unit 5 Key Concepts: Central Dogma & Nucleus Essentials

Central Dogma

  • Molecular biology focuses on processes inside the cell that build biological molecules.

  • Central dogma (DNA → RNA → Protein): transcription converts DNA to mRNA; translation converts mRNA to protein; ultimate goal is functional protein synthesis.

  • First described by Francis Crick in 1957.

Nuclear Organization and Function

  • DNA is housed in the nucleus; transcription occurs here, replication occurs here.

  • Nuclear envelope has three parts: outer nuclear membrane, perinuclear (perinuclear) space, inner nuclear membrane.

  • Lamins (e.g., lamin A/C, lamin B) provide rigidity to the nuclear envelope.

  • Linkage between nucleus and cytoskeleton via the LINC complex (SUN proteins in the inner membrane; KASH domain proteins in the outer membrane).

  • Nuclear pore complex (NPC) forms through nucleoporins (nups).

  • Nucleoplasm is a gelatinous interior containing chromatin and other factors.

  • NPCs regulate traffic between nucleus and cytoplasm and also participate in gene regulation.

  • Neuronoporin (nup) family examples: Nup53, Nup155, etc.; these proteins have broader roles in gene transcription and DNA interactions beyond transport.

Chromatin and Chromosome Packaging

  • DNA is wrapped around histone proteins to form nucleosomes (core particle).

  • Core particle + histone organization compacts DNA into chromatin; further folding forms chromatids and chromosomes.

  • Chromatin conformation enables DNA replication and transcription by making regions accessible or inaccessible.

  • Chromatin packing explains why DNA fits in the nucleus and how gene expression is regulated.

DNA vs RNA: Structure and Key Differences

  • DNA is a double helix with a sugar–phosphate backbone and base pairs: A–T (2 H-bonds) and C–G (3 H-bonds).

  • Bases: A, C, G, T in DNA; RNA replaces T with U (A–U, C–G).

  • DNA is antiparallel: strands run in opposite directions (5'→3' and 3'→5').

  • RNA is typically single-stranded with ribose sugar and can form complex structures.

  • Nucleotides: DNA has four bases; RNA also has four bases; in RNA, uracil replaces thymine.

  • Directionality for reading: always read from 5' end to 3' end.

Genetic Content and Coding Potential

  • Humans have about 2×1042\times 10^4 genes (~20,000).

  • Coding genes constitute roughly 5%–6%5\%\text{–}6\% of all genes; the rest are noncoding.

  • Coding genes are transcribed into mRNA; noncoding genes have regulatory or other roles.

DNA Structure Details and Nucleotide Rules

  • DNA bases: A, C, G, T; RNA bases: A, C, G, U.

  • Base-pair rules: A↔T(2  H-bonds),C↔G(3  H-bonds)A\leftrightarrow T\quad (2\;\text{H-bonds}),\quad C\leftrightarrow G\quad (3\;\text{H-bonds}).

  • DNA is read 5'→3' on each strand, with strands complementary and antiparallel.

  • The two strands together form the double helix with a sugar-phosphate backbone on the outside.

Transcription and Translation Overview

  • Transcription occurs in the nucleus to produce mature messenger RNA (mRNA).

  • The mRNA exits the nucleus through the nuclear pore complex to the cytoplasm for translation.

  • Translation requires three RNA types:

    • mRNA: carries sequence information for the protein.

    • rRNA: forms the ribosome, the site of protein synthesis.

    • tRNA: brings amino acids to the ribosome and matches them to the codons on mRNA.

DNA Replication: Semiconservative and Key Enzymes

  • Purpose: copy the entire genome for cell division (mitosis).

  • Semiconservative replication: each new DNA molecule has one old strand and one new strand.

  • Steps:

    • Unwinding: histones must be displaced to expose DNA.

    • Unzipping: hydrogen bonds between base pairs are broken by helicase to form a replication fork.

    • Complementary base pairing: primers laid by primase; DNA polymerase extends new strands.

    • Ligation: gaps filled and strands joined by ligase.

  • Leading strand: synthesized continuously 5'→3' toward the replication fork.

  • Lagging strand: synthesized discontinuously in Okazaki fragments (also 5'→3', but away from fork).

  • Okazaki fragments are initiated by RNA primers; later replaced with DNA.

  • RNA primers removed by exonuclease; gaps filled by DNA polymerase; fragments sealed by DNA ligase.

  • Directionality reminder: DNA synthesis is always 5'→3'.

  • Common errors occur but are corrected by proofreading and repair mechanisms; unresolved damage can trigger checkpoints or apoptosis.

DNA Damage and Repair Checkpoints

  • DNA damage can arise from mispaired bases or environmental factors.

  • Repair mechanisms include proofreading during replication, mismatch repair, and cellular checkpoints.

  • If damage is irreparable, cells may undergo apoptosis (programmed cell death).

Transcription through to Protein Synthesis

  • After replication, DNA must be transcribed; mRNA is produced in the nucleus and exported to the cytoplasm.

  • Translation uses the ribosome and tRNA to assemble amino acids into polypeptides according to the mRNA sequence.

  • This completes the flow from gene to functional protein.