Ribosomes and the Endomembrane System Overview

Ribosomes

Small, non-membranous organelles made of protein and ribosomal RNA (rRNA) that are the sites of protein synthesis.

Free Ribosomes

Float freely in the cytoplasm. Synthesize soluble proteins for the cytosol or other organelles.

Membrane-bound Ribosomes

Attached to the membrane of the rough endoplasmic reticulum (Rough ER). Synthesize proteins for membrane incorporation, lysosomes, and export from the cell.

Eukaryotic ribosomes

Consist of a large (60S) ribosomal subunit, a small (40S) ribosomal subunit, and a complete functional ribosome (80S).

Svedberg unit (S)

Measures how quickly something settles during centrifugation.

Endomembrane System

Components include Rough ER, Smooth ER, Golgi apparatus, Secretory vesicles, Lysosomes, and nuclear and plasma membranes.

Functions of Endomembrane System

Produce, degrade, store, and export biological molecules. Degrade harmful substances like pathogens and toxins.

Endoplasmic Reticulum (ER)

An extensive system of tubes and membranes enclosing fluid-filled cavities (cisterns or cisternae) throughout the cytosol.

Rough ER

Studded with ribosomes. Manufactures proteins for packaging into vesicles, incorporation into the cell membrane, and secretion from the cell.

Smooth ER

Lacks ribosomes. Involved in lipid and glycogen metabolism, cholesterol and steroid hormone synthesis, and detoxification of chemicals (e.g., drugs, pesticides).

Golgi Apparatus

A series of stacked, flattened membranous sacs associated with vesicles.

Entry Face of Golgi Apparatus

Proteins enter the Golgi apparatus.

Processing in Golgi Apparatus

Proteins are modified and packaged.

Exit Face of Golgi Apparatus

Modifies, concentrates, and packages proteins and lipids made in the ER.

Secretory Vesicles

Contain proteins for export.

DNA

Carries instructions for building proteins, composed of four nucleotide bases: A (adenine), G (guanine), T (thymine), and C (cytosine).

Codons

Each set of three nucleotide bases is called a codon (or triplet) and specifies a particular amino acid.

Gene Expression

Uses a gene's DNA sequence to build proteins with specific amino acids.

Transcription

The process of converting DNA to RNA.

Translation

The process of converting RNA to amino acids.

Exons

Coding segments that specify amino acids in eukaryotic organisms.

Introns

Non-coding segments interspersed among exons in eukaryotic organisms.

Role of RNA

Acts as a 'go-between' molecule, translating genetic information from DNA into proteins.

Messenger RNA (mRNA)

Copies the DNA sequence.

Transfer RNA (tRNA)

Has anti-codons on one end and corresponding amino acids on the other.

Ribosomal RNA (rRNA)

Binds to mRNA and tRNA in the ribosome, facilitating the binding of amino acids.

Transcription Factor

Coordinates the binding of RNA polymerase.

Gene

A segment of DNA that codes for a protein.

Promoter

A region of DNA that initiates transcription of a particular gene.

Terminator

A sequence that signals the end of transcription.

Splicing RNA

The process of removing noncoding introns and reconnecting exons before mRNA exits the nucleus.

Translation Process

The conversion of nucleotide sequences into amino acid sequences.

Ribosome Structure

Composed of a large subunit and a small subunit, containing three binding sites: P site (peptidyl), A site (aminoacyl), E site (exit).

Initiation

tRNA binds to mRNA start codon.

Elongation

tRNA pairs with mRNA, forming peptide bonds.

Termination

Protein synthesis halts at stop codon.

Sense strand

DNA strand transcribed into mRNA.

Antisense strand

Non-coding DNA complementary to sense strand.

snRNP

Small nuclear ribonucleoproteins for splicing.

Functional mRNA

Processed mRNA that exits nucleus for translation.

Nucleus

Cell organelle containing genetic material.

Nuclear Envelope

Double-membrane barrier surrounding the nucleus.

Nucleoli

Sites for ribosomal RNA assembly in nucleus.

Chromatin

DNA and proteins, condenses into chromosomes.

DNA Analogy

DNA likened to a book with chapters.

Noncoding RNAs

Regulate DNA/RNA functions, previously 'junk DNA'.

Activator Proteins

Enhancers that promote gene transcription.

Repressors

Proteins that inhibit gene expression.

Vesicles

Membrane-bound sacs transporting materials in cells.

Exocytosis

Process of vesicles releasing contents outside cell.

Lysosomes

Organelles containing enzymes for digestion.

Peroxisomes

Organelles that oxidize and detoxify substances.

Ubiquitin

Protein tag signaling destruction of other proteins.

Proteasomes

Barrel-shaped structures degrading unneeded proteins.

Mitochondria

Produce ATP by breaking down food molecules.

Cell Types

Most cells are uninucleate; some are multinucleate.

Chromosomes

Condensed chromatin structures during cell division.

Nuclear Pores

Regulate passage of large particles in nucleus.

Transport Vesicles

Deliver proteins to specific cellular locations.