Eukaryotic genes: general structures and organization. DNA packaging in chromatin fibers.
- Genome of eukaryotes have diploid number of chromosomes.
- Genome = info in 1 complete haploid chromosome set: total amount of DNA in haploid genome of species = C value.
- The structural complexity and C value of an organism are NOT related = C-value paradox.
- C value / 1 (n) genome = total amount of DNA in haploid.
- No relationship to structural/organizational complexity → paradox: less complex organisms may have more C value.
- A typical eukaryotic gene consists of exons (sequences appearing in mature mRNA) interrupted by introns.
- Regions between genes are not expressed but may help with chromatin assembly or contain promoters.
- Eukaryotic genes may be clustered (e.g., same metabolic pathway) but are independently controlled.
- Operons / polycistronic mRNAs do NOT exist in eukaryotes (unlike prokaryotes).
- The eukaryotic nucleus contains the organism’s genetic information and insulates it from the cytosol.
- DNA replication and RNA synthesis occur in the nucleus.
- Nuclear envelope = double-layered membrane:
- Outer membrane continuous with rough ER
- Inner membrane continuous with nuclear lamina
- Nuclear pores connect inner + outer membranes and regulate movement of mRNA, proteins, and macromolecules.
- Chromosomes condense DNA in an orderly fashion so it fits in the nucleus.
- Chromosomes contain genetic information and transmit hereditary information to offspring.
- Each species has a specific chromosome number (humans: 46; gametes: 23).
- Nucleoplasm stores chromatin (DNA + proteins).
- Chromatin = material making up chromosomes.
- In prokaryotes DNA is circular; in eukaryotes chromosomes are linear.
- DNA packaging:
- Nucleosome assembly: 8 histone proteins attach to DNA
- Nucleosomes coil together and stack → chromatin fiber (~30 nm)
- Fiber loops and folds further into chromosomes
- Chromosomes are only clearly visible during metaphase.
- DNA + histones form nucleosomes (“beads on a string”).
- Chromatin = protein–chromosome complex.
- Nucleolus = condensed chromatin region where rRNA is transcribed and ribosomes assembled.
- After transcription, mRNA leaves nucleus to ribosomes for translation.
- Ribosomes:
- Made of RNA + proteins
- Small subunit binds mRNA
- Large subunit binds tRNA + amino acids
- Subunits separate after translation ends
- Two forms of chromatin:
Euchromatin – condenses and decondenses with cell cycle; actively transcribed
Heterochromatin – remains condensed; inactive
- Types of heterochromatin:
- Constitutive: same sites on both homologs
- Facultative: varies with cell type/development; contains condensed, inactive euchromatin
- Example: Barr body = inactive X chromosome; densely packed