● A `color{Violet}"gene"` is defined as the functional `color{Violet}"unit of inheritance"`.

● Though there is `color{Violet}"no ambiguity"` that the genes are located on the DNA, it is difficult to literally define a gene in terms of DNA sequence.

● The DNA sequence coding for `color{Violet}"tRNA or rRNA"` molecule also define a gene.

● However by defining a `color{Violet}"cistron"` as a segment of `color{Violet}"DNA coding for a polypeptide"`, the structural gene in a transcription unit could be said as:

`star` `color{Violet}"Monocistronic"` (mostly in `color{Violet}"Eukaryotes"`)
`star` `color{Violet}"Polycistronic"` (mostly in bacteria or `color{Violet}"Prokaryotes"`).



● In `color{Violet}"eukaryotes"`, the monocistronic structural genes have `color{Violet}"interrupted coding sequences"` – the genes in eukaryotes are `color{Violet}"split"`.

● The coding sequences or `color{Violet}"expressed sequences"` are defined as `color{Violet}"exons"`.

● `color{Violet}"Exons"` are said to be those sequence that appear in `color{Violet}"mature or processed RNA"`.

● The exons are `color{Violet}"interrupted"` by `color{Violet}"introns"`.

● Introns or `color{Violet}"intervening sequences"` do not appear in mature or processed RNA.

● The `color{Violet}"split-gene arrangement"` further complicates the definition of a gene in terms of a DNA segment.

● Inheritance of a character is also affected by `color{Violet}"promoter"` and `color{Violet}"regulatory sequences"` of a structural gene.

● Hence, sometime the `color{Violet}"regulatory sequences"` are loosely defined as `color{Violet}"regulatory genes"`, even though these sequences `color{Violet}"do not code"` for any RNA or protein.

● Types of RNA In bacteria, there are three major types of RNAs:
`star` `color{Violet}"mRNA"` (messenger RNA)

`star` `color{Violet}"tRNA"` (transfer RNA)

`star` `color{Violet}"rRNA"` (ribosomal RNA)

● All `color{Violet}"three RNAs"` are needed to `color{Violet}"synthesise a protein"` in a cell.

● The `color{brown}"mRNA"` provides the `color{Violet}"template"`, `color{brown}"tRNA"` brings `color{Violet}"aminoacids"` and reads the `color{Violet}"genetic code"`, and `color{brown}"rRNAs"` play `color{Violet}"structural and catalytic role"` during translation.

● There is `color{violet}"single"` `color{violet}"DNA-dependent RNA polymerase"` that catalyses transcription of `color{violet}"all types of RNA"` in bacteria.

● `color{brown}"Initiation"`: RNA polymerase binds to `color{violet}"promoter"` and initiates transcription.
It uses `color{violet}"nucleoside triphosphates"` as substrate and polymerises in a `color{violet}"template depended fashion"`.

● It somehow also facilitates `color{violet}"opening of the helix"` and `color{violet}"continues elongation"`.

● Only a `color{violet}"short stretch of RNA"` remains bound to the enzyme.

● Once the polymerases reaches the `color{violet}"terminator region"`, the `color{violet}"nascent RNA falls"` off, so also does the RNA polymerase.

● This results in `color{violet}"termination of transcription"`.

● The three steps are `color{violet}"initiation"`, `color{violet}"elongation"` and `color{violet}"termination"`.

● The `color{violet}"RNA polymerase"` is only capable of catalysing the `color{violet}"process of elongation"`.

● It associates transiently with `color{violet}"initiation-factor (σ)"` and `color{violet}"termination-factor (ρ)"` to initiate and terminate the transcription, respectively.

● Association with these factors `color{violet}"alter the specificity"` of the RNA polymerase to either initiate or terminate.

● In bacteria, the mRNA `color{violet}"does not require"` any `color{violet}"processing"` to become active, and also transcription and translation take place in the `color{violet}"same compartment"` (there is no separation of cytosol and nucleus in bacteria).

● Due to this many times the `color{violet}"translation"` can `color{violet}"begin much before"` the mRNA is fully transcribed.

● Consequently, the `color{violet}"transcription and translation"` can be `color{violet}"coupled"` in bacteria.