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Questions and Answers

What is the primary function of the nucleolus within the nucleus of eukaryotic cells?

  • Degradation of proteins
  • Production of ribosomal RNA (correct)
  • DNA replication
  • Synthesis of lipids

    • Which of the following types of RNA is NOT produced by the nucleolus?

  • mRNA (correct)
  • 5S rRNA
  • 18S rRNA
  • 28S rRNA

    • Which cells would most likely have a well-developed nucleolus?

  • Inactive fibroblasts
  • Fat cells
  • Muscle cells during rest
  • Malignant tumor cells (correct)

    • How are the genes for rRNA arranged in human chromosomes?

    <p>Clustered in tandem arrays on specific chromosomes</p> Signup and view all the answers

    Which RNA polymerase is responsible for transcribing rRNA in the nucleolus?

    <p>RNA polymerase I</p> Signup and view all the answers

    What ratio of ribosomes can actively growing cells contain?

    <p>5-10 million</p> Signup and view all the answers

    What cellular feature is the nucleolus most associated with?

    <p>Protein synthesis</p> Signup and view all the answers

    Which of the following statements about nucleoli is true?

    <p>Their size can vary depending on the activity of the cell.</p> Signup and view all the answers

    What components primarily make up ribosomes?

    <p>Proteins and nucleic acids</p> Signup and view all the answers

    What is the primary function of both prokaryotic and eukaryotic ribosomes?

    <p>Assembling amino acids into proteins</p> Signup and view all the answers

    How do the sizes of mitochondrial and chloroplast ribosomes compare to those in the cytoplasm?

    <p>Similar to prokaryotic ribosomes</p> Signup and view all the answers

    Where are bound ribosomes primarily located?

    <p>On the exterior of the endoplasmic reticulum</p> Signup and view all the answers

    In which type of cells are free ribosomes more numerous?

    <p>Cells that retain most of their manufactured proteins</p> Signup and view all the answers

    What is the primary purpose of the ribosomes found in pancreatic cells?

    <p>Produce digestive enzymes for secretion</p> Signup and view all the answers

    Which histone binds to the linker DNA between nucleosomes?

    <p>H1</p> Signup and view all the answers

    Which of the following statements about ribosomes is true?

    <p>Ribosomes are involved in the production of proteins.</p> Signup and view all the answers

    How many base pairs of DNA wrap around the nucleosomal core?

    <p>146 bp</p> Signup and view all the answers

    What process leads to the formation of mRNA from DNA?

    <p>Transcription</p> Signup and view all the answers

    What is the thickness of chromatin fibers in the second level of DNA packaging?

    <p>30 nm</p> Signup and view all the answers

    Which statement about the structure of the 30 nm chromatin fiber is true?

    <p>It consists of approximately 6 nucleosomes in each turn.</p> Signup and view all the answers

    What role do lysine and arginine residues play in histones?

    <p>They make histones positively charged.</p> Signup and view all the answers

    Where are the 40S and 60S ribosomal subunits assembled?

    <p>Nucleolus</p> Signup and view all the answers

    What catalyzes the transcription of 5S rRNA?

    <p>RNA polymerase III</p> Signup and view all the answers

    Which rRNAs are derived from the large 45S pre-rRNA molecule?

    <p>18S, 5.8S, and 28S rRNAs</p> Signup and view all the answers

    What is the primary role of small nucleolar RNAs (snoRNAs) in the nucleolus?

    <p>Processing pre-rRNA</p> Signup and view all the answers

    Which type of chromatin is transcriptionally active and less condensed?

    <p>Euchromatin</p> Signup and view all the answers

    What is not a characteristic of euchromatin?

    <p>Visible with light microscopy</p> Signup and view all the answers

    What function does the nucleolus serve apart from ribosomal RNA production?

    <p>Assembly of RNA-protein complexes</p> Signup and view all the answers

    How many copies of the gene that encodes 5S rRNA are present in human cells?

    <p>2000 copies</p> Signup and view all the answers

    What is the primary purpose of ribosomes in protein synthesis?

    <p>To synthesize proteins from the mRNA sequence</p> Signup and view all the answers

    Which subunit of prokaryotic ribosomes is designated as 30S?

    <p>Small subunit composed of 16S rRNA and 21 proteins</p> Signup and view all the answers

    How does the composition of eukaryotic ribosomes differ from that of prokaryotic ribosomes?

    <p>Eukaryotic ribosomes have larger subunits and more proteins</p> Signup and view all the answers

    What is the composition of the small subunit (40S) in eukaryotic ribosomes?

