L1- Nucleic acids and chromatin

Explain the structure of DNA

• Double helix- strands are antiparallel (3’—→5’ and 5’ —→3’)

• A —→T (forms 2 hydrogen bonds)

• C —-→G (forms 3 hydrogen bonds)

• Phosphodiester binds between nucleotides to form the backbone of te DNA

WHat did Francis Crick and James Watson do?

First described the molecular structure of DNA

What did Crick, Watson and Maurice Wilkins do?

Won a nobel prize in 1962

What did Rosalind Franklin do?

Generated the X-ray diagram, but she died from cancer

How many chromosomes do humans have?

46 chromosomes, 23 pairs

What percentage of base pairs of DNA are used as coding proteins?

2%

What percentage of genes do humans share? What changes are different as a result?

99.9%- the 0.1% counts for differences in appearance, race etc

What is a CpG island?

A region of DNA with a high frequency of cytosine and guanine bases, commonly found near promoter regions

Usually where DNA methylation occurs (CH3 added to cytosine base)

What is the functional difference between methylated and unmethylated CpG sites?

Mehtylated- less accessible

Unmethylated- more open, transcription machinery can bind and genes are more active

What is epigenetic memory?

The ability of a cell to remember which genes should be switched on or off

What is a gene?

A length of DNA that contains the coding sequence for protein

What is chromatin?

A complex of DNA and proteins packaging genetic material inside the nucleus of eukaryotic cells

Two forms: euchromatin and heterochromatin

Describe the difference between the two forms of chromatin

Euchromatin- less condensed, can be transcribed. Usually genetically active (gene expressed)

Heterochromatin- highly condensed, not typically transcribed. Usually genetically inactive (gene silenced)

What is a nucelosome?

DNA (negatively charged) wrapped around a histone protein (positively charged)

What is a chromatosome?

Nucelosomes are folded around each other to form a fibre- nucleosome + another histone protein

Describe the structure of a chromosome

Densely packed structure of DNA and associated proteins, usually appearing during cell division

What is the purpose of chromatin remodelling?

Dynamic modification to control DNA accessibility for transcription, replication and repair

Allows cells to control which genes are expressed

What are the two mechanisms of chromatin remodelling?

1. Histone modification- acetylation, methylation, phosphorylation

2. ATP dependent remodelling- energy from ATP used to physically move, remove or restructure nucleosomes

What is DNA methylation?

CH3 groups are added to CpG sites (not necessarily CpG islands) by DNA methyltransferase

Results in gene silencing as it promotes chromatin compaction, and transcription factors cannot bind to promoter regions

What is the definition of epigenetic effects?

Influence the phenotypic changes by changing gene activity, without altering the genotype (DNA sequences)

Inherited during cell division (epigenetic memory)