Genome size refers to the total amount of DNA in an organism and can vary widely between species. Prokaryotic genomes typically consist of a single circular chromosome between 0.6-10 megabases in length, and sometimes plasmids up to 1.7 megabases. Gene regulation in prokaryotes occurs at the transcriptional level through operons, which contain multiple genes regulated by a single promoter. The lac operon in E. coli contains genes to break down lactose which are regulated by a repressor protein; in the presence of lactose or its isomer allolactose, the repressor detaches from the operator and allows transcription.

1. GENOME SIZE & ORGANIZATION,
& GENE REGULATION IN
PROKARYOTES
PRESENTED BY IQRA WAZIR

2. YOU WILL BE LEARNING
• Genome size
• Genome organization
• Gene regulation in prokaryotes

3. Genome Size
• Genome size is the total amount of DNA contained within one
copy of a single genome.
• Measured in terms of mass in picograms (trillionths (10−12) of a
gram, abbreviated pg)
• In Daltons or
• As the total number of nucleotide base pairs typically in
megabases (millions of base pairs, abbreviated Mb or Mbp).
• One picogram equals 978 megabases.

4. Genome Size
• In diploid organisms genome size is used interchangeably with the term
C-Value.
• An organism's complexity is not directly proportional to its genome size;
some single cell organisms have much more DNA than humans.
• Genome size can increase by
1. duplication,
2. insertion, or
3. polyploidization (hybridization that leads to polyploidy) and
4. the process of recombination can lead to both DNA loss and gain.
• It is also possible that genomes can shrink due to deletions.

5. Genomic Organization
• Genome organization refers to the sequential, not the structural
organization of the genome.
• Besides the coding exons, the non-coding DNA in Eukaryotes may fall
in the following classes:
Introns: non coding regions
Pseudogenes: non functional copies of gene
Retropseudogenes: like pseudogenes but lack introns
Transposons: jumping genes
Retrotransposons: encodes reverse transcriptase enzyme (RT)

6. Genomic Organization
• In humans only 1.5% of the entire genome length corresponds to coding
DNA.
• This 1.5% codes for about 27,000 genes which in turn code for proteins
that are responsible for all the cellular processes.
• Most of the well-characterized prokaryotic genomes consist of double-
stranded DNA organized as:
1. A single circular chromosome 0.6–10 Mb in length &
2. One or more circular plasmid species of 2 kb-1.7 Mb.

7. Gene Regulation In Prokaryotes
• No bound nucleus containing genetic material
• Genetic material dispersed in cytoplasm
• Transcription and translation takes place simultaneously in the cytoplasm
• Gene regulation takes place at the transcriptional level

8. Terminology To understand
• Operon:
An operon is made up of several structural genes arranged under a
common promoter and regulated by a common operator

9. Terminology To understand
• Promoter
 A promoter is a region of DNA that initiates transcription of a particular gene.
 Promoters are located near the transcription start sites of genes, on the same strand and
upstream on the DNA (towards the 5' region of the sense strand).

10. Terminology To understand
Operator
These are segments of DNA that regulate the activity of the structural genes of the operon.
If the operator is not bound by a repressor molecule, then the RNA polymerase can pass over the
operator and move to the protein coding genes
If the operator, on the other hand, is bound by a repressor molecule, then the RNA polymerase is
blocked behind the repressor molecule.
However, when an "Inducer molecule" is present, it binds the repressor molecule causing it to
change shape, rendering it incapable of binding the operator. The RNA polymerase moves freely.

11. Terminology To understand
• Regulator gene:
 A regulator gene, regulator, or regulatory gene is a gene involved in controlling the
expression of one or more other genes.
 Regulatory sequences, which encode regulatory genes, are often 5' to the start site of
transcription of the gene they regulate.

12. Terminology To understand
• Structural genes:
 A structural gene is a gene that codes for any RNA or protein product other
than a regulatory factor (i.e. regulatory protein).
 It may code for a structural protein, an enzyme, or an RNA molecule not
involved in regulation.

13. Terminology To understand
• Polycistronic mRNA
 a type of messenger RNA that can encode more than one polypeptide separately within the
same RNA molecule

14. Types of Operons
• Inducible operons:
They include genes that encode for enzymes that take part in metabolic
pathways and the expression of the gene is controlled by the substrate
Example: the "Lac Operon"
• Repressible operons:
They include genes that encode for enzymes involved in biosynthetic
pathways, and the expression of the gene is controlled by the end-product
of the pathway.
Example: the "Trp Operon"

15. Lac Operon In Ecoli
• Function - to produce enzymes which break down lactose (milk
sugar)
• lactose is not a common sugar, so there is not a great need for these
enzymes
• When lactose is present, they turn on and produce enzymes for its
metabolism.

16. Lac Operon In Ecoli
• Two components - repressor genes and functional genes
• Three functional genes:
1. lacZ produces B-galactosidase: This enzyme hydrolyzes the bond between the two
sugars, glucose and galactose
2. lacY produces permease: This enzyme spans the cell membrane and brings lactose into
the cell from the outside environment. The membrane is otherwise essentially impermeable
to lactose.
3. lacA produces B-galactosidase transacetylase: The function of this enzyme is still not
known.

17. Lac Operon In Ecoli
• Promoter (P) - aids in RNA polymerase binding
• Operator (O) - "on/off" switch - binding site for the repressor protein
• Repressor (lacI) gene
Repressor gene (lacI) - produces repressor protein with two binding sites, one for the operator
and one for lactose

18. Lac Operon In Ecoli
Repressor protein (produced by repressor gene)
• The repressor protein is under allosteric control - when not bound to lactose, the repressor
protein can bind to the operator
• When lactose is present, an isomer of lactose, allolactose, will also be present in small
amounts.
• Allolactose binds to the allosteric site, changes the conformation of the repressor protein so
that it is no longer capable of binding to the operator.

19. Operation of lac operon

20. YOU CAN ASK QUESTIONS!