Genome Organisation
In Prokaryotes and Eukaryotes
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Contents
Molecular structure of prokaryotes and eukaryotes
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Gene organization / gene structure
• Gene structure in prokaryotes
• Gene structure in eukaryotes
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Genome: complexity & organization
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Prokaryotic vs Eukaryotic cell
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Prokaryotic and Eukaryotic Chromosomal differences
● The genome of E.coli contains amount of
● 4x10⁶ base pairs.
● > 90% of DNA encode protein.
● Lacks a membrane-bound nucleus.
Circular DNA and supercoiled domain.
● Histones are not present.
Many bacteria have small circular DNA structures called plasmids which can be swapped between neighbors and across bacterial species.
Prokaryotes
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● The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in 1952.
● A plasmid is separated from, and can replicate independently of the
chromosomal DNA.
● Plasmid size varies from 1 to over 1,000 (kbp).
Plasmid
Prokaryotic and Eukaryotic Chromosomal differences
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● A small fraction of the total DNA encodes protein.
Many repeats of non-coding sequences.
● All chromosomes are contained in a membrane bound nucleus DNA is divided between two or more chromosomes.
● A set of five histones
DNA packaging and gene expression regulation.
Eukaryotes
Prokaryotic and Eukaryotic Chromosomal differences
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Chromosome organization
Double Helix DNA
Nucleosomes forming the
“beads-on-string” structure
Metaphase chromosome Chromatin
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Chromosome structure
Chromosome structure
Metacentric Submetacentric Acrocentric Telocentric
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The study of chromosomes, their structure and their inheritance, is known as cytogenetics.•
Each species has a characteristic number of chromosomes and this is known as karyotype.Karyotype
Organism No. of chromsomes
Fruit fly 8
Garden pea 14
Yeast 16
Frog 26
Cat 38
Organism No. of chromsomes
Mouse 40
Rat 42
Rabbit 44
Human 46
Chicken 78
National Genome Research Institute
Contents
Gene organization / gene structure
• Gene structure in prokaryotes
• Gene structure in eukaryotes
2
Molecular structure of prokaryotes and eukaryotes
1
Genome: complexity & organization
3
Gene Organization
Gene 🡪 basic unit of genetic information.
Exceptions are genes for RNA molecules (such as rRNA and tRNA), which are not translated.
Genetic information transcribed into a single RNA molecule, which is in turn translated into a single protein; located on
chromosomes at a particular genetic locus.
Gene Organization
In diploid organisms:
chromosomes arranged as
homologous pairs, different forms of the same genes is called allele.
Allele
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● Genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins.
● Proteins 🡪 responsible for biological functions.
● A gene is the sequence of
nucleotides within a portion of DNA that codes for a peptide or a
functional RNA.
● Sum of all genes = genome.
Gene Organization
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Gene structure in prokaryotes
OPERON 🡪 cluster of genes that are related and that are under the control
of single promoter/regulatory region. Structural genes: encode proteins lacZ encode B-galactosidase lacY for galactosidase permease lacA for transacetylase
Control region: controls the cluster using promoter (P) and operator (O) region.
Repressor gene: lies outside the operon itself and is controlled by promoter Pi .
In prokaryotes, structural genes are often grouped together 🡪 transcribed mRNA may contain information for more than one protein, known as polycistronic mRNA.
E.g.: lac operon (encode enzymes responsible for lactose catabolism).
https://thebiologynotes.com/gene-a-comprehensive-guide/
Gene structure in eukaryotes
Gene structure and function in eukaryotes are more complex than in prokaryotes. They consist of:
Intervening sequences, does not encode proteins.
In many cases number and total length of introns exceed that of the exons.
Introns
Encodes Protein Exons
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Introns must be removed before mRNA can be translated:
Fully functional mRNA that is ready for export to the
cyptolasm for translation.
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Introns are spliced out of the primary transcript.
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addition of ‘cap’ at the 5’ terminus and
‘tail’ of adenine residues at the 3’
terminus (Poly(A)).
RNA Processing
End Product
Gene structure in eukaryotes
Prokaryotes and Eukaryotes genome
Prokaryotes Eukaryotes
Single cell Single or multi cell
No nucleus Nucleus
One piece of circular DNA Chromosomes
No mRNA post transcriptional
modification Exons/Introns splicing
Contents
Genome: complexity & organization
3
Molecular structure of prokaryotes and eukaryotes
1
Gene organization / gene structure
• Gene structure in prokaryotes
• Gene structure in eukaryotes
2
Genome
Is the total complement of DNA in the cell.
Include coding & noncoding region.
Genome size tend to increase with:
● Organismal complexity
● More complex organization
Genome: Organization
While in viral and bacterial genomes are very efficient in using DNA for encoding their genes.
This raises question about how genomes are organized.
In human genome, only about 3% of the total amount of
DNA is involved in coding for proteins. Even when the introns
and control sequence are added, the majority of DNA has
no obvious function 🡪 termed as junk DNA.
● Genome sequencing has greatly improved our understanding of how genomes work.
● Eukaryotes: many genes are single copy 🡪 tend to be dispersed across multiple chromosomes in nuclei.
● Other: Multigene families 🡪 grouped at a particular chromosomal location, or may be dispersed.
● Indicator features important in studying gene organization: gene density, gene size, mRNA size, intergenic distance, and intron/exon sizes.
● E.g. Early analysis of human DNA 🡪 average size of coding region is 1500 bp, average size of gene is 10-15 kbp, gene density is about one gene per 40-45 kbp.
Genome: Organization
DNA Extrachromosomal
● DNA molecule that is not part of the host cell chromosome.
● Also referred to extranuclear DNA or cytoplasmic DNA
Plant and animal cell mtDNA or mitochondrial DNA and chloroplasts DNA (plant cells only), which have their own separate genomes that specify many of the components required by these organelles.
Mitochondrial DNA
• Mitochondrial DNA is a double stranded circular molecule.
• There are several copies in each mitochondrion and there are many mitochondria in each of the cells.
• Mitochondrial DNA is similar to prokaryotic DNA. There are no histones or any other protein associated with mtDNA.
• The genes contain no introns.
Maternal inheritance.
• Because it is in a highly
oxidizing environment it has a much higher rate of mutations than nuclear DNA.
• The genes in mtDNA code for mitochondrial ribosomes and transfer RNAs.
• Some genes code for polypeptide subunits of the electron transport chain common to all
mitochondria.
Mitochondrial DNA location
National Human Genome Research Institute
References
● Schleif R. Genetics and Molecular Biology, Second Edition. The Johns Hopkins University Press.
● Weaver, R.F. 2001. Molecular Biology. McGraw-Hill.
