Comprehensive Notes on Transcription and mRNA Processing

Occurs in the cytoplasm and ribosomes.
Starts with transcription in the nucleus, where DNA is converted into mRNA.
mRNA then moves to the cytoplasm, where it's translated into a sequence of amino acids, forming a protein.

Definition: synthesis of mRNA using one side of the DNA strand as a template.
mRNA is built by reading one side of the DNA strand.
Base Pairing: In mRNA, adenine (A) pairs with uracil (U) instead of thymine (T).
DNA serves as the blueprint for making mRNA during transcription.

Sense Strand: The coding strand that contains the genes with information to make a protein.
Antisense Strand: The non-coding strand used as a template for transcription.
To create an exact copy of the sense strand in mRNA, the antisense strand is transcribed.
Transcribing the antisense strand results in an mRNA sequence identical to the sense strand, except uracil (U) replaces thymine (T).
Sense strand runs 5' to 3', while the antisense strand runs 3' to 5'.
The antisense strand is used to create the sense strand.
Example: If the sense strand is $AGCCGA$ , the mRNA sequence will be $AGCCGA$ (with U replacing T).
Reading the sense strand directly would result in an incorrect, complementary sequence.
If the sense strand is $A G C C T A$ , reading it directly would yield $U C G G A U$ (incorrect).
The correct procedure involves transcribing the antisense strand (3' to 5') and then creating the mRNA sequence.

Definition: A specific region found at the beginning of a gene in the DNA sequence that signals the start of transcription.
Functions as a flag indicating where transcription should begin.
Different promoters signal the production of different molecules (e.g., eye color, hair color).
RNA polymerase, the enzyme responsible for transcription, binds to the promoter.
After binding, RNA polymerase opens the DNA and begins transcribing the coding sequence into mRNA.
Genes that are the same often share common promoters.
Hormones can trigger promoters to produce specific molecules (e.g., testosterone triggering muscle production).

Enzyme that opens the DNA for transcription (not helicase, which is used in DNA replication).
Creates mRNA by reading DNA nucleotides.
mRNA is created in the 5' to 3' direction.
Free nucleotides, called nucleosides, are present in the nucleus.
Nucleosides have three phosphate groups; nucleotides have one phosphate group.
RNA polymerase adds nucleosides to the template, releasing two phosphates to provide energy for bonding.
The process continues until RNA polymerase reaches the terminator region, where it detaches, releasing the mRNA strand.

Initiation: RNA polymerase binds to the promoter and starts adding RNA nucleotides.
Elongation: RNA polymerase continues reading the DNA and building the mRNA strand.
Termination: RNA polymerase reaches the terminator, detaches from the template, and releases the mRNA.

Introns: Non-coding sections of mRNA that are removed during processing.
Exons: Coding sections of mRNA that contain the necessary information for protein synthesis.
Introns must be removed to ensure the mRNA can be read correctly and produce the correct protein.
Introns are spliced out (cut) by an enzyme called spliceosome, leaving only the exons.

Allows for the creation of multiple protein versions from a single mRNA strand.
Different exons can be selected and combined in various ways.
Example: Exons 1, 2, 3, 4, and 5 can be combined to create different protein versions by including or excluding certain exons.
The specific exons included depend on communication from outside the cell, signaling the nucleus to produce a certain protein.

mRNA needs protection before leaving the nucleus to prevent damage in the cytoplasm.
A cap (methyl group) is added to one end of the mRNA, and a poly-A tail (multiple adenine bases) is added to the other end.
These protect the mRNA during its journey to the ribosome.

Telomeres: Protective ends of DNA strands that do not code for anything.
- They shorten with each mitosis, contributing to aging.
Regulatory Genes: Found before promoters and control gene expression.
- They regulate transcription by acting as repressors (slowing down) or activators (speeding up).
Sequences Coding for tRNA and rRNA: These sections code for tRNA (used for translation) and rRNA (makes up ribosomes).
- Important for translation but don't code for proteins.
Introns: As mentioned, non-coding sections spliced out during mRNA processing.

Transcription occurs in the nucleus, and translation occurs in the cytoplasm.
mRNA editing (removing introns, adding caps and tails) is necessary.
The nucleus separates transcription from translation, making it a slower process.

Lack a nucleus, so transcription and translation occur simultaneously.
mRNA editing is unnecessary due to the absence of organelles that could harm the mRNA.
The process is quicker because there is no separation between transcription and translation.