Lecture 16: Chromosomes

Comprehensive practice questions covering chromosome structure, DNA packaging, and transposable elements based on Lecture 18 by Dr. Polly Gravells.

What is the term for the organized representation of all the chromosomes in a eukaryotic cell during metaphase?

The karyotype.

According to Figure 4-63, what do individual chromosomes occupy even within interphase nuclei?

Distinct subnuclear territories.

Under an electron microscope, what structure does interphase chromatin resemble?

Beads on a string, where the beads are nucleosomes.

What is the relationship between the $30\,nm$ fibre and the "beads on a string" interphase chromatin?

The $30\,nm$ fibre is a supercoiled array of nucleosomes.

Which part of the 8 core histone subunits projects out from the nucleosome core to interact with other proteins?

The N-terminal tails.

What is the role of linker histones such as H1?

They strap DNA onto histone octamers, limit DNA movement, stabilize the $30\,nm$ fibre, and facilitate transcriptionally silent heterochromatin.

Which dynamic structure allows interphase chromatin to condense and decondense reversibly without becoming knotted?

Fractal globules (globules within globules).

In interphase cells, what type of DNA composition is typically found at the nuclear periphery?

Transcriptionally inactive DNA.

What enzyme is responsible for replicating specialized repetitive DNA sequences at chromosome ends?

Telomerase.

What is the specific repetitive sequence found in human telomeres, and how long can the overhanging arrays be?

$5'\, \text{TTAGGG}\, 3'$; they can be several hundred nucleotides long.

What type of specialized DNA repeats are found in centromeres that help form condensed chromatin?

Alpha-satellite DNA repeats.

Differentiate the binding targets of Kinetochore Inner Plate and Outer Plate proteins.

Inner Plate proteins bind to chromatin containing alpha-satellite DNA, while Outer Plate proteins bind to microtubules of the mitotic spindle.

Describe the structure of the kinetochore in yeast as shown in Figure 4-48.

It is a basket that links a single nucleosome of centromeric chromatin to a single microtubule.

What percentage of the human genome is composed of genes, and what percentage actually encodes information for making cellular proteins?

Genes account for $20\%$; only $1.5\%$ encodes for protein information.

What two factors accompany increasing biological complexity in organisms?

(a) Increasing numbers of protein-coding genes and (b) increasing amounts of non-protein-coding DNA.

What is the function of cis-regulatory information in non-protein-coding DNA?

It determines where and when in the body adjacent protein-coding genes are transcribed.

Approximately what percentage of the Human genome is composed of repeated DNA sequence elements?

$\sim 50\%$.

Identify the three main types of repeated DNA sequences (Transposons) in the human genome.

1) DNA Transposons, 2) Retroviral retrotransposons, and 3) Non-retroviral polyA retrotransposons.

By what mechanism do DNA transposons move, and what enzyme do they require?

They move via a cut-and-paste mechanism requiring the enzyme Transposase.

What are three examples given of DNA transposons?

P-element (fly), Activator-Dissociator (maize), and Tn3 / Tn10 (E. coli).

Who discovered the Activator-Dissociator (Ac-Ds) DNA transposon of Maize in 1952?

Barbara McClintock.

How do retroviral retrotransposons move throughout the genome?

They replicate via RNA intermediates and produce new DNA copies using self-encoded Reverse Transcriptase.

Name the specific example provided for a Human non-retroviral PolyA retrotransposon that uses a copy-and-paste mechanism.

Human L1 elements (LINE-1 elements).

What human disease is associated with insertions of L1 elements disrupting genes?

Haemophilia.

What do the acronyms LINEs and SINEs represent?

Long Interspersed Elements and Short Interspersed Elements.

Most copies of transposable elements in the genome are described as what?

Defective, ancient relics of formerly functional elements with mutations that prevent protein expression.

From which single copy gene did Human Alu and Mouse B1 elements originally evolve?

The $7SL\, \text{RNA}$ gene.

What are two potential adaptive advantages of transposable element expansion in higher organisms?

Increasing raw material for evolution and creating novel phenotypes that can accelerate adaptation and speciation.

How do transposable elements contribute to gene regulation?

Some have evolved into regulatory elements that control gene expression.

What are the three specialized DNA sequences mentioned that facilitate replication and segregation?

Telomeres, Centromeres, and Origins of replication.