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Questions and Answers

What are chromosomes?

Chromosomes are the structures that contain the genetic material.

What is the genome?

The genome comprises all the genetic material that an organism possesses.

In bacteria, the genome is typically a single circular chromosome.

True

In eukaryotes, the genome refers to one complete set of mitochondrial chromosomes.

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

The main function of the genetic material is to store information required to produce an organism.

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

Which of the following are necessary for DNA sequences?

<p>Compaction of chromosomes</p> Signup and view all the answers

What are viruses?

<p>Viruses are small infectious particles containing nucleic acid surrounded by a capsid of proteins.</p> Signup and view all the answers

Viruses can replicate independently without relying on host cells.

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

Most viruses exhibit a wide host range, infecting various types of cells.

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

What is a viral genome?

<p>A viral genome is the genetic material of the virus, also known as the viral chromosome.</p> Signup and view all the answers

What can a viral genome be composed of?

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

Viral genomes are always the same size across all viruses.

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

What is the nucleoid?

<p>The nucleoid is the region within a bacterial cell where the bacterial chromosome is located.</p> Signup and view all the answers

A typical bacterial chromosome contains a few hundred different genes.

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

What are structural gene sequences?

<p>Structural gene sequences are DNA sequences that encode proteins.</p> Signup and view all the answers

What are intergenic regions?

<p>Intergenic regions are nontranscribed DNA sequences located between adjacent genes.</p> Signup and view all the answers

Most bacterial species contain a single type of chromosome.

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

A single origin of replication is required for each chromosome in a bacterial cell.

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

Short repetitive sequences in bacterial chromosomes play a role in DNA folding, replication, and gene expression.

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

How much must the bacterial chromosomal DNA be compacted to fit within the bacterial cell?

<p>The bacterial chromosomal DNA must be compacted about 1000-fold.</p> Signup and view all the answers

What are loop domains?

<p>Loop domains are formed when the bacterial chromosome is organized into loops that are anchored to the nuclear matrix.</p> Signup and view all the answers

The number of loops in bacterial cells varies according to the size of the bacterial chromosome.

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

What is DNA supercoiling?

<p>DNA supercoiling is a second important way to compact the bacterial chromosome, involving the twisting of the DNA helix.</p> Signup and view all the answers

Supercoiling can only reduce the compaction of DNA.

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

What are the two main enzymes involved in controlling DNA supercoiling in bacteria?

<p>The two main enzymes involved in controlling DNA supercoiling in bacteria are DNA gyrase and DNA topoisomerase I.</p> Signup and view all the answers

DNA gyrase introduces positive supercoils.

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

DNA topoisomerase I relaxes both positive and negative supercoils.

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

The total amount of DNA in eukaryotic species is typically smaller than that in bacterial cells.

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

Where are chromosomes located in eukaryotic cells?

<p>Chromosomes in eukaryotes are located in the nucleus.</p> Signup and view all the answers

How are chromosomes compacted in eukaryotic cells?

<p>Chromosomes in eukaryotes are highly compacted by the binding of many proteins to the DNA.</p> Signup and view all the answers

Eukaryotic genomes are always the same size across all species.

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

The difference in genome size between closely related species is always due to the presence of extra genes.

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

Eukaryotic chromosomes are always circular.

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

What are the three types of DNA sequences required for chromosomal replication and segregation in eukaryotes?

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

What is the function of the centromere?

<p>The centromere acts as a recognition site for the kinetochore proteins, which attach to the spindle fibers during cell division, ensuring accurate chromosome segregation.</p> Signup and view all the answers

What are telomeres?

<p>Telomeres are specialized DNA sequences located at both ends of the linear chromosome.</p> Signup and view all the answers

Genes are only found in the centromeric and telomeric regions of chromosomes.

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

Genes in lower eukaryotes are typically longer and have more introns than genes in higher eukaryotes.

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

What is sequence complexity?

<p>Sequence complexity refers to the number of times a particular base sequence appears in the genome.</p> Signup and view all the answers

What are the three main types of repetitive sequences?

<p>Highly repetitive</p> Signup and view all the answers

Moderately repetitive sequences are found a few times in the genome.

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

Highly repetitive sequences are always several hundred base pairs in length or longer.

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

Explain the process of renaturation of DNA strands.

<p>Renaturation of DNA strands is a process where complementary single-stranded DNA molecules re-associate to form a double helix.</p> Signup and view all the answers

The rate of renaturation is influenced by the concentration of the complementary partner.

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

Unique sequences renature faster than highly repetitive sequences.

