Chapter 11- Chromosome structure

Supercoiled DNA is ___ or ___, causing it to twist on itself and occupy less space.

overwound; underwound

What type of enzyme adds or removes rotations by cutting the DNA strand?

topoisomerases

Why is most DNA supercoiled and underrotated?

so it is easier to open for replication and transcription

When does positive supercoiling occur?

when DNA is overrotated; the helix twists on itself

When does negative supercoiling occur?

when DNA is underrotated; the helix twists on itself in the opposite direction

What is chromatin?

DNA closely associated with protein

Euchromatin

undergoes processes of condensation and decompensation throughout the cell cycle, contains most of the functional genes; unique sequences

Heterochromatin

remains highly condensed throughout the cell cycle; tends to have few genes; tends to be very repetitive

Histone proteins

are most abundant in chromatin: small, positively charged, five main types

What are the five main types of histone proteins?

H2A, H2B, H3, and H4 form the octamer nucleosome core; H1 binds to the outside and "locks" DNA in place

Chromatin condensation in Euchromatin and Heterochromatin

Less condensed in Euchromatin, more condensed in Heterochromatin

Location of Euchromatin and Heterochromatin

Euchromatin on chromosome arms, Heterochromatin at centromeres, telometes, and other specific places

Type of sequences in Euchromatin and Heterochromatin

Unique sequences in Euchromatin, Repeated sequences in Heterochromatin

Presence of genes in Euchromatin and Heterochromatin

Many genes in Euchromatin, Few genes in Heterochromatin

When are Euchromatin and Heterochromatin replicated?

Euchromatin throughout S phase, Heterochromatin during late S phase

When does transcription occur in Euchromatin and Heterochromatin?

Often in Euchromatin, Infrequent in Heterochromatin

What is the frequency of crossing over in Euchromatin and Heterochromatin?

Common in Euchromatin, Uncommon in Heterochromatin

What is chromatin composed of?

nucleosome, linker DNA, high-order chromatin structure

Nucleosome

DNA wrapped around a histone core

Histone H1

"clamp" on the outside of the nucleosome

Linker DNA

DNA between nucleosomes

High-order chromatin structure

30 nm fiber

300 nm loops

250 nm wide fiber

At the simplest level, chromatin is a

double-stranded helical structure of DNA

DNA is complexed with histones to form

nucleosomes

Each nucleosome consists of _ histone proteins around with the DNA wraps _.__ times.

8; 1.65

The nucleosomes fold up to produce a

30 nm fiber

What does the 30 nm fiber form?

loops averaging 300 nm in length

The 300 nm loops are compressed and folded to produce

a 250 nm wide fiber

Tight coiling of the 250 nm fiber produces what?

the chromatid of a chromosome

What is the fundamental repeating unit of chromatin?

the nucleosome

DNA isolated from nuclei looks like what under a microscope?

beads on a string

How are polytene chromosomes created?

repeated rounds of DNA replication with no cell division

Where are polytene chromosomes located in drosophila larvae?

salivary glands

What are chromosomal puffs?

decondensed areas of chromatin where transcription is taking place

Is chromatin structure altered in transcription?

Sensitivity of DNA to digestion by DNase I is correlated with gene expression, suggesting that chromatin structure changes in the course of transcription

Eukaryotic chromosomes possess centromeres and telomeres; both are mostly ______

heterochromatin

What happens to chromosome fragments that lack centromeres?

they are lost in mitosis

Telomere structure: ends of chromosomes; provide a means to

replicate the ends of linear chromosomes

What does telomerase do?

replicates the ends of chromosomes (telomeres), active in germline cells and some somatic cells

Highly repetitive DNA sequences are found in

telomeres

What is the C value?

"constant value"; technically the haploid genome size per nucleus in picograms

What does heating DNA cause?

denaturation

What does denaturation define?

the melting temperature

Cooling DNA slowly allows what?

complementary strands to renature

Unique-sequence DNA

Gene family: similar but not identical copies of unique DNA sequences that arose through duplication of an existing gene

Repetitive DNA

Moderately repetitive DNA, Highly repetitive DNA

Moderately repetitive DNA

(around 150-300 bp long)

tandem repeat sequences

interspersed repeat sequences (SINES and LINES)

Highly repetitive DNA

less than 10 bp long

microsatellite DNA, centromeres and telomeres

Which organelles contain DNA?

mitochondria, chloroplasts, nucleus

Where are genes required for organelle function encoded?

nucleus

What does the endosymbiotic theory propose?

mitochondria and chloroplasts in eukaryotic cells arose from bacteria

Many modern protists are hosts to

endosymbiotic bacteria

Mitochondria and chloroplasts are similar in size to bacteria and

have their own DNA

Gene sequences in mtDNA and cpDNA are most similar to

bacterial DNA sequences

The ribosomes (essential for protein translation) are similar in

size, shape, and sequence to bacterial ribosomes