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Description
Flashcards by J yadonknow, updated more than 1 year ago
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Created by J yadonknow
almost 7 years ago
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| Question | Answer |
| What is the nucleoid? | The nucleoid is a circular strand of supercoiled DNA in prokaryotes which is anchored in place by proteins which comprise ~2% of the mass of the nucleoid and isn't enclosed by a membrane |
| What is meant by the term "genophore"? | The genophore is all the genetic information of a prokaryote |
| Why is the nucleoid necessary? | The nucleoid is necessary as DNA needs to be compressed |
| What is typical of bacterial genomes? | Most often prokaryotes have only 1 chromosome that is circular and therefore has no free ends, most prokaryotic DNA is organised into genes and encodes proteins i.e. exons |
| What are the features of a plasmid?(7) | 1. Accessory circular DNA molecules seperate from the chromosome 2.Size range from 1kb-400kb 3.Cells may contain 1-200 copies of a particular plasmid 4.Plasmids carry non-essential genes 5.They are readily conjugated between cells 6.Have an origin of replication |
| Describe eukaryotic nuclear genomes | Most eukaryotic DNA are introns - non-coding DNA ~50% of the human genome is repetitive DNA i.e. non-coding sequences present in thousands of copies. |
| Describe Eukaryotic chromosomes | Eukaryotes have a number of linear chromosomes that are only visible at mitosis/meiosis. Two copies of a particular chromosomes are homologous to one another |
| Compare eukaroytic genome/prokaryotic genome (5) | (Usually) multiple chromo / (usually) one chromo Linear chromo / circular chromo (usually) Plasmids very rare / plasmids common Many introns / few introns Gene numbers similar between both organisms i.e. yeast ~4000 / E.Coli ~ 4300 |
| What is the composition and function of a mitotic chromosome?(4) | Compresses DNA 1/3 DNA (one long molecule) 1/3 histone proteins 1/3 non-histone proteins |
| Draw a chromosome | (chromosome/chromatid/centromere labelled) |
| Name the 5 relevant histone proteins | H1/H2A/H2B/H3/H4 |
| What do all histone proteins have in common? (5) | Basic:~20% of amino acids are arginine or lysine and are charged Leaving plenty of room for hydrostatic interactions between DNA and histone proteins Highly conserved between species |
| What is the packing ratio of DNA during mitosis? | packing ratio=length of DNA/Length of structure DNA is packed into e.g. smallest human chromo during mitosis=2um, contains 14mm DNA P.R.=14000/2=7000. |
| How is compression achieved during mitosis? | Formation of nucleosomes. When interphase nuclei are lysed in low ionic strength buffer and viewed through an electron microscope a "beads on a string" structure known as the '10mm filament' is observed. The beads are known as a nucleosome |
| Describe nucleosome structure | A nucleosome is 14bp of DNA wrapped around a core particle consisting of:2 H2A molecules / 2 H2B molecules / 2 H3 molecules/ 2 H4 molceules |
| Draw the structure of a nucleosome | |
| What is the role of Histone H1? | H1 binds to DNA outside core particle, sealing DNA to particle |
| How is a higher order structure achieved?(3) | Long regular structures made from nucleosomes (e.g. a helix with 6 nucleosomes per turn) are formed. This form doesn't exist in vivo. Nucleosomes are packed into the nucleus of an interphase cell in disorded chains with varying degrees of nucleosome densities |
| What are mitotic chromosomes? | Chromosomes visible at mitosis/meiosis consisting of a scaffold of non-histone proteins (e.g. condensins) that anchors long loops of nucleosomes in the 10mm filament form |
| Draw a schematic diagram of this compression | (10mm filament->nucleosome->scaffold ->mitotic chromosome |
| How can eukaryotic gene expression be regulated? | By covalent modification of histone proteins i.e. acetylation |
| What does acetylation do? | Acetylation removes positive charges from side chains and weakens interactions between histones and negatively charged DNA DNA associated with acetylated histones are much more readily transcribed than DNA associated with unmodified histone proteins. |
| Why are unmodified histone proteins less likely to lead to gene expression? | As DNA is tightly associated with nucleosomes and is not very accessible for transcription - The gene is switched off. DNA that has a looser association with nucleosomes are more accessible for transcription - gene is switched on. In vivo this is always due to DNA partially unwinding to form a replication fork. |
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