Chromosome structure

Centromere

Kinetochore proteins

Origin of replication

Origin of replication

Origin of replication

Origin of replication

Telomere

Telomere

GenesRepetitive sequences

Chromosome Organization• Genes located between centromere & telomeres

– hundreds to thousands of genes– lower eukaryotes (i.e. yeast)

• Genes are relatively small• Very few introns

– higher eukaryotes (i.e. mammals)• Genes are long• Have many introns

• Non-gene sequences– Repetitive DNA

• Telomere• Centromere• Satellite

Chromosomes vs. Chromatin

Chromosomes• Tightly packaged DNA• Found only during cell

division• DNA is not being used for

macromolecule synthesis

Chromatin• Unwound DNA• Found throughout

Interphase

• DNA is being used for macromolecule synthesis

Homologous Chromosome Pairs• Eukaryote chromosomes come in pairs (homologues)

• Normal humans have 46 chromosomes in 23 pairs

• One chromosome of each pair comes from an individual’s

mother, the other comes from their father

• Homologous Chromosomes (chromosome pairs) carry genes

that control the same Characters, e.g., eye color, blood type,

flower color, height, etc.

• Homologous chromosomes have nearly identical structure,

banding patterns, and nucleotide sequences

• Locus: Physical site on chromosomes where given gene is located

• Allele: Different forms of the same gene, e.g., A, B, or O blood-type alleles

Autosomes & Sex ChromosomesAutosomes: Found in both males and females

• In humans there are 22 pairs of autosomes

• Autosomes that are the same size (& structure) are called homologues

Sex Chromosomes: Determine an individual’s gender

• One pair of chromosomes (X and Y)

• The X and Y chromosomes are not homologous

• The X chromosome is much larger than the Y chromosome

and contains many genes

• The Y chromosome has only a small number of genes

• In humans and other mammals females are XX and males

are XY

• Lesson 2 - Chromosome structure• The DNA compaction problem• The nucleosome histones (H2A, H2B, H3, H4)• The histone octamere • Histone H1 the linker histone• Higher order compactions• Chromatin loops and scaffolds (SAR)• Non histone chromatin proteins• Heterochromatin and euchromatin• Chromosome G and R bands• Centromere

• Take 4 meters of DNA (string) and compact them

into a ball of 10M. Now 10M are 1/100 of a

mm and a bit small to imagine – so now walk

from here to the main entrance let say 400 meters

and try to compact it all into 1 mm.

• Histone proteins basic (+ charged lysine & arginine)amino acids that bind DNA backbone

• Four core histones in nucleosome– Two of each of H2A, H2B, H3 & H4

• Fifth histone, H1 is the linker histone

Figure 10.14

Nucleosomes

Figure 10.14

Nucleosomes

Figure 10.21

10-64Figure 10.21

Compaction level in euchromatin

Compaction level in heterochromatin

During interphase most chromosomal

regions are euchromatic

Compaction level of interphase chromosomes is not uniform

Euchromatin Less condensed regions of chromosomes Transcriptionally active Regions where 30 nm fiber forms radial loop domains

Heterochromatin Tightly compacted regions of chromosomes Transcriptionally inactive (in general) Radial loop domains compacted even further

Heterochromatin vs Euchromatin

Types of Heterochromatin

Figure 10.20

• Constitutive heterochromatin– Always heterochromatic– Permanently inactive with regard to transcription

• Facultative heterochromatin– Regions that can interconvert between euchromatin and

heterochromatin– Example: Barr body