LECTURE CONNECTIONS

11 | Chromosome Structure

© 2009 W. H. Freeman and Companyand Transposable Elements

11.1 Large Amounts of DNA Are Packed into a Cell

• One type of tertiary structure is supercoiling.

• Supercoiling

• Positive supercoiling

• Negative supercoiling

• Topoisomerase: the enzyme responsible for adding and removing turns in the coil

11.2 A Bacterial Chromosome Consists of a Single Circular DNA Molecule

11.3 Eukaryotic Chromosomes Are DNA Complexes to Histone Proteins

• Each eukaryotic chromosome consists of a single, extremely long molecule of DNA.

• The packing of eukaryotic DNA is not static but changes regularly in response to cellular processes.

• Functional DNA must be capable of partly unfolding and expanding so that individual genes can undergo replication and transcription.

• Humans have 3.2 billion base pairs of DNA

Chromatin Structure

• Euchromatin- undergoes the normal process of condensation and decondensation in the cell cycle;

• Heterochromatin- remains in a highly condensed state throughout the cell cycle;

• Histone proteins: small, positively charged. Five major types: H1, H2A, H2B, H3, and H4.

H1: acts as a clump around the nucleosome

• A heterogeneous assortment of nonhistone chromosomal proteins make up about half of the protein mass of the chromosome.

Chromatin Structure

• Nucleosome is the fundamental repeating unit of chromatin.

• The core particle and its associated H1 histone are called the chromatosome.

• Linker DNA: stretch of DNA separating two nucleosomes.

• High-order chromatin structure: when nucleosomes fold on themselves to form a dense, tightly packed structure.

Changes in Chromatin Structure

• Polytene chromosome: giant chromosomes found in certain tissues of Drosophila and some other organisms.

• Chromosomal puffs: localized swellings of the chromosome. Chromatin have a relaxed structure.

DNase I sensitivity is correlated with the transcription of globin genes in erythroblasts of chick embryos. The U gene encodes embryonic hemoglobin; the αD and αA genes encode adult hemoglobin.

What is the nature of the change in chromatin structure that produces chromosome puffs and DNA I sensitivity?

• Histones might loose their grip on the DNA.

• Acetylation alters chromosome structure.

- Enzymes called acetyltranferases attached acetyl groups to lysine amino acids on the histone tails.

• Methylation and phosphorylation also alter chomatin structure.

• Modified histones are often found in regions of active gene expression.

Changes in Chromatin Structure

• Centromere structure

Centromeric sequences are the binding sites for the kinetochore, to which spindle fibers attach.

Some organisms have chromosomes with diffuse centromeres, and spindle fibers attach along the entire length of each chromosome.

Centromeres consist of particular sequences repeated many times.

• Telomeres are the natural ends of a chromosome.

• Each telomere must serve as a cap that stabilizes the chromosome.

• Telomeres also provide a means of replicating the ends of chromosomes (Chapter 12).

• Telomeres sequences usually consist of a series of C nucleotides followed by several A or T nucletiodes or both.

end of chromosome 5’-CCCTAA toward centromere 3’-GGGATT

• The G-rich strand often protrudes beyond the complementary C-rich strand at the end of the chromosome.

• The G-rich strand at the telomere is longer than the C-rich strand.

Special POT proteins bind to G-rich single-stranded sequence, protecting the telomere from degradation and preventing the ends of the chromosomes from sticking together.

In mammalian cells, the G-rich strand folds over pairs with a short stretch of DNA to forma t-loop to protect the telomeres from degradation.

Concept Check 3

Neutralizing their positive charges would have which effect on the histone proteins?

a. They would bind the DNA tighter.

b. They would separate from the DNA.

c. They would no longer be attracted to each other.

d. They would cause supercoiling of the DNA.

11.4 Eukaryotic DNA Contains Several Classes of Sequence Variation

• Denaturation- when the DNA in solution is heated, the two strands together are weakened and, with enough heat, the two nucleotide strands separate completely.

Melting temperature: depends on the base sequence of the particular sample.

(DNA sequence in C-G or A-T????)

• Renaturation- if single-stranded DNA is slowly cooled, single strands will collide and _________ bonds will again form between complementary base pairs.

Types of DNA Sequences in Eukaryotes

• Unique sequence DNA: sequences that are present only once or, at most, a few times in the genome.

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

(example: in humans there are seven β-globin genes, clustered together on chromosome 11).

Types of DNA sequences in Eukaryotes

• Repetitive DNA

• Moderately repetitive DNA: 150 ~ 300 bp long that are repeated many thousand of times.

(example: genes for rRNAs and tRNAs)

• Tandem repeat sequences: appear one after another and tend to be clustered.

• Interspersed repeat sequences: are scattered throughout the genome

• Short interspersed elements: SINEs: Alu element• Long interspersed elements: LINEs

Types of DNA Sequences in Eukaryotes

• Repetitive DNA

• Highly repetitive DNA: less than 10 bp long

These short sequences are present in hundreds of thousands to millions of copies that are repeated in tandem and clustered in certain regions of the chromosome (examples: centromeres and telomeres).

•Microsatellite DNA