Unit 8 -> Cell division

Can all eukaryotic cells divide?

No

What is a homologous pair of chromosomes?

-a pair of chromosomes: 1 maternal (from mother)/1 paternal (from father)
-carries same genes but different alleles - there are 23 pairs in humans

What are the stages of the cell cycle?

interphase, mitosis, cytokinesis
-the longest part is interphase

What causes movement from one stage of the cell cycle to the next?

Chemicals called cyclins and they are proteins

What happens during interphase?

-proteins needed for growth are synthesised
-DNA is copied, chromosomes are replicated
-checks for incorrectly replicated DNA

What are the stages of mitosis?

prophase, metaphase, anaphase, telophase

What happens during prophase?

-chromosomes condense
-spindle fibres form
-nuclear envelope breaks down

What happens during metaphase?

-chromosomes line up in the middle of the cell
-spindle fibres connect to the centromeres of the sister chromatids

What happens during anaphase?

-spindle fibres shorted
-centromeres of sister chromatids split and 2 chromosomes produced
-these chromosomes are then pulled to opposite poles of the cell

What happens during telophase?

-Chromosomes uncoil and become thinner
-spindle fibres break down
-nuclear envelope reforms.

What happens during cytokenesis?

cytoplasm divides and produces 2 new genetically identical daughter cells

What can uncontrolled mitosis lead to?

The formation of tumours and cancers

What is a tumour?

A mass of abnormal cells undergoing uncontrolled mitosis

How do cancer treatments work?

-They are targeted at controlling the rate of cell division.
-They may affect DNA synthesis, spindle formation or anything else that will cause DNA replication to be inhibited
-They mainly affect cells that divide rapidly such as cancerous cells with uncontrolled mitosis, but this also affects hair cells which is why things such as chemo cause hair loss

What happens during binary fission?

-The single, circular DNA molecule undergoes DNA replication
-Any plasmids present undergo DNA replication
-The parent cell divides into two cells, with the cytoplasm roughly halved between the two daughter cells
-The two daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids

Are the daughter cells produced in binary fission still genetically identical?

Yes, they may not have the same number of plasmids but they still contain the same genetic code and just have more/less copies of it.

Why does binary fission provide proof that mitochondria/chloroplasts were once prokaryotes?

They replicate in the same way as binary fission during interphase

Why don't viruses undergo cell division?

- non-living and inject genetic information into host cell instead as they are parasitic

What happens during viral replication (GENERAL)?

-Virus uses attachment proteins to attach to the host cell via complementary receptor proteins on hosts cell surface
-Virus injects its RNA/DNA into the host cell
-Host cells nucleic acid and ribosomes are then used to produce viral proteins

How do the newly formed viral proteins leave the cell?

-The cell bursts open and they are all released at once
-They leave the cell by 'budding out' which is where the particles leave the cell and take a piece of the host cells membrane with them to act as an envelope
-> over time this causes the cell to shrink and die

How does HIV replicate (IMMUNOLOGY)?

-HIV attachment proteins binds to a CD4 receptor on Helper T Cells
-Protein capsid fuses with cell-surface membrane, RNA & enzymes of HIV enter the Helper T Cell
-Viral RNA is used as template by Reverse Transcriptase to produce viral DNA which is inserted into host DNA
-HIV DNA in the nucleus creates mRNA using the cells enzymes and i used to make new viral proteins and mRNA
-New HIV viruses are assembled.
-The HIV bud out of T Helper with a piece of its cell surface membrane surrounding them to Forms Lipids Envelope

What is meiosis?

-Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells when the cell divides twice and produces gametes and spores(in plants)

What is a haploid cell?

a cell with one set of chromosomes(23 in humans or n)

What is a diploid cell?

A cell with two sets of chromosomes(46 in humans or 2n)

What happens during the meiosis 1?

-Homologous chromosomes pair up to form bivalents
-crossing over of homologous pairs occurs at the chiasmata
-cell then divides and homologous chromosomes separate randomly
-each daughter cell contains either a maternal or paternal copy

What happens during the meiosis 2?

-independent segregation occurs
-cells divide to produce 4 genetically different daughter cells

What happens during crossing over?

-homologous chromosomes come together and pair up and exchange parts
-the chromatids then have a different combination of alleles but contain the same genes
-this increases genetic variety

What happens during independent segregation?

-each chromosome aligns with its homologous partner at the equator of the cell
-the combination of maternal/paternal chromosomes aligns randomly
-one chromosome of each pair goes to daughter cell- by chance
-leads to genetic variation in offspring

How do you calculate the number of different combos of chromosomes following meiosis and after fertilisation?

2*n -> after meiosis
n=the haploid number of chromosomes
EG- in humans the haploid number is 23 so 2*23=8,388,608 different combinations
(2n)2 -> after fertilisation

What is a non-disjunction mutation?

-a mutation where chromosome don’t separate correctly during meiosis
-gametes with this have wither one more or one less chromosome than normal

Why are non-disjunction mutations found in all cells of offspring from the mutated gamete?

-all cells come from diploid zygotes by mitosis which produces genetically identical cells
-all cells will be genetically identical so they will all have the mutation

What is the difference in chromosome number in mitosis and meiosis?

Mitosis- cell chromosomenumber maintained
EG -> Haploid - Haploid
Meiosis- cell chromosome number halved
EG -> Haploid - Diploid