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

 

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 promoters and housekeeping promoters, transcribed by RNA pol II

 

 

 

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

Chi

 

In embryo

Psi-alpha-

Pseudo gene

 

chi

 

 

 

Psi-alpha-1

Pseudo gene

 

Alpha-1

 

Fetus and adult

Alpha-2

 

Fetus and adult

Alpha-1

 

Fetus and adult

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

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.

 

Description: Figure 7.1. Organization of the human genome.

Classification of human genome into structural and functional forms

 

This diagram shows the organization eukaryotic genes for ribosomal proteins

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

 

 

 

Skin color,

 

Hairs color

 

Height,

 

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,

 

ABCA

 

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

 

T and B cell receptors

 

T cell receptors

 

Cytokine /Lymphokines

 

There are many

protein families

too

 

Signal receptor s

 

Motor

 

Membrane transporters

 

 

Protein kinases and

other kinases

 

Phosphatase

 

 

Structural proteins

 

Metallothionein

 

Histone

 

 

 

 

 

 

 

 

 

 

Distribution of Some Gene Families on the Chromosomes

Gene family

Gene count

Chromosomes

Additional information

Calmodulin

3

2, 14, 19

identical protein sequences, many other related proteins

Enolase

3

1, 12, 17

Actinins

4

1, 11, 14, 19

Notch

4

1, 6, 9, 19

also smaller related protein on chromosome 1

Amylase

5

1

cluster spans about 205 kb, also pseudogene

G β subunits

5

1, 7, 9, 12, 15

Actin

6

1, 2, 7, 10, 15, 17

also highly similar ACTBL2 and many other related proteins

Polycomb PCGF

6

2, 4, 10 (3), 17

three genes on chromosome 10 not closely linked

Alcohol dehydrogenase

7

4

cluster spans about 365 kb

Metallothionein

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 CommitteeThis link leads to a site outside Genetics Home Reference. are included in Genetics Home Reference.