Comprehensive Cell Biology: Central Dogma, Gene Regulation & Cell Communication (28-32)

Central Dogma

DNA → mRNA → Protein

Transcription

DNA is copied into mRNA

Translation

mRNA is read to build a protein

Codon

3 RNA nucleotides (A, U, C, G); each codon → 1 amino acid

Universal Genetic Code

Almost all organisms use the same codons

Unambiguous Genetic Code

Each codon gives only ONE amino acid

Redundant Genetic Code

Multiple codons can code for the same amino acid

Start Codon

AUG (methionine)

Stop Codons

UAA, UAG, UGA

Template Strand

Strand used to build RNA

Coding Strand

Same sequence as RNA except T→U

RNA Polymerase

Enzyme that builds RNA

Promoter

DNA sequence where RNA polymerase binds

Initiation of Transcription

RNA polymerase + sigma factor bind promoter; DNA unwinds

Elongation of Transcription

RNA polymerase moves 5'→3' and builds RNA complementary to the DNA template

Termination of Transcription

RNA forms a hairpin loop; polymerase falls off; transcription ends

Pre-mRNA Processing

Modifications after transcription in eukaryotes

5' Cap

Protects RNA, helps ribosome bind, helps leave nucleus

3' Poly-A Tail

Stabilizes RNA, helps leave nucleus

Splicing

Removes introns and keeps exons; alternative splicing allows 1 gene to make several proteins

Key Players in Translation

mRNA, tRNA, anticodon, ribosome

A Site

New tRNA enters during translation

P Site

Growing chain during translation

E Site

Exit site for tRNA during translation

Initiation of Translation

Small ribosomal subunit binds 5' UTR; initiator tRNA binds AUG start codon

Elongation of Translation

New tRNA enters A site; peptide bond forms; ribosome shifts forward

Stop codon

Ribosome hits UAA, UGA, UAG.

Release factor

No tRNA matches → binds to the ribosome.

Chemical modifications

Modifications such as phosphate, methyl, acetyl groups after translation.

Folding

Process sometimes assisted by chaperones after translation.

Prokaryotes

Transcription and translation occur together.

Eukaryotes

Transcription and translation occur separately.

mRNA in Prokaryotes

No introns present.

mRNA in Eukaryotes

Contains introns and requires splicing.

Open chromatin (euchromatin)

Genes are ON.

Closed chromatin (heterochromatin)

Genes are OFF.

Acetylation

Modification that opens chromatin and increases gene expression.

Methylation

Modification that can activate or repress gene expression depending on location.

Epigenetics

Changes in gene expression without changing DNA sequence.

Structural genes

Genes that make proteins such as enzymes.

Regulatory genes

Genes that control other genes, e.g., lac repressor.

Transcriptional regulation

Mostly occurs in prokaryotes, e.g., lac operon.

Levels of Gene Regulation in Eukaryotes

Includes transcriptional, post-transcriptional, translational, and post-translational.

Ligand

Signaling molecule in cell communication.

Receptor

Receives the ligand in cell signaling.

Signaling cell

Cell that sends the signal.

Target cell

Cell that has the receptor and responds.

Autocrine signaling

Cell signals itself.

Direct signaling

Involves direct cell-to-cell passage through gap junctions.

Paracrine signaling

Short distance signaling, e.g., growth factors, neurotransmitters.

Endocrine signaling

Long distance signaling via bloodstream, e.g., hormones.

Membrane receptors

Bind hydrophilic ligands and cannot cross the membrane.

Intracellular receptors

Bind hydrophobic ligands and can cross the membrane, usually changing gene expression.

Receptor Activation

Ligand binds and receptor changes shape.

Signal Transduction

A chain reaction inside the cell involving kinases and phosphatases.

Cellular Response

Could include changes in gene expression, metabolism, cell growth, apoptosis, movement, secretion, contraction, etc.

Signal Amplification

One ligand activates many molecules, leading to a huge response from tiny amounts of signal.