Date post: | 17-Jul-2015 |
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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