Genome size and complexity of eukaryotic genome.pptx

Genome size and complexity of the eukaryotic genome

1. Genomic size and genetic content, complexity of eukaryotic genome 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 , Duplication Insertion Deletion Polyploidization 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 .

12. 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

13. 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.

14. 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.

15. 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

16. 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.