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

About 60-70% of human DNA consists of unique sequences.

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

What is the repeating structural unit within eukaryotic chromatin?

<p>The repeating structural unit within eukaryotic chromatin is the nucleosome.</p> Signup and view all the answers

What are histone proteins?

<p>Histone proteins are basic proteins that bind to DNA and play a crucial role in the packaging and compaction of chromosomes.</p> Signup and view all the answers

An octamer of histone proteins consists of four copies of each of the four different histones (H2A, H2B, H3, and H4).

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

The linker histone H1 binds to both linker DNA and nucleosomes.

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

Nonhistone proteins have a minimal role in chromosome organization and compaction.

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

The 30 nm fiber shortens the total length of DNA about five-fold.

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

The 30 nm fiber structure has been fully determined.

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

Which of the following models have been proposed to explain the structure of the 30 nm fiber?

<p>Solenoid model</p> Signup and view all the answers

The two events, formation of nucleosomes and the 30 nm fiber, have shortened the DNA length about 100-fold.

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

What is the nuclear matrix?

<p>The nuclear matrix is a network of protein fibers that provides structural support to the nucleus and helps to organize the chromosomes.</p> Signup and view all the answers

Radial loop domains are formed when DNA is attached to the nuclear lamina.

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

The attachment of radial loops to the nuclear matrix plays a role in gene regulation.

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

Each chromosome in the nucleus is randomly located within the nuclear space.

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

What are the two main types of chromatin?

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

Euchromatin is tightly compacted and transcriptionally inactive.

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

Heterochromatin compacts even further than euchromatin.

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

What are the two types of heterochromatin?

<p>Constitutive heterochromatin</p> Signup and view all the answers

Constitutive heterochromatin can be converted to euchromatin.

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

Facultative heterochromatin can reversibly switch between euchromatin and heterochromatin.

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

The Barr body is an example of constitutive heterochromatin.

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

During interphase, most chromosomal regions are heterochromatic.

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

Metaphase chromosomes are highly condensed and undergo significant gene transcription.

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

The scaffold that anchors radial loops in metaphase chromosomes is formed from histone proteins.

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

Histone proteins are not required for the compaction of radial loops.

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

What are the two multiprotein complexes involved in forming and organizing metaphase chromosomes?

<p>The two multiprotein complexes involved in forming and organizing metaphase chromosomes are condensin and cohesin.</p> Signup and view all the answers

Condensin plays a critical role in sister chromatid alignment.

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

What does the acronym SMC stand for?

<p>The acronym SMC stands for Structural Maintenance of Chromosomes.</p> Signup and view all the answers

SMC proteins utilize energy from ATP to catalyze changes in chromosome structure.

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

During interphase, condensin is located in the nucleus.

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

Condensin compacts radial loops by reducing their diameter.

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

Cohesin remains attached at the centromere throughout mitosis.

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

Cohesin is responsible for holding sister chromatids together during the S phase.

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

The separation of sister chromatids in anaphase is facilitated by condensin.

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

Study Notes

Chromosome Organization and Molecular Structure

  • Chromosomes are structures containing genetic material, complexes of DNA and proteins.
  • The genome is all the genetic material an organism possesses.
  • Bacterial genomes are typically single, circular chromosomes.
  • Eukaryotic genomes refer to complete sets of nuclear chromosomes, and also include mitochondrial and chloroplast genomes in eukaryotes (plants also have a chloroplast genome).
  • The function of genetic material is to store information necessary for producing an organism; DNA accomplishes this via its base sequence.
  • DNA sequences are necessary for:
    • Synthesizing RNA and cellular proteins.
    • Proper segregation of chromosomes.
    • Replication of chromosomes.
    • Compaction of chromosomes (so they fit within living cells).

Viral Genomes

  • Viruses are small, infectious particles containing nucleic acid (DNA or RNA, single-stranded or double-stranded, circular or linear) surrounded by a protein capsid.
  • Viral genomes range in size from a few thousand to more than a hundred thousand nucleotides.
  • Viruses rely on host cells for replication.
  • Most viruses have a limited host range, infecting only specific host cell types.
  • Bacteriophages may have a sheath, base plate, and tail fibers.

