Genome size and complexity of the eukaryotic genome

1. Presented by,
ARYA KRISHNAN
IInd MSc Zoology

2. GENE
Gene is a unit of heredity
which is transferred from a
parent to offspring and held to
determine some characteristics
to offspring.
 A gene is a section of DNA
made up of a sequence of As,
Cs, Ts and Gs.
It holds the information for the
production of a particular
protein.

3. GENOME
Genome is the entire set of genetic information in an
organism.
It is encoded either in DNA or for many type of virus, in RNA.
The name genome was given by HANS WINKLER in 1920.
Genome is defined as the entire collection of genes and all
other functional and non-functional DNA sequence in a
haploid set of chromosomes.
It includes structural genes, regulatory genes and non-
functional nucleotide sequence.

4. STRUCTURAL GENES – DNA segments that code for
some specific RNAs of proteins. It encode for mRNAs,
tRNAS, SnRNAs, ScRNAs.
FUNCTIONAL SEQUENCES (Regulatory sequences )-
Regulatory sequences occur as regulatory elements
(initiation sites, promoter sites, operator sites etc.)
NON FUNCTIONAL SEQUENCES –It includes introns
and repetitive sequences .It is needed for coding
regulation and replication of DNA. It is much more in
number than functional sequences.

5. In Prokaryotic cells Genomic DNA forms a single circular
chromosome, without basic proteins, lies in the cell
cytoplasm in nucleoid region.
In Eukaryotic cells DNA is associated with basic
proteins(histones), form long chromatin fibers. Chromatin
fibers form a network, enclosed in a double layered
nuclear envelop, condenses into chromosomes during cell
division.

6. GENOME SIZE AND
GENETIC CONTENT
Genome size is the total amount of DNA contained within one
copy of a single complete genome.
 It is typically measured in terms of mass in picograms
(trillionths (10-12) of a gram, abbreviated pg) or less
frequently in daltons, or as the total number of nucleotide
base pairs, usually in megabases (millions of base pairs,
abbreviated Mb or Mbp).
One picogram is equal to 978 megabases.
In diploid organisms, genome size is often used
interchangeably with the term C-value.

7. C-VALUE
C-value or genomic value (Swift-1950) is the total amount
of DNA per genome or haploid set of chromosomes in an
organism.
It is characteristic of each species.
C value is usually expressed in picograms per cell
(pg/cell).
The DNA content of a diploid cell is referred to as 2C
value.Haploid nuclei of sperms, ova and the spores of
higher plants will contain IC amount of DNA.

8. The C-value of a species is usually constant and, in
general, it increases with increase in the genetic
complexity of species.
The C-value of eukaryotes is higher than that of
prokaryotes by a few times (as in yeasts) or several million
times (as in some salamanders).
 The increased C-value of eukaryotes is believed to
indicate their high genetic complexity

9. C-VALUE PARADOX
 C-value paradox is the paradox that though C-value is an index of
genetic complexity, there is no direct correlation between the
comparative C-values of different species and their relative
organizational complexities and evolutionary status.
 The term C –Value paradox was used to describe this situation by
C.A.THOMAS.Jr in 1971.
 There is no obvious and direct correlation between different species
of organisms with regard to their relative C values and organic
complexities.
 C –Value paradox means that a larger genome doesn’t always lead to
more complexity.

10. For example ,Human beings have only about 3.3 billion
base-pairs in the haploid genome in place of more than
200 billion base-pairs of Amoeba, and over the 300 billion
base-pairs of an average bony fish.
It is also paradoxical that very closely related species have
considerably different C- Values.
The basic reason for the C-value paradox is that in species
with very high C-values , a large portion of the genomic
DNA contains repetitive sequences, introns and non
coding sequences.

11. Genome size can increase by ,
1. Duplication
2. Insertion
3. Deletion
4. Polyploidization
5. Recombination etc.
 The Genome size and the number of genes it encodes varies
widely between organisms. The Smallest genomes occur in
viruses and viroids. Plans can have extremely large genomes.
Lung fishes have the largest genomes among animals ranging
from 40 pg in Protopterus annectens to 133 pg in P. aethiopicus.

13. COMPLEXITY
OF
EUKARYOTIC
GENOME
 Complexity is the number of
base pairs of unique DNA in
a given segment of DNA.
 The genomes of most
eukaryotes are larger and
more complex than those of
prokaryotes.
 The genome size of many
eukaryotes does not appear
to be related to genetic
complexity

14. The presence of large amounts of noncoding sequences is a
general property of the genomes of complex eukaryotes.
Much of the complexity of eukaryotic genomes thus results
from the abundance of several different types of noncoding
sequences, which constitute most of the DNA of higher
eukaryotic cells.
large amounts of noncoding DNA are also found within
most eukaryotic genes. Such genes have a split structure
in which segments of coding sequence (called exons) are
separated by noncoding sequences (intervening sequences,
or introns) some of the noncoding DNA in eukaryotes is
accounted for by long DNA sequences that lie between
genes (spacer sequences)The introns are then removed by
splicing to form the mature mRNA.

15. On average, introns are estimated to account for about 10
times more DNA than times in the genes of eukaryotes.
(Eg-human gene that encode the blood clotting protein
factor Vlll. This gene spans approximately 186 kb of DNA
and is divided into 26 exons. The mRNA is only about 9 kb
long, so The gene contains introns totaling more than 175
kb.)
Another factor contributing to the large size of eukaryotic
genomes is that some genes are repeated many times.
Many eukaryotic genes are present in multiple copies
called gene families while most prokaryotic genes are
represented only once in the genome.

16. Eukaryotic nuclear genome is linear. It
conforms to the Watson-Crick double-helix
structural model Furthermore, it is
embedded in nucleosomes-complex DNA-
protein structures that pack together to form
chromosomes. Beyond these basic, universal
features, eukaryotic genomes vary
dramatically in terms of size and gene
counts. Even so, genome size and the number
of genes present in an organism reveal little
about that organism’s complexity

18. The genomes of higher animals (such as humans) are more complex
and contain large amounts of noncoding DNA.
Thus, only a small fraction of the 3 x 10° base pairs of the human
genome is expected to correspond to protein-coding sequence.
Approximately one-third of the genome corresponds to highly
repetitive sequences. Remaining estimated 2 x 10” base pairs for
functional genes, pseudogenes, and non repetitive spacer sequences.
It is expected that the human genome to consist of about 100,000
genes, with protein-coding sequences corresponding to only about 3%
of human DNA.