Gene expression

• Transcription involves making an RNA copy of a bit of DNA code.

• In replication we end up with a complete copy of the cell’s DNA, in transcription we end up with only a tiny specific section copied into an mRNA.

• Transcription begins at special sequences of the DNA strand called promoters.

• The official starting point is called the start site.

• We copy only one of the two DNA strands.

• The strand that serves as the template is known as the antisense strand.

• The other strand that lies dormant is the sense strand, or the coding strand.

• RNA polymerase builds RNA by adding nucleotides only to the 3′ side, therefore building a new molecule from 5′ to 3′

• Translation: mRNA —> protein

• Process occurs on ribosomes in cytoplasm and on the rough endoplasmic reticulum

• 3 nucleotides is called a codon. Each codon corresponds to a particular amino acid.

• One end of the tRNA carries an amino acid. The other end, called an anticodon, has three nitrogenous bases that can complementarily base pair with the codon in the mRNA.

• The third position is said to experience wobble pairing. Things that don’t normally bind will pair up, like guanine and uracil.

• Translation also involves three phases: initiation, elongation, and termination.

• Transcription factors can encourage or inhibit this from happening.

• Sometimes changes to the packaging of DNA will alter the ability of the transcription machinery to access a gene, this is called epigenetic changes.

• In bacteria, a cluster of genes can be under the control of a single promoter; these functioning units of DNA are called operons.

• The operon consists of four major parts:

structural genes, promoter genes, the operator, and the regulatory gene:

• Structural genes code for enzymes needed in a chemical reaction. These genes will be transcribed at the same time to produce particular enzymes.

• The promoter gene is the region where the RNA polymerase binds to begin transcription.

• The operator is a region that controls whether transcription will occur; this is where the repressor binds.

• The regulatory gene codes for a specific regulatory protein called the repressor. The repressor is capable of attaching to the operator and blocking transcription.

• Post-transcriptional regulation occurs when the cell creates an RNA, but then decides that it should not be translated into a protein. This is where RNAi comes into play.

• RNAi molecules can bind to an RNA via complementary base pairing. This creates a double-stranded RNA

• Post-translational regulation can also occur if a cell has already made a protein, but doesn’t yet need to use it.