WEEK 15 - chromosome structure + basic transcription

what is chromatin

DNA packaged with histone proteins

how does 2 metres of DNA fit in a nucleus

chromatin packed into highly ordered structures, which condenses when chromosomes divide

chromosome compaction in each stage of the cell cycle

metaphase - chromosomes highly compacted

interphase - less distinct, variable compaction (euchromatin vs heterochromatin

what are nucleosomes

basic units of chromatin - 147 bp of DNA wrap around histone octamer

which histones are involved in nucleosomes

H2A, H2B, H3, H4

what protein does chromatin condensation involve

histone H1

euchromatin

largely decompacted, light staining, actively transcribed genes, concentrates in middle of nucleus

heterochromatin

highly compacted, dark-staining, genes not transcribed, repetitive sequences

what is chromatin remodelling

alteration of chromatin structure to increase DNA accessibility for transcription

how can histone be remodelled

enzymes (HATs and HDACs) add or remove acetyl groups to lysine residues - reduce positive charge

how does chromatin remodelling affect dna accessibility

HATs add acetyl groups to positive lysine residues - reduces positive charge so reduces interaction with negative DNA

what enzymes involved in chromatin remodelling

HATs - histone acetyltransferases

HDACs - histone deacytlases

what does histone acetylation do

open up nucleosomes, increasing DNA accessibility for transcription factors

Beadle and Tatum experiment

proved that one gene leads to one enzyme/polypeptide

• each mutation caused a defect in only one enzyme

how do cells have different cell functions (process)

differential gene expression

how is differential gene expression regulated

combinations of different transcription factors

where is the primary control point for gene expression

at the initiation of transcription of a gene - very beginning of the protein production process

why is gene expression usually controlled at transcription

multiple proteins can be made from one mRNA

how is eukaryotic gene expression controlled

many regulatory proteins - greater flexibility

• prokaryotes - regulatory proteins controlled by nutrient availability

what is transcription + translation

produces mRNA complementary to a strand of DNA

converts RNA to amino acid sequence

how does RNA differ from DNA

single-stranded (with secondary structure)

ribose sugar

uracil instead of thymine

internal base pairing - fold into complex shapes

why does DNA use thymine instead of uracil

cytosine is unstable and can deaminate into uracil

components needed for transcription

nucleotide triphosphates (ATP,GTP,CTP,UTP)

DNA-dependent RNA polymerase

DNA template for base pairing

basic process of transcription

RNA polymerase links nucleotides together by condensation reactions - 5’ phosphate is added to 3’ OH end

characteristics of RNA polymerase

processive - a single binding results in hundreds of RNA bases polymerised

no primers

no proofreading

what are the 3 steps of transcription

initiation

elongation

termination

what are promotor sequences

sequencing signalling the start of translation

how is RNA polymerase recruited

sigma factors (transcription factors) bind to promotor sequences

what directs enzymes which strand of dna to translate

promotor sequences

transcription - process of initation

recruitment of RNA polymerase to promotor

• sigma factors bind to promotor sequences, recruiting rna polymerase, determining which genes are expressed

• promotors direct enzymes where to start and which strand/direction to translate

• dna unwinds, transcription begins at initiation site

transcription - process of elongation

RNA polymerase unwinds and reads DNA strand, adding nucleotides

• produced transcript is antiparallel to template

• no proofreading or mistake correction

  • ribonucleotide triphosphates as substrates, 2 phosphates are removed = energy

transcription - process of termination

specified by a specific DNA sequence

• the transcript forms a loop and falls away

• or, protein binding makes it detach