Lecture 5.docx

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Biological Sciences
Rahul Warrior

Lecture 5 • Chromatin consists of DNA and proteins • Chromatin is the material of which chromosomes are composed • Chromatin is only visible by light microscopy during mitosis when it condenses and compacts G1 – everything multiples except for chromosomes S – chromosomes duplicates G2 – checks if everything is duplicated M – mitosis The amount of compaction of DNA differs from one from another Some parts are replicated earlier than others. For ex. Some bubbles are larger than others in the other diagram Even during interphase, you can see different chromosomes associated with different regions Chromosome appear to be anchored by their centromeres.Adjacent interchromosomal domains that are DNA scarce separate chromosomal territories.Active genes appear to be near these boundaries perhaps because transcription components have easier access or RNA diffusion is more rapid. Chromatin – DNA + protein Chromosome is made up of chromatin. The proteins are diff. types. The main types are histone proteins. Other proteins are nonhistone proteins. Just focus on histone proteins. Chromosome compostion • Each chromosome is ~ half DNA and half protein • About half of the proteins are histone proteins, small baasic proteins that tightly bind DNA • The remaining proteins, the nonhistone proteins, are very diverse and perform a variety of tasks in the nucleus Histones • Histones are the major class of proteins associated with DNA • There are 5 major types of histones in all eukaryotes H1, H2A, H2B, H3 and H4 • 20-30% of the amino acids in histones are lysine and arginine—DNA has a negative charge • The remaining nonhistone proteins, are very diverse and perform a variety of tasks in the nucleus • Histones form circular structures called nucleosomes and DNA is wound around them. 8 protein structure. 2 H2A, H2B, H3, and H4 • 2 H3/H4 dimers assemble to form a tetramer. Once this is formed, 2 H2A/H2B dimers associate with the tetramer to form the octamer. • H1 is not part of the core nucleosome H1 helps pack the histone octomers together. Looks like beads on a string Chromatin loops of 20 to 100 kb are anchored to the chromosome scaffold by nonhistone proteins at sites called MARs (matrix attachment regions) The radial loop-scaffold model suggests that the loops gather into “rosettes” and are further compressed by nonhistone proteins Metaphase chromatin is compacted 250-fold compared to the 300-nm fiber The regions of DNA that has to be copied into RNA has to be more relaxed. The part of has to decondensed so RNA poly can access that region. It is the region being actively transcribed. Chromosome compaction allows for efficient separation of chromosomes at anaphase. The chromatin loops formed during cond
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