nucleic acids and chromatin

describe the structure of DNA

• double stranded

• building blocks = nucleotides

• base pairing rules: A,T = 2 hydrogen bonds, C,G = 3 hydrogen bonds

• double stranded DNA is antiparallel, one strand runs 5’ to 3’ and one runs 3’ to 5’

• contain phosphodiester bonds that link two nucleotides to form the backbone, hydroxyl group on 3’ carbon of one sugar joins phosphate group on the 5’ carbon of the next nucleotide

why in junk DNA useful

• CpG islands - DNA methylation can make regions less accessible, unmethylated regions are more open to transcription machinery.

• Histone modifications - create open or closed chromatin, affecting gene activity.

• Non-coding RNAs - affect the stability and activity of mRNA.

• Control elements and enhancers - regulate transcription by binding transcription factors.

• Introns - contribute to gene regulation and chromatin organisation.

what information is encoded within DNA

• gene

• transcriptional factor binding site

• CpG island

what is epigenetic memory

• how cells remember their identity

• epigenetic modifications allow individual cells to maintain a specific gene expression

• modifications can be copied during DNA replication

describe DNA

• molecule that stores genetic information

• made of nucleotides

• exists as a double helix

• serves as a template for making proteins and provides instructions for cell function

summary of structure of chromosomes

• a highly organised, compact structure of DNA and histone proteins

• packages long DNA molecules to fit inside the nucleus

• becomes highly condensed and visible during cell division

• contains many coding regions, regulatory regions and other non coding sequences

• contains centromeres, telomeres and sister chromatids

what is chromatin

• a complex of DNA and proteins that packages genetic material inside the nucleus of eukaryotic cells

• exists in two forms - euchromatin + heterochromatin

• euchromatin - less condensed, can be transcribed

• heterochromatin - highly condensed, not transcribed

what are histones

• basic protein (positively charged) that DNA wraps around to form nucleosomes

• can form dimers and then octamers

what is a nucleosome

• DNA wrapped around histone octamer, each unit of tiny beads on a string with a diameter of approximately of 11nm

• less than two turns of DNA wrapped around the histone octamer

• subunit of chromatin

what is a chromatosome

• nucleosomes are then folded around each other to form a fibre

• e.g. histone H1 + nucleosome

• chromatin fibres are then folded into loops

DNA packaging

• chromatin is a double stranded helical structure of DNA

• DNA is complexed with histones to form nucleosomes

• each nucleosomes consists of 8 histone proteins which the DNA wraps around

• a chromatosome consists of a nucleotide plus a histone

• the nucleosomes fold to produce a fibre that forms loops

• fibres are compressed and folded to produce a wide fibre

• tight coiling of the 250nm fibre produces the chromatid of a chromosome

what is chromatin remodelling

• the dynamic alteration of chromatin structure to control how accessible DNA is - regulates whether DNA can be used for transcription, replication and repair.

• ATP-dependent chromatin-remodelling complexes move, eject or restructure nucleosomes.

• Histone modifications - acetylation, deacetylation, methylation and phosphorylation.

• changes can expose a gene’s promoter, allowing the transcription machinery to bind more easily and increasing transcription - chromatin may also become more compact, reducing DNA accessibility and gene activity.

why is it important to be able to control how DNA is used

• different cells in an animal have different functions

• an animal has different needs throughout its life

• animals need to adapt to different environments

what is histone modification

• when histone tails are chemically modified - affects gene expression

describe DNA methylation

• occurs in CpG sites to add a methyl group to cytosine bases in DNA

• CpG island are regions of DNA rich in C-G sequences often near gene promoters

• DNA methyltransferases - add methyl group, TET - remove the methyl group

• results in gene silencing

what is the impact of epigenetic effects

• influence the phenotypic changes by changing the gene activity without altering the genotype

• major mechanisms - DNA methylation, histone methylation/acetylation.phosphorylation, chromatin remodelling

• turns genes on and off

• inherited during cell division (epigenetic memory), inherited to offspring if happened in germ cells, inherited changes without changing genetic information