Mitochondrial and chloroplast proteins are synthesized outside of the organelles in the cytoplasm. They are taken up by the organelles through receptor proteins that recognize targeting sequences - for mitochondria this is a sequence of 3-5 Arg/Lys residues in the N-terminal region, while chloroplast targeting involves a sequence rich in Ser/Thr and hydrophobic residues. The precursor proteins are cleaved within the organelles to produce the functional proteins that carry out the roles of mitochondria and chloroplasts.

1. a) The proteins of mitochondria and chloroplast are synthesized out side the cell, in the
cytoplasm. These proteins are synthesized on ribosomes that are not attached to rough
endoplasmic reticulum. The proteins after synthesize are released from ribosomes into cytoplasm
and are then uptaken by respective organelles. The proteins are uptaken by binding to specific
receptor proteins on the organelles. The receptor proteins recognize specific targeting sequences
in newly synthesized proteins. Genelly, the uptake target signal sequence for mitochondrial
proteins is located in the N terminal region. The uptake target signal sequence contains 3 – 5
nonconsecutive Arg or Lys residues, often with Ser and Thr; no Glu or Asp residues. For
chloroplast, the uptake target signal sequence is located in the N terminal region. The uptake
target signal sequence contains no common sequence motifs but is rich in Ser, Thr, and small
hydrophobic amino acid residues and poor in Glu and Asp residues.
b) The proteins are targeted as preproteins and the precursor is cleaved to produce the functional
protein in mitochondria and chloroplast and then proteins perform the respectivew functions of
these cell organelles.
Solution
a) The proteins of mitochondria and chloroplast are synthesized out side the cell, in the
cytoplasm. These proteins are synthesized on ribosomes that are not attached to rough
endoplasmic reticulum. The proteins after synthesize are released from ribosomes into cytoplasm
and are then uptaken by respective organelles. The proteins are uptaken by binding to specific
receptor proteins on the organelles. The receptor proteins recognize specific targeting sequences
in newly synthesized proteins. Genelly, the uptake target signal sequence for mitochondrial
proteins is located in the N terminal region. The uptake target signal sequence contains 3 – 5
nonconsecutive Arg or Lys residues, often with Ser and Thr; no Glu or Asp residues. For
chloroplast, the uptake target signal sequence is located in the N terminal region. The uptake
target signal sequence contains no common sequence motifs but is rich in Ser, Thr, and small
hydrophobic amino acid residues and poor in Glu and Asp residues.
b) The proteins are targeted as preproteins and the precursor is cleaved to produce the functional
protein in mitochondria and chloroplast and then proteins perform the respectivew functions of
these cell organelles.