Gene Structure: Family of genes:
A set of genes duplicated, slightly modified and inherited, over a time course of evolution, are called gene family or a family of genes. A gene may exist in multiple copies with certain subtle variations, some may be active some may be inactive as pseudo genes or cryptic genes. A gene family is a group of genes that share important characteristics. In many cases, genes in a family share a similar sequence of DNA building blocks (nucleotides). These genes provide instructions for making products (such as proteins) that have a similar structure or function. In other cases, dissimilar genes are grouped together in a family because proteins produced from these genes work together as a unit or participate in the same process.
Genes belonging to multiple alleles fall into a family of genes. They may be clustered in a region or dispersed in the genome. The members of this gene family may express at different stages of development and some may remain silent. Often genes that are involved in a particular structure or function can be deemed as a family of genes, ex. DNA pol and RNA pol involved in DNA replication and transcription. There are Gene families-allelic family, functional families and Protein family of genes.
Basic structural features of Eukaryotic Genes
https://www.slideshare.net
During the course of evolution, spanning several billions of years, the genes might have gone through duplication and certain degree of changes in their sequences, but not drastically. Some of those, which have lost functions, are called pseudo genes. Some of the related genes form a kind of super family.
Human immunoglobulin genes form a super family of genes; they are IgG, IgE, IgA, IgD, and IgI. The MHC class of genes also belongs to a family of genes.
These diagrams represent just basic components of eukaryotic promoters, such as specific gene promoters and housekeeping gene promoters, transcribed by RNA pol II
http://slideplayer.com
Globin Genes:
Globin genes exist as clusters of alpha Globin and beta Globin family of genes. They are expressed at different stages of the development of the organism.
Alpha Globin:
They are organized in a cluster of 30kbp; they are organized in the following sequence.
----chi-I-psi-alpha--chi—psi-alpha-1---alpha-2—alpha-1-alpha-1—
Alpha clusters:
Gene |
Type |
Expression |
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Chi |
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In embryo |
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Psi-alpha- |
Pseudo gene |
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chi |
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Psi-alpha-1 |
Pseudo gene |
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Alpha-1 |
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Fetus and adult |
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Alpha-2 |
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Fetus and adult |
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Alpha-1 |
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Fetus and adult |
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Beta clusters:
Beta clusters: organized in the following sequence. They are in a region of 50kbp locus, between the genes there are spaces, which are not transcribed, it holds good for both clusters. The table shows the expression of genes with time.
Gene |
Type |
Expressed in |
Epsilon |
- |
Embryonic stage |
G-gamma |
- |
Fetus stage |
A-gamma |
- |
Fetus stage |
Psi-beta0-1 |
Pseudo gene |
|
Delta |
- |
In adults |
Beta |
- |
In adults |
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|
This line diagram depicts the organization alpha and beta globin gene families
Early Embryonic means: less than eight weeks.
Embryonic: means less than eight weeks of pregnancy, expressed genes are - Chi2, chi-2, gamma-2, alpha-2, and epsilon-2
Fetal means: 3-9 months, but expressed are alpha-2, gamma-2.
Adult means: from birth, but the expressed genes are alpha-2 delta-2, alpha-2, and beta2.
Chi and alpha are like alpha.
Epsilon, gamma, delta and beta are like beta.
Histone gene family:
Histone genes exist not as single genes but as a family of genes and they are clustered together in certain loci of Human chromosomes.
Clustering of Heat shock (Hs) and Globin genes on chromosome 11; http://www.web-books.com/
Sea urchin:
The repeat length is 6300 bp and the number of repeats is 300 in sea urchins and 600 in newts.
http://genome.cbs.dtu.dk/
Drosophila:
4800bp long and there are 100 repeats.
This diagram shows how histone genes as clusters organized as loci in chromosomes. http://genome.cbs.dtu.dk/
Humans: Human genome contains a family of Histone genes.
I--H1-->I--I--H3-->I---I<--H2b---I--I-->H2A-->I---I--H4->->
Yeasts: They contain two copies each of histone genes. Birds contain 10 to 20 repeats of each cluster. In mammals the number of repeats is 600 to 800.
Classification of human genome into structural and functional forms
rRNA gene family; http://www.rochester.edu/
Some of the gene families |
Number of genes
|
Actin |
5-30 |
|
3-10 |
Keratin |
>20 |
Myosin (heavy chain) |
5-10 |
Protein kinase? |
10-100 |
Human Ig variable |
>500 |
Chick Ovalbumin |
3 |
Tubulin alpha and beta |
3-15 |
Visual pigment gene (human) |
4 |
Chick vitellogenin |
5 |
Insect egg shell protein |
- |
Silk fibroin (silk moth and fruit fly) |
50 |
Transplantation human antigen |
50-100 |
Globin genes |
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Skin color, |
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Hairs color |
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Height, |
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MHC class |
MHC I, II and III -200 genes |
Ig Family |
The Ig like domains can be classified as IgV, IgC1, IgC2, or IgI |
Homeobox, |
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ABCA |
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DNA Pol |
Prokaryotic, Archaea, eukaryotic |
RNA PoL |
Prokaryotic, Eukaryotic-RNAP I, II and III |
HSPs |
HSP 60, 70 and 90, HSP 90 has 17 or are more in human genome |
Ribosome Protein family |
60-70 |
tRNA family |
22-30 |
rRNA family |
PK and EK |
snRNA family |
5-8 |
Sigma factor family |
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T and B cell receptors |
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T cell receptors |
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Cytokine /Lymphokines |
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There are many protein families too |
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Signal receptor s |
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Motor |
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Membrane transporters
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Protein kinases and other kinases |
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Phosphatase
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Structural proteins |
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Metallothionein |
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Histone |
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Distribution of Some Gene Families on the Chromosomes |
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Gene family |
Gene count |
Chromosomes |
Additional information |
3 |
2, 14, 19 |
identical protein sequences, many other related proteins |
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3 |
1, 12, 17 |
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4 |
1, 11, 14, 19 |
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4 |
1, 6, 9, 19 |
also smaller related protein on chromosome 1 |
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5 |
1 |
cluster spans about 205 kb, also pseudogene |
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5 |
1, 7, 9, 12, 15 |
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6 |
1, 2, 7, 10, 15, 17 |
also highly similar ACTBL2 and many other related proteins |
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6 |
2, 4, 10 (3), 17 |
three genes on chromosome 10 not closely linked |
|
7 |
4 |
cluster spans about 365 kb |
|
11 |
16 |
cluster spans about 120 kb, also related genes / pseudogenes |
A gene family is a group of genes that share important characteristics. In many cases, genes in a family share a similar sequence of DNA building blocks (nucleotides). These genes provide instructions for making products (such as proteins) that have a similar structure or function.
In other cases, dissimilar genes are grouped together in a family because proteins produced from these genes work together as a unit or participate in the same process. Some of the genes found in duplicates and located on chromosomes, not on the same but can be on different chromosomes.
The following families, defined by the HUGO Gene Nomenclature Committee
are included in Genetics Home Reference.