This document discusses several gene families and related concepts. It describes how a gene family is a set of similar genes formed by duplication of a single original gene, often with similar functions. It provides examples like the human hemoglobin gene family. It also discusses histone gene families, which package DNA into nucleosomes and are present in tandem repeats due to high demand during DNA replication.

1. SUBMITTED TO :- SUBMITTED BY :-
Mr. P.C.GUPTA NAVREET KAUR RAI
(M.Sc. (Ag.) Pre. Yr. GPB Sem-I)
Deptt. Of Genetics And Plant Breeding
COLLEGE OF AGRICULTURE SWAMI KESHWANAND
RAJASTHAN AGRICULTURE UNIVERSITY

2.  A gene family is a set of several similar
genes, formed by duplication of a
single original gene, and generally with
similar biochemical functions.
 One such family are the genes for
human haemoglobin subunits; the ten
genes are in two clusters on different
chromosomes, called the alpha-globin
and beta-globin loci.

3.  A gene family is a set of homologous genes
within one organism.
 Genes are categorised into families based on
shared nucleotide or protein sequences.
 The position of exons within the coding
sequence can be used to infer common
ancestry.
 When a gene is present in two or more copies
per genome, the condition is known as
“redundancy”.

4.  The members of a gene family may be either
clustered together, dispersed on different
chromosomes or present in a combination
of both.
 If the gene of a gene family encode proteins,
the term “protein family” is often used in an
analogous manner to gene family.
 One example for such family are the genes
for Human haemoglobin subunits.

5.  Gene families are group of related genes
that share a common ancestor.
 Members of gene families may be paralogs
or orthologs.
 Gene paralogs- genes with similar sequence
from within the same species.
 Gene orthologs- genes with similar
sequence in different species.
 Gene families are highly variable in size,
sequence diversity and arrangement.

6.  Depending on the diversity and functions of the genes
within the family, families can be classified as a
multigene families or superfamilies.
MULTIGENE FAMILIES-
 Typically consist of members with similar sequences
and functions, though a high degree of divergence (
at the seq. &/or functional level ) doesnot lead to
the removal of a gene from a gene family.
 Individual genes in the family may be arranged
close together on the same chromosome or
dispersed throughout the genome on different
chromosomes.

7. SUPERFAMILIES –
 These families are much larger than single
multigene families.
 Superfamilies contain upto hundreds of
genes, including multiple families as well as
single, individual gene members.
 The genes are diverse in sequence and
function displaying various levels of
expression and seperate regulation controls.
 Some gene families also contain
pseudogenes, sequences of DNA that closely
resemble established gene sequences but
are non-functional.

8.  A gene cluster is a part of a gene family.
 A gene cluster is a group of two or more genes
found within an organism’s DNA that encode
for similar polypeptides or proteins,which
collectively share a generalised function and
are located within a few thousand base pairs of
each other.
 The size of gene clusters can vary significantly,
from a few genes to several hundred genes.

9.  Genes found in a gene cluster may be
observed near one another on the same
chromosome or on different, but homologous
chromosomes.
 Extensive tanden repetition of a gene
normally occurs when the gene product is
needed in unusually large amounts.
Eg., genes for rRNA, histone genes, etc.
 Sometimes all the members of a gene family
are functional, but often some members are
nonfunctional pseudogenes.

10.  In a tandem repeat, the nucleotide sequence
is repeated in the same orientation.
 For example, the trinucleotide sequence
GAA is repeated two times in the DNA
segment- GAAGAA-
GAA GAA
CTT CTT
tandem repeat

11.  Genes encoding the various globin
proteins evolved from one common
ancestral globin gene,which duplicated
and diverged about 450-500 million
years ago.
 After the duplication events,
differences between the genes in globin
family arose from the accumulation of
mutations.

14.  The haemoglobin molecule is a tetramer and is
composed of two similar polypeptides, the alpha
and beta chains,encoded by two distinct genes.
 Each polypeptide incorporates a hemi-group,
that reversibly binds oxygen.
 The genes are co-ordinatedly turned on and
turned off during the embryonic, foetal and
adult stages of development.
 The genes for alpha-globin lie in a cluster on
chromosome 16, while those for beta-globin are
located on chromosome 11.

20.  Myoglobin is an iron- and oxygen- binding
protein found in the muscle tissue of
vertebrates in general and in almost all
mammals.
 It is releated to haemoglobin, which is the iron-
and oxygen- binding protein in blood,
specifically in the red blood cells.
 In humans, myoglobin is only found in the
bloodstream after muscle injury.

21.  Myoglobin is the primary oxygen- carrying
pigment of muscle tissues.
 High concentrations of myoglobin in muscle
cells allow organisms to hold their breath
for a longer period of time.
 Diving mammals such as whales and seals
have muscles with particularly high
abundance of myoglobin
 In humans,myoglobin is encoded by MB
gene.

22.  Histones are highly alkaline proteins found in
eukaryotic cell nuclei that package and order
the DNA into structural units called
“nucleosomes”.
 They are the cheif protein components of
chromatin,acting as spools around which DNA
winds, and play a role in gene regulation.
 Without histones, the unwounded DNA in
chromosomes would be very long.

23.  Histones are found in the nuclei of the
eukaryotic cells and in certain Archaea, but
not in bacteria.
 The unicellular algae known as
dinoflagellates are the only eukaryotes that
are known to completely lack histones.
 Histone protein are among the highly
conserved proteins in eukaryotes.
 Histone genes are of 5 types namely, H1,
H2A, H2B, H3 and H4.

24.  Large quantities of histones are required during
the S phase of cell cycle to package the newly
synthesised DNA.
 To meet this demand multiple copies of each of
the five histone genes are present per genome
in clusters of tandemly repeated units.
 A repeat unit contains one copy each of five
histone genes seperated from each other by non
transcribed spacer regions.
 The histone genes don’t have intron sequences.
All the exon sequences are highly conserved.

25. 1. HISTONE H1:-
 H1 is one of the five main histone
protein families which are components of
chromatin in eukaryotic cells.
 Unlike other histones,H1 doesn’t make
up the nucleosome bead. It sits on the
top of the structure, keeping in place the
DNA that has wrapped around the
nucleosome.

26.  Histone H2A is one of the five main
histone proteins involved in the
structure of the chromatin in eukaryotic
cells.
 H2A is important for packaging DNA into
chromatin.
 H2A plays a major role in determining
the overall structure of the chromatin.

27.  It is also a main component of histone
protein involved in structure of chromatin in
eukaryotic cells.
 H2B is involved with the structure of the
nucleosomes of the ‘beads on a string’
structure.

28.  Histone H3 is one of the five main histone
proteins involved in the structure of
chromatin in eukaryotic cells.
 H3 is involved with the structure of the
nucleosomes of the ‘beads on a string’
structure.

29.  Histone H4 is one of the five main histone
proteins involved in the structure of
chromatin in eukaryotic cells.
 H4 is structural component of the
nucleosome, and is subjected to some
modifications including acetylation and
methylation, which may alter expression of
genes located on DNA associated with its
parent histone octamer.