Organelles Involved in Protein Synthesis

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  • 0:01 The Cell
  • 0:20 The Nucleus and DNA
  • 1:28 Transcription and MRNA
  • 2:38 Ribosomes and…
  • 4:10 Golgi Apparatus
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: Joseph Said
This lesson provides an overview of the process of protein synthesis and the organelles it involves. It follows the path of an mRNA molecule from the nucleus to the ribosomes, where proteins are synthesized.

The Cell

The cell functions much like a factory for building various types of molecules that the cell needs. Like a factory, each organelle has a specific function much like organs in the body. These organelles work together to perform all of the necessary functions of the cell, like making proteins.

Cell Diagram
Cell diagram

The Nucleus and DNA

Let's start with the nucleus, the organelle that houses the cell's DNA or genetic information. First, we need to define transcription versus translation. Transcription is the process where DNA code creates a template complementary RNA molecule. Translation is the process where the RNA code is converted to a protein molecule.

We can think of the nucleus as a genetic library that stores all of the necessary information to make proteins. The nucleus is composed of a lipid bilayer, which forms a protective barrier around the cell's DNA. Inside the nucleus DNA segments called genes are transcribed as needed by RNA polymerases, which code a complementary strand of mRNA.

DNA is composed of the four nucleotides:

  • Adenine (A)
  • Thymine (T)
  • Cytosine (C)
  • and Guanine (G), and A always bonds with T while G bonds with C.

These four nucleotides are found in the DNA of every living thing on Earth. They are responsible for storing all of the information necessary for life.

Transcription and mRNA

DNA is used to transcribe messenger RNA (mRNA), which when translated codes for proteins. Messenger RNA is composed of:

  • Adenine (A)
  • Uricil (U)
  • Cytosine (C)
  • and Guanine (G), where A bonds to U instead of T.

The gene is transcribed from the start codon until one of the three stop codons is encountered by the RNA polymerase. Codons are composed of three nucleotides, and they either code for an amino acid or they are stop codons, signaling the end of transcription. For example, AUG is a codon that codes for the amino acid methionine.

The following is a codon chart showing which three nucleotides code for which amino acids.

Codon Chart
Codon chart

Once the mRNA molecule is transcribed, it is fitted with a poly-A tail, and a 5' cap is added to the molecule, which allows it to leave the nucleus. The poly-A tail is a long string of adenine molecules and the cap is an altered nucleotide, and these two features act as tags showing the mRNA is ready to leave the nucleus.

Ribosomes and Endoplasmic Reticulum

Ribosomes are the organelles responsible for protein translation and are composed of ribosomal RNA (rRNA) and proteins. Some ribosomes are found in the cytoplasm, a gel-like substance that organelles float in and some are found in the rough endoplasmic reticulum. If the protein in question is a cytoplasmic protein that will not be transported to the cell membrane or secreted, it will be translated on a cytoplasmic ribosome.

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