Nucleus and Ribosomes Notes

The Nucleus

  • The nucleus is the command center of the cell.

  • It contains chromatin, a combination of proteins and nucleic acids.

  • Diameter of about 5 \mu m.

  • Oval structure located near the center of most cells.

  • Some cells, like skeletal muscle cells, can have more than one nucleus.

  • The interior contains a semifluid matrix called the nucleoplasm.

  • Chromatin has a grainy appearance but is a network of strands that condense and coil into rod-like chromosomes before cell division.

  • Chromosomes carry genetic information organized as genes.

  • All cells of an individual have the same number of chromosomes, except for egg and sperm cells, which have half the number.

  • Egg + Sperm = Zygote

    • Egg with 23 chromosomes + Sperm with 23 chromosomes = Zygote with 46 chromosomes

  • Three types of ribonucleic acid (RNA) are produced in the nucleus:

    • messenger RNA (mRNA)

    • ribosomal RNA (rRNA)

    • transfer RNA (tRNA)

  • Ribosomal RNA (rRNA) is produced in the nucleolus, a dark region of chromatin, where rRNA joins with proteins to form the subunits of ribosomes.

  • Transfer RNA (tRNA) participates in the assembly of amino acids into a polypeptide by recognizing both mRNA and amino acids during protein synthesis.

  • Messenger RNA (mRNA), a mobile molecule, acts as an intermediary for DNA, specifying the sequence of amino acids in a protein.

  • The nucleus is physically separated from the cytoplasm by a double membrane called the nuclear envelope, yet it communicates through nuclear pores.

  • Nuclear pores (100 nm) allow passage of ribosomal subunits and mRNA out of the nucleus and proteins into the nucleus.

  • Non-membrane components associated with the pores form a nuclear pore complex, acting as gatekeepers.

Ribosomes

  • Ribosomes are the site of protein synthesis.

  • Composed of large and small ribosomal subunits, made of proteins and rRNA.

  • In eukaryotes, ribosomes are 20 nm by 30 nm; prokaryotes have slightly smaller ribosomes.

  • Pancreatic cells and other glands have many ribosomes for producing protein-containing secretions.

  • In eukaryotic cells, some ribosomes occur freely in the cytoplasm (singly or in groups called polyribosomes), while others are attached to the endoplasmic reticulum (ER).

  • Function of ribosomes:

    • mRNA, copied from a gene, exits the nucleus through a pore complex.

    • mRNA is read by a ribosome in the cytoplasm.

    • Amino acids are connected together by the ribosome in a sequence specified by the mRNA.

    • When a polypeptide is first translated, it begins with a signal peptide; this combines with a signal recognition particle (SRP), which brought to the rough ER.

    • The SRP leaves, and the polypeptide is made and pushed into the ER lumen.

    • The signal peptide is removed and the polypeptide folds into its final protein shape.

  • Ribosomes receive mRNA, which carries a coded message from DNA indicating the correct sequence of amino acids in a particular protein.

  • Free ribosomes in cytosol

  • Membrane-bound ribosomes

  • Binding occurs only if the protein being synthesized by a ribosome begins with a sequence of amino acids called a signal peptide.

  • The signal peptide binds a particle (signal recognition particle, SRP), which then binds to a receptor on the ER.

  • Once the protein enters the ER, an enzyme cleaves off the signal peptide. The protein ends up within the lumen (interior) of the ER, where it folds into its final shape.

  • Central dogma of molecular biology:

    • DNA --(Replication)--> DNA

    • DNA --(Transcription)--> RNA

    • RNA --(Translation)--> Protein