Protein Synthesis

Nucleus Structure and Function

  • Components of the Nucleus:

    • Nucleoplasm

    • Chromatin

    • Nucleolus

    • Nuclear envelope

    • Nuclear pore

    • Perinuclear space

    • Transmission Electron Microscopy (TEM) at × 4800 magnification.

DNA and RNA Structure

  • Importance: Understanding DNA and RNA structure is crucial for comprehending protein synthesis due to their roles in encoding genetic information.

  • Key Terms:

    • DNA Structure: Double-stranded helical structure composed of nucleotides.

    • Components of a nucleotide:

      • One sugar (deoxyribose)

      • One phosphate group

      • One nitrogenous base (A, T, C, G in DNA)

    • RNA Structure: Single-stranded nucleotide chain with three types:

    • Ribosomal RNA (rRNA): Site of protein synthesis

    • Transfer RNA (tRNA): Delivers appropriate amino acids for protein construction

    • Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes, containing uracil instead of thymine.

The Nucleus and DNA

  • DNA Packaging:

    • DNA strands are coiled around proteins called histones to form nucleosomes.

    • Nucleosomes create a complex structure known as chromatin, which becomes tightly coiled to form chromosomes during cell division.

    • In humans, somatic cells contain 23 pairs of chromosomes.

    • DNA encodes the recipe for amino acids that synthesize proteins, known as the genetic code.

Chromatin Structure

  • Structure Levels:

    • 2 nm: DNA double helix

    • 11 nm: DNA winds around histones to form core particles

    • 30 nm: Nucleosomes organized into zigzag fibers

    • 300 nm to 700 nm: Further coiling in chromatin leads to visible chromatids during cell division.

DNA Replication Process

  • Initiation of Replication:

    • Begins with DNA helicase, which unwinds DNA strands and disrupts hydrogen bonds between bases.

    • DNA polymerase binds to exposed nitrogenous bases and facilitates binding with complementary nucleotides in the nucleoplasm, creating covalent bonds between nucleotides.

Genes and the Genetic Code

  • Functional Units:

    • Genes are sequences of nucleotides in DNA that specify amino acids for proteins.

    • The genetic code uses triplet codes (base triplets) where groups of three bases correspond to amino acids.

  • Codons:

    • A codon is a sequence of three bases in mRNA.

    • There are 64 possible codons for 20 amino acids, with 61 coding for amino acids and 3 acting as stop codons.

    • The start codon AUG codes for methionine. Stop codons are UAG, UGA, UAA, signaling the end of translation.