Bacterial Chromosomes

  • Bacterial chromosomes are located in a region called the nucleoid, not a membrane-bound nucleus.
  • Bacterial DNA is in direct contact with cytoplasm.
  • Bacteria can have one to four identical copies of the same chromosome.
  • The number of chromosome copies depends on species and growth conditions.
  • Bacterial DNA is typically circular and contains a few thousand different genes, with structural genes (encoding proteins) representing the majority.
  • Intergenic regions (nontranscribed DNA between genes) are also present.
  • The chromosome has an origin of replication, genes, repetitive sequences and numerous hundreds of nucleotides in length.
  • To fit in the bacterial cell, the chromosomal DNA must be compacted about 1,000-fold.
  • This compaction occurs via the formation of loop domains (loops number by species size dependent) and helps to condense it about 10-fold.
  • Supercoiling further compacts the DNA and involves two main enzymes:
    • DNA gyrase (topoisomerase II) introduces negative supercoils using energy from ATP and relaxes positive supercoils.
    • DNA topoisomerase I relaxes negative supercoils.

Eukaryotic Chromosomes

  • Eukaryotic species have one or more sets of linear chromosomes, with each set composed of several different, linear chromosomes.
  • Eukaryotic genomes are generally larger than bacterial genomes, often due to accumulated repetitive DNA sequences.
  • Eukaryotic chromosomes are housed within the nucleus.
  • To fit in the nucleus, eukaryotic DNA must be highly compacted into chromatin.
  • Chromosomes contain origins of replication, centromeres, and telomeres for replication and segregation.
  • Genes are found between telomeric and centromeric regions, with a few hundred to several thousand per chromosome.
  • Lower eukaryotes have smaller genes with fewer introns, while higher eukaryotes have larger genes with many introns.

Repetitive Sequences

  • Sequence complexity refers to the number of times a base sequence appears in the genome.
  • There are three main types of repetitive sequences:
    • Unique or non-repetitive sequences found once or a few times in the genome.
    • Moderately repetitive sequences repeated several hundred to a few thousand times and include genes for rRNA and histones, origins of replication, and transposable elements.
    • Highly repetitive sequences found tens of thousands to millions of times, often short, clustered in tandem arrays (e.g. centromeres).

Renaturation Experiments

  • The rate of renaturation (pairing of complementary DNA strands) distinguishes types of repetitive sequences.
  • Highly repetitive sequences renature quickly due to many copies.
  • Unique sequences renature slowly due to the fewer copies.

Eukaryotic Chromatin Compaction

  • Eukaryotic chromosomes are highly compacted, and must be organized within the compact cell nucleus.
  • A single set of human chromosomes stretched end-to-end can exceed 1 meter, but cells are tightly compacted.
  • Chromatin compaction involves interactions between different types and families of DNA and proteins.
  • The basic repeating structural unit of eukaryotic chromatin is the nucleosome, wrapped around histones in a double-stranded DNA structure, consisting of an array of eight histone proteins.
  • Nucleosomes create a 'beads-on-a-string' morphology.
  • Histones are basic proteins containing positively-charged amino acids (lysin and arginine) binding the negatively charged phosphates along the DNA backbone, resulting in a compact morphology.
  • Five types of histones exist, with H2A, H2B, H3, and H4 core histones forming the octamer, and H1 linker histone further binding to DNA and nucleosomes.
  • Nucleosomes join to form 30 nm fibers.
  • The 30 nm fiber shortens the DNA another seven-fold.
  • This fiber undergoes further compaction through interactions with the nuclear matrix and the formation of radial loop domains to achieve substantial levels of compaction.
  • Interactions between the 30nm fiber and nuclear matrix proteins, and the radial loop domains shorten the DNA an additional 50-fold.

Heterochromatin vs Euchromatin

  • Eukaryotic chromatin is not uniformly compacted - Euchromatin are less condensed and transcriptionally active regions.
  • Heterochromatin are tightly compacted, transcriptionally inactive regions.
  • Two types of heterochromatin exist
    • Constitutive heterochromatin: permanently inactive regions.
    • Facultative heterochromatin: regions that can interconvert between euchromatin and heterochromatin (e.g. Barr body).

Metaphase Chromosomes

  • During M phase, chromosomes become highly compacted. Condensation involves two multiprotein complexes, condensin and cohesin:
    • Condensins: plays a critical role in chromosome condensation, and travels to the nucleus for M-stage chromosome assembly.
    • Cohesins: plays a crucial role in sister chromatid alignment.
  • Both condensins and cohesins contain a category of proteins called SMC (structural maintenance of chromosomes) proteins that change chromosome structure using energy from ATP.
  • At metaphase, sister chromatids are entirely heterochromatic, and the overall diameter of parallel chromatids is 1400 nm, before separating.
  • Metaphase chromosomes undergo little gene transcription.

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