Cell Biology Chapter 5

What is the function of the DNA component of chromosomes?

DNA stores and transmits genetic information.

What is teh function of the protein component of chromosomes?

Proteins package and organize DNA, regulate gene expression, and support chromosome structure.

What are the three components of nucleotides?

A phosphate group, deoxyribose sugar, and a nitrogenous base.

What is the base-pairing rule in DNA?

A pairs with T (2 H-bonds); g pairs with C (3 H-bonds)

What are the major and minor grooves in DNA?

Spaces where proteins interact with bases; major groove = wider, more accessible; minor groove = narrower

How many base pairs per turn in B-DNA?

around 10 base pairs per turn

What is the approximate width of the DNA double helix?

around 2 nm

What experiment demonstrated with DNA carried genetic information?

Avery-Macleod-McCarty (showed DNA is the “transforming principle”) and Hersey-Chase (phage DNA enters cells, not protein0.

What bonds link nucleotides in a single strand?

Covalent phosphodiester bonds.

What bonds hold the two DNA strands together?

Hydrogen bonds between complementary bases.

How do these bonds affect DNA behavior?

Covalent bonds give stability; hydrogen bonds allow strands to separate easily during replication/transcription.

how does complentary base pairing lead to uniform double helix?

Purine— pyrimidine pairing keeps width constant and causes twisting due to interaction between stacked bases

What is karyotyping?

A method of visualizing chromosomes under a microscope to detect number and structural abnormalities

How is karyotype prepared?

Cells arrested in metaphase → swollen → stained → chromosomes photographed and arranged by size and shape

Define gene

A DNA segment that encodes a functional RNA or protein

Define genome

The entire DNA content of an organism

Define chromosome

A single DNA molecule with proteins, containing part of the genome

How do genome differ among species?

Species vary widely in genome size and chromosome number; complexity does not strictly correlate with genome size

Function of centromere?

Attachment site for kinetochore; ensured chromosome segregation

Function of telomeres?

Protect chromosome ends; prevent degradation and fusion

Function of replication origin?

Sites where DNA replication begins

What makes certain interphase chromosomes unique in their location in the nucleolus?

rRNA gene clusters localize in the nucleolus for ribosome assembly

Does genome size correlate with organism complexity?

Not reliably; many simple organisms have large genomes

Does gene number correlate with complexity?

Only partially; regulatory complexity matter more than gene count

Why is “junk DNA” thought to have function?

It includes regulatory elements, noncoding RNAs, structural sequences, and influences gene expression and chromatin architecture

What is the structure and function of the nucleolus?

Dense region of rRNA transcription and ribosomal subunit assembly; formed around rDNA clusters.

Compare interphase vs mitotic chromosome compaction.

Interphase: loosely organized, partially condensed

Mitotic: highly condensed, visible under light microscope

Role of histone proteins?

Package DNA into nucleosomes; regulate access to DNA. H1 secures DNa entry/exit from nucleosome.

role of non-histone proteins?

Regulate gene expression, replication, repair, and higher-order chromosome structure

What is a nucleosome?

DNA wrapped around histone core + linker DNA (around 200 bp)

What is a nucleosome core particle?

Only the 147 bp of DNA wrapped around the histone octamer.

Describe the structural organization of eukaryotic chromosomes.

DNA → nucleosome → 30 nm-fiber → looped domains → chromatin → chromosome

How can components be isolated?

Nuclease digestion → removes linker DNA → core particles isolated; salt extraction→ releases histones

Define nucleosome.

DNA wrapped around histone octamer.

Define chromatin.

DNA + proteins forming chromsomes.

Define chromosomes

Fully packaged DNA unit visible during mitosis

Define euchromatin.

Loosely packed, transcriptionally inactive chromatin.

Define heterochromatin.

Tighly packed, transcriptionally inactive chromatin.

Why would a cell decondense a DNA region?

Euchromatin = open

to allow transcription, replication, or repair proteins access

Structural differences between euchromatin and heterochromatin?

Euchromatin: open

Heterochromatin: compact

Gene activity in euchromatin and heterochromatin.

Euchromatin: active

Heterochromatin: silent

Location of euchromatin and heterochromatin along chromosomes

Euchromatin: throughout arms

Heterochromatin: centromeres and telomeres

How does heterochromatin silence genes?

Spreads compact structure, preventing transcription factor binding; often via histone modifications

Example of heterochromatin-mediated silencing?

X-chromosome inactivation (Barr Body)