the mRNAs for the protein haemoglobin polypeptides are unusually stable and are translated repeatedly in developing red blood cells