Download The human genome: gene structure and function

Gene structure and function
Nabil bashir
Fall, 2015
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Introduction:
• Is it a simple one-to-one correspondence between genes and proteins,?
• Is the 25,000 genes sufficient to account for the vast array of functions
that occur in human cells.
• Are the 2 copies of a gene on an autosomal chromosome expressed and
generate product?
• Are different chromosomes have the same amount of genes?
• Are genes the same size on different chromosomes?
• Do genes have the same number of exones and intrones?
• Are all human genes authentic?
• Are there genes that do not produce proteins?
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 Alternative coding segments : the 25,000 human genes can encode as many
as a million different proteins
 Combinatorial nature of gene networks : They form elaborate networks of
functions, involving many different proteins and responding in a
coordinated fashion to many different genetic, developmental, or
environmental signals.
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• For genes located on the autosomes, there are two copies of each gene,
one on the chromosome inherited from the mother and one on the
chromosome inherited from the father.
• For most autosomal genes, both copies are expressed and generate a
product.
• There are, however, a small number of genes in the genome that are
exceptions to this general rule and are expressed only from one of the two
copies. Examples of this unusual form of genetic regulation, called
genomic imprinting,
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.The Central Dogma: DNA → RNA → Protein
• The informational relationships among DNA, RNA, and protein are
depndent
• structure of genes in the genome is a foundation for discussion of the
genetic code, transcription, and translation.
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Gene structure
promoter
region
exons (filled and unfilled boxed regions)
+1
introns (between exons)
transcribed region
mRNA structure
5’
3’
translated region
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The (exon-intron-exon)n structure of various genes
• Some (very few) genes do not have introns. One example is
the histone genes, which encode the small DNA-binding
proteins, histones H1, H2A, H2B, H3, and H4.
• Shown here is a histone gene that is only 400 base pairs (bp)
in length and is composed of only one exon.
• The beta-globin gene has three exons and two introns.
• The hypoxanthine-guanine phosphoribosyl transferase
(HGPRT or HPRT) gene has nine exons and is over 100times larger than the histone gene, yet has an mRNA that is
only about 3-times larger than the histone mRNA (total exon
length is 1,263 bp). This is due to the fact that introns can be
very long, while exons are usually relatively short. An
extreme example of this is the factor VIII gene which has
numerous exons (the blue boxes and blue vertical lines).
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The (exon-intron-exon)n structure of various genes
histone
total = 400 bp; exon = 400 bp
b-globin
total = 1,660 bp; exons = 990 bp
HGPRT
(HPRT)
total = 42,830 bp; exons = 1263 bp
factor VIII
total = ~186,000 bp; exons = ~9,000 bp
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• Structural Features of a Typical Human Gene
 a gene as a sequence of DNA in the genome that is required for
production of a functional product, be it a polypeptide or a functional
RNA molecule. A gene includes not only the actual coding sequences
but also adjacent nucleotide sequences required for the proper
expression of the gene—that is, for the production of a normal mRNA
molecule, in the correct amount, in the correct place, and at the correct
time during development or during the cell cycle.
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Pseudogenes:
• DNA sequences that closely resemble known genes but are
nonfunctional
• Pseudogenes are widespread in the genome and are of two
general types, processed and nonprocessed.
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Nonprocessed pseudogenes
• are thought to be byproducts of evolution, representing
“dead” genes that were once functional, having been
inactivated by mutations in coding or regulatory sequences.
• In some cases, as in the pseudo–α-globin and pseudo–βglobin genes, the pseudogenes presumably arose through
gene duplication, followed by the accumulation of
numerous mutations in the extra copies of the oncefunctional gene
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processed pseudogenes
• are pseudogenes that have been formed, not by mutation,
but by a process called retrotransposition, and reverse
transcription , and finally integration of such DNA copies
back into the genome.
• Because such pseudogenes are created by
retrotransposition of a DNA copy of processed mRNA,
they lack introns and are not necessarily or usually on the
same chromosome (or chromosomal region) as their
progenitor gene.
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Noncoding RNA Genes
• Not all genes in the human genome encode proteins.
• Chromosome 11, for example, in addition to its 1300
protein-coding genes, has an estimated 200 noncoding
RNA genes, whose final product is an RNA, not a protein.
• Although the functions of these genes are incompletely
understood, some are involved in the regulation of other
genes, whereas others may play structural roles in various
nuclear or cytoplasmic processes.
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• microRNA (miRNA) genes : An important class of
noncoding RNA genes , of which there are at least 250 in
the human genome; miRNAs are short 22-nucleotide-long
noncoding RNAs, at least some of which control the
expression or repression of other genes during
development.
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