    <p>18S rRNA and approximately 30 proteins</p> Signup and view all the answers

    What role does transfer RNA (tRNA) play in protein synthesis?

    <p>To transport amino acids to the ribosome for protein synthesis</p> Signup and view all the answers

    Which of the following statements about ribosome structure is true?

    <p>A ribosome is made up of two subunits formed by rRNA and proteins</p> Signup and view all the answers

    Which rRNA molecules are included in the large subunit (60S) of eukaryotic ribosomes?

    <p>28S, 5.8S, and 5S rRNAs</p> Signup and view all the answers

    What happens to proteins synthesized by ribosomes in the cytoplasm?

    <p>They are used within the cytoplasm itself</p> Signup and view all the answers

    What is the primary characteristic of heterochromatin?

    <p>It is dark staining and condensed.</p> Signup and view all the answers

    Which of the following describes the structure of a nucleosome?

    <p>It is formed by DNA coiled around a histone octamer.</p> Signup and view all the answers

    What percentage of the genome is typically packaged into heterochromatin in mammalian cells?

    <p>More than 10%</p> Signup and view all the answers

    Which proteins are most abundant in chromatin?

    <p>Histones</p> Signup and view all the answers

    What is the diameter of a chromatin fiber composed of nucleosomes?

    <p>10 nm</p> Signup and view all the answers

    The first level of DNA folding occurs when DNA coils around which structure?

    <p>Nucleosome core</p> Signup and view all the answers

    What is the role of linker DNA in a nucleosomal structure?

    <p>To separate nucleosomes</p> Signup and view all the answers

    Which of the following histones is categorized as a linker histone?

    <p>H1</p> Signup and view all the answers

    Study Notes

    Internal Organization of the Nucleus and Ribosome Biosynthesis

    • The nucleus has a double membrane nuclear envelope
    • The nuclear envelope has pores (0.25 µm surface area) for ribosome passage
    • There are ribosome pore complexes (1 µm) on the nuclear envelope and nuclear lamina (TEM).
    • The rough endoplasmic reticulum(ER) is connected to the nuclear envelope
    • The structure of ribosome pore complex is visible in Transmission Electron Microscopy (TEM) images.

    Nucleolus

    • The nucleolus is the largest structure in the nucleus of eukaryotic cells

    • It is 1 μm in diameter

    • It is stained with basic dyes

    • It can be difficult to observe if nuclear chromatin is very condensed

    • The nucleolus is composed of proteins + RNA (1940)

    • It is the site of ribosomal RNA production (1960)

    • RNA transcription, processing, and ribosome assembly take place within the nucleolus

    • Actively growing cells contain 5-10 million ribosomes

    • Nucleoli are well developed in cells active in protein synthesis. This is especially true during rapid cell growth (like in young cells, cells of the pancreas, and various cancers)

    Nucleolus Contains

    • rRNA genes (DNA regions extending into the nucleolus)
    • Pre-rRNA transcripts
    • Mature rRNA transcripts
    • Pre-ribosomal subunits

    Nucleolus Organization

    • The nucleolus is organized around specific chromosomal regions (13, 14, 15, 21, 22)
    • These chromosomes contain the genes for 5.8S, 18S, and 28S rRNAs.
    • Cells contain repeated rRNA genes (multiple copy genes).
    • The rRNA genes, including repeats and the resulting ribosomal RNA molecules, are crucial for ribosome function.

    Ribosomes

    • Ribosomes are the sites of protein synthesis in both prokaryotic and eukaryotic cells

    • Prokaryotic and eukaryotic ribosomes are composed of two distinct subunits (each with specific proteins and rRNA components.

    • E. coli ribosomes account for 25% of the dry weight of the cell.

    • Ribosomes are found in mitochondria and chloroplasts (of eukaryotes). Mitochondria and chloroplast ribosomes resemble bacterial ribosomes

    • Ribosomes have a function in protein production

    • Ribosomes are composed of rRNA + proteins

    Types of Ribosomes

    • Free Ribosomes:

      • Located in the cytosol
      • Occur in greater numbers in cells that retain most of their manufactured protein
      • For proteins that stay in the cytoplasm or form cytoplasmic structural or motile elements

    • Bound Ribosomes:

      • Attached to the endoplasmic reticulum (ER)
      • Occur in greater numbers in cells that secrete their produced proteins (like in pancreatic cells).
      • For proteins packaged into vesicles (for storage/export) or form membranes

    Functions of Ribosomes

    • Assemble amino acids into proteins (proteins are necessary for cellular functions)
    • The process of protein production proceeds via DNA transcription and mRNA translation
    • The genetic message from mRNA is translated into proteins during DNA translation
    • Protein assembly sequences are in the mRNA
    • mRNA is synthesized in the nucleus and exported to the cytoplasm for protein synthesis
    • Ribosomes located in the cytoplasm complete protein production.

