What is a signal sequence?

A signal sequence is a short (3-60 amino acids long) peptide chain that directs the post-translational transport of a protein. These sequences are usually found at the N-terminus of the protein sequence and are cleaved off by a signal peptidase once the protein is transported.

Here are the steps to understand what a signal sequence is:

1. Understand Proteins: Proteins are large, complex molecules that play many critical roles in the body. They are required for the structure, function, and regulation of the body's tissues and organs.

2. Know about Amino Acids: Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains.

3. Learn about Peptide Chains: A peptide chain is a string of amino acids. The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active.

4. Signal Sequence: A signal sequence is a short peptide chain that directs the post-translational transport of a protein. It's like a 'tag' that tells the cell where the protein needs to go.

5. Post-Translational Transport: After a protein is synthesized by a ribosome, it needs to be transported to its proper location in the cell. This is what the signal sequence directs.

6. N-Terminus of the Protein: Proteins have a directionality, with an amino (N) terminus and a carboxy (C) terminus. The signal sequence is usually found at the N-terminus, or the beginning of the protein sequence.

7. Cleavage by Signal Peptidase: Once the protein is transported to its proper location, the signal sequence is cleaved off, or removed, by an enzyme called a signal peptidase. This is because the signal sequence is no longer needed once the protein is in place.