Genetic Code, Gene Expression, and RNA Processing

How Genes Work

DNA carries instructions for cell structure and function, passing them to new cells.
The question is: How are these instructions carried out?

DNA, with its four nitrogenous bases, codes for all proteins made in a cell.
Genes, made of DNA, are coded instructions controlling specific protein production (e.g., enzymes, structural proteins).

Gene expression: DNA directs protein synthesis through transcription and translation.

Proteins are made of amino acids.
There are 21 amino acids and four nucleotides.
Three nucleotides in sequence specify each amino acid, providing 64 possible combinations.
Each nucleotide triplet is a codon.
Each codon calls for a specific amino acid; linked amino acids form a protein.

Genes provide instructions for making specific proteins but don't build them directly.
RNA is the bridge between DNA and protein synthesis.
A gene on a chromosome consists of many codons and codes for a particular protein.

Ribosomes in the cytoplasm make proteins.
DNA determines which proteins are made.
A gene on DNA is copied into RNA.
RNA carries the message from DNA (in the nucleus) to ribosomes (in the cytoplasm).
RNA tells ribosomes which proteins to make and how.

mRNA travels from the nucleus to the cytoplasm (ribosomes) with protein-making instructions.
mRNA is the "messenger" between DNA in the nucleus and ribosomes in the cytoplasm.
Instructions are carried in codons.
The first codon is the "start" codon, where mRNA attaches to the ribosome to begin protein-building.
The rest of the molecule is a sequence of nucleotides that dictates the sequence of amino acids for a particular protein.
The last codon is the “stop” codon, which tells the ribosome to stop protein production.

Each tRNA has an anticodon at one end and an amino acid binding site at the other end.
Anticodon bases are complementary to mRNA codons.
tRNA reads the mRNA message and gathers amino acids for making proteins.
tRNA transfers amino acids from the cytoplasmic pool to a ribosome.
The cell keeps cytoplasm stocked with all 21 amino acids.
One end of tRNA attaches to an amino acid and carries it to the ribosome.

Transcription forms an RNA strand from a DNA strand.
This process occurs in the nucleus.
The cell makes RNA to send to the cytoplasm to tell ribosomes how and which proteins to make.
The RNA molecule is a faithful copy of a gene’s protein-building instructions; this RNA is mRNA.
RNA polymerase catalyzes the reaction.
Transcription copies one gene from the DNA molecule.
A promoter is a DNA sequence where RNA polymerase attaches and initiates transcription.
A terminator is the DNA sequence signaling the end of transcription.

RNA polymerase binds to the promoter on DNA.
RNA polymerase separates DNA strands.
One DNA strand is used as a template.
New nucleotides are inserted according to base pairing rules: adenine pairs with uracil (in RNA), and cytosine pairs with guanine.
Transcription continues until the terminator is reached.
As RNA polymerase moves, hydrogen bonds reform between DNA strands.
A single-stranded RNA molecule (mRNA transcript) is transcribed.
Transcription copies only a single gene.
The mRNA transcript is sent to the ribosome for protein synthesis.

Introns are non-coding sequences that are cut out of the RNA before it goes to ribosomes.
Exons are coding sequences involved in protein making.
When mRNA forms, both introns and exons are copied from DNA.
Introns are cut out while RNA is in the nucleus.
The remaining exons are spliced back together to form the final RNA.
Edited mRNA is sent to ribosomes after adding a cap and tail.

Introns allow a single gene to code for more than one type of protein, depending on which segments are treated as introns and exons.
Cutting out different segments results in different proteins.