    Bacterial Ribosomes

    • The general structure of prokaryotic and eukaryotic ribosomes are similar.
    • E. coli ribosome small subunit (30S), consists of 16S rRNA and 21 proteins
    • E. coli ribosome large subunit (50S), consists 23S rRNA and 5S rRNA and 34 proteins.
    • A ribosome is composed of large and small subunits; each comprises proteins + rRNA molecules.

    Eukaryotic Ribosomes

    • Eukaryotic ribosomes' subunits are larger and have more proteins than prokaryotic ones
    • The small subunit (40S) of eukaryotic ribosomes consists of 18S rRNA and ~30 proteins.
    • The large subunit (60S) contains 28S, 5.8S, 5S rRNAs, and 45 proteins.
    • Proteins and RNA make up the subunits.

    5S rRNA

    • 5S rRNA is not located in the nucleolus
    • There are ~2000 copies of the gene that encodes 5S rRNA
    • 5S rRNA genes are present in a single cluster on chromosome 1.
    • 5S rRNA transcription takes place outside the nucleolus (done by RNA polymerase III).

    Transcription & Processing of rRNAs

    • Transcription of large 45S pre-rRNA molecules (includes 18S, 5.8S, 28S rRNAs)
    • Processing involves
      • Cleavages to make precursors for the large and small ribosomal subunits
      • Chemical modifications
    • Pre-rRNA is modified to create the individual rRNAs that go on to combine with proteins to create the ribosome components.

    Chemical Modifications and Nucleolytic Processing

    • pre-rRNA is chemically modified (involving RNA molecules and proteins)
    • pre-rRNA is processed through nucleolytic digestion, creating 18S, 5.8S and 28S rRNA molecules.
    • There are ~300 proteins involved.

    Other Nucleolus Functions

    • Site of other RNA production
    • Site of other RNA-protein complex assembly
    • U6 SnRNP (small nuclear ribonucleoprotein) part of mRNA splicing processes is an example.

    Chromatin

    • Chromatin is the packaging material for DNA

    • Chromatin is composed of DNA + proteins (histones and non-histone proteins)

    • Chromatin structure is important for cell division, preventing tangles and damage to DNA during division

    • Basic dyes bind to it; the Feulgen reaction is specific for identifying DNA

    Euchromatin

    • Lightly stained (invisible with light microscopy; visible with electron microscopy)
    • Less condensed; uncoiled and dispersed form of chromatin
    • Active chromatin; actively transcribed sequences
    • Contains many of the genes that are actively transcribed in a cell

    Heterochromatin

    • Dark staining

    • Dense, tightly packed form

    • In the nuclear periphery

    • Contains inactive, condensed DNA

    • Located at centromeres and telomeres

    • More than 10% of the mammalian genome is packaged into heterochromatin

    • The cell must condense and de-condense chromatin for DNA transcription

    Fine Structure of Chromatin

    • Basic structural unit = nucleosome

    • Roger Kornberg described the nucleosome in 1974

    • Chromatin spreads over a lower salt concentration solution to study via transmission electron microscopy (EM)

    • A 10 nm chromatin fiber has a beaded appearance

    • Nucleosome = a disc-shaped structure

    • DNA is coiled around the core of an octamer of histone molecules.

    • 5 major types of histones exist

      • H2A, H2B, H3, and H4 (nucleosomal)
      • H1 (linker)

    • Extended DNA length = 2 meters for a human somatic cell

    • The DNA needs to fit in a cell nucleus = only 5-10 µm in diameter

    • Histones create order and tightly wrap DNA within the cell nucleus

    • A histone octamer is formed via 2 copies of H2A, H2B, H3, and H4

    • DNA winds around the octamer core (approximately 146 bp and ~1.75 turns)

    • DNA connecting two nucleosomes = Linker DNA (~80 bp)

    • Histone H1 attaches to the Linker DNA, changing conformation.

    • Chromatin's further condensation is achieved through 30 nm chromatin fiber formation

    • 10 nm chromatin fiber folds into a thicker filament = 30 nm chromatin fibre

    • Interactions between H1 histones are important for 30 nm chromatin condensation structure

    • The 30 nm fibers coil into a more tightly condensed form.

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