Chromosomes Flashcards
Describe chromatin structure.
DNA is condensed into chromatin (means it cannot be expressed)
If chromosomes were de-condensed it would be a bead on a string structure. Each bead is a nucleosome
DNA is wrapped around the nucleosome
Nucleosomes are made up of 8 histone proteins.
Chromosome structure is dynamic and heterogeneous.
Histone structure and modification?
DNA wraps tightly around the histone proteins
Histone proteins are made of a high proportion of positively charged AA so it attracts DNA.
it has a positive N-terminus AA tail that will attract medications (methylation, acetylation and phosphorylation)
How do histone modifications effect DNA expression?
When DNA is bound to histone proteins the gene is not expressed.
Some acetyl groups are negatively charged so it will repel DNA, DNA does not bind and so is expressed.
Methyl groups are positively charged so will attract the DNA, which will bind and so the gene will not be expressed.
Why does DNA form a helix structure?
Double stranded with purine paired with pyrimidines, AT = 2 H bonds, GC = 3 H bonds. Purines are bigger than pyrimidines so it forms a helix.
DNA replication is bidirectional, what does this mean?
DNA is synthesised in both directions from the replication fork.
What is the function of helicase and topoisomerase?
Helices unwinds the double strand. As the double strand starts to unwind the rest of the strand supercoils, topoisomerase forms cuts in the supercoil so it doesn’t get too tight.
What is the function of DNA polymerase?
DNA polymerase makes the new strand.
Add dNTP to the 3’ end of the growing strand, triphosphate acts as an energy store - when the dNTP joins 2 phosphates are given off = energy.
phosphodiester bonds form.
DNA polymerase needs a primer on the single strand to start replication. Ligase binds the adjacent bases.
How do Okazaki fragments allow replication in 3’ to 5’ direction?
DNA polymerase can only synthesise DNA in 5’ to 3’ direction. Okazaki fragments act as primers on the 3’ to 5’ strand and DNA polymerase works to synthesise the strand between fragments in 5’ to 3’ direction. Ligase binds the Okazaki fragments.
