2/13/16
BIOLOGY 207 - Dr. Locke
Lecture#8 - -globin gene structure and expression.
Required readings and problems:
Reading: Open Genetics, Chapter 12
Problems: Chapter 12
Optional
Griffiths (2008) 9th Ed. Readings: pp 225-226; 321-324; 700-702
Problems: Same as previous lecture
Campbell (2008) 8th Ed. Readings: Concept 17.1-3, 18.2, 21.4-5
Concepts:
How are the beta-globin genes organized and expressed?
1. The forms of hemoglobin expressed changes during development.
2. The globin genes are similar in structure - a gene family
3. The genes encoding the globins are clustered.
4. Genes in the cluster are arranged in the order in which they are expressed.
5. Higher level chromatin structure affects gene expression in eukaryotes.
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
Hemoglobin proteins
1- Vertebrate hemoglobin is a protein involved in the transport of oxygen in the blood.
2- It is the major component of red blood cells
3- Hemoglobin protein is a hetero-tetramers, consisting of 4 polypeptides, each with a
heme group (Iron containing compound).
4- In the adult the 4 polypeptides are composed of 2 identical alpha and 2 identical
beta globin polypeptides. Fig
5- alpha and beta polypeptides are encoded by different genes
6- alpha-globin gene and a beta-globin-gene.
Globin genes are similar in structure
Each gene encodes a similar but different protein.
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
Hemoglobin genes - Common Basic structure:
_________________________________________________________
Positions of the introns/exons are very similar in alpha & beta globin genes -> family of
genes in most vertebrates.
Human alpha-globin 141 amino acids long; gene has 3 exons and 2 introns
Human beta-globin 146 amino acids long; gene has 3 exons and 2 introns
Comparison to other alpha and beta globin genes from other species shows the intron
positions are conserved.
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
During human development there are 3 distinct time periods that
differ in globin gene expression.
Fig
Early = Embryo (< 8 weeks old). Globins epsilon and zeta are most prominent.
Middle = Fetal (39 months).
Globins have
gamma (G+A)
and alpha are
most
prominent.
Late = Adult
(After birth)
Globins are
alpha and beta
are most
prominent
(97%), with
some delta and
gamma.
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
All Hemoglobins have
2 alpha-like and 2 beta-like polypeptides.
In humans there are two classes of globins:
alpha-like: alpha, zeta, 50%
beta-like: beta, epsilon, gamma, and delta 50%
Ratio of polypeptide synthesis is 1:1
1 alpha-like : 1 beta-like.
Question:
Why might there be different types of globin proteins expressed at different stages of
development?
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
Each type is located as a single gene cluster.
Division into alpha-like and beta-like gene types is reflected in the organization of
the genes.
Fig
beta-globin cluster:
epsilon, G-gamma, A-gamma, delta, beta (5 genes)
alpha-globin cluster:
zeta, alpha-2, alpha-1 (3 genes)
Other vertebrates - have similar clusters of alpha- and beta-like genes.
Pseudo-genes
- are DNA sequences that share substantial sequence similarity with a functional
(expressed) gene(s), but they, themselves, are not expressed at all.
- frequently lack the cis-acting regulatory elements (promoter and enhancer
sequences) that are required for expression,
- retain the protein coding portion sequences which permits gene (pseudo-gene)
identification.
- are present in both human clusters and in the clusters of other organisms.
- psi (trident shaped character) means “pseudo” genes.
Last Note: There can be more genes (members of a gene family) than might be
expected based on standard protein analysis.
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
beta-globin genes are clustered and work as a unit of gene
regulation
beta-globin locus
epsilon
gamma
G A
-->
--> -->
embryo YES
fetal
no
adult
no
delta
no
YES
no
Beta
-->
-->
no
no
little
no
no
YES
Beta-globin locus
Alpha-globin locus
Stem cells -->
Stem cells -->
embryo --> express epsilon
embryo --> express zeta
fetal --> express gamma
fetal --> express alpha
adult --> express beta (and delta) adult --> express alpha
The order of genes along the chromosome is the same order of expression
during development.
This generalization is not entirely true for all genes/organisms.
Conclusion :
Gene order/position/location can be important for regulation.
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
Higher level chromatin structure affects beta globin gene
expression - a major difference from prokaryotes
In eukaryotes gene expression is also influenced by chromatin
Adapted from: Driscoll MC, Dobkin CS, Alter BP. 1989. Gamma delta beta-thalassemia due to a de novo mutation deleting the 5' beta-globin gene activation-region
hypersensitive sites. Proc Natl Acad Sci U S A. 86:7470-4.
Locus control region (LCR) of the beta globin gene cluster
- Assay  nuclease hypersensitive sites
- sites develop in erythroid precursor cells before globin gene transcription
- 5' sites develop first, then the 3' sites
- mutations that prevent the 5' site formation lack subsequent 3' site formation  need
5’ sites for 3’ site appearance
Regional change in chromatin conformation then permits the globin genes to be
regulated by their own individual promoters, enhancers, etc
Biol207 Dr. Locke section
Lecture#8
Fall'11
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2/13/16
Summary of Globin gene regulation (eukaryote genes)
Expression is:
1- Tissue specific
- globin is expressed in red blood cells
- myoglobin (different globin-like gene) is expressed only in muscle cells
2- Developmental specific
- each globin gene is expressed at a limited time during development
- on/off at the correct times
3- Coordinately controlled
- alpha and beta genes are expressed to the same level so that there is a 1:1 ratio
of globin polypeptides.
Gene structure is related to expression:
1- Promoter – proximal sequences
2- Enhancer/silencer elements – distal (far away)
3- Locus control region (LCR) – large-scale chromatin changes for large regions of
DNA
Gene therapy implications……………
Biol207 Dr. Locke section
Lecture#8
Fall'11
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