D2.1 Cell and Nuclear Division

Cell cycle vs Mitosis

Interphase - cell growth and normal function (NOT Mitosis)
Prophase - The Chromosomes condense and the nucleus dissolves

Metaphase - The Chromosomes align on the equator

Anaphase - The Chromosomes are pulled apart to the poles by spindle fibers (microtubules)

Telophase - The Chromosomes are at the poles and the nuclei reform. 

Cytokinesis - The cell splits (NOT Mitosis)

Mitosis

PMAT

Mitotic Index

Cells in mitosis / total number of cells

Histones and gene expression

Histone proteins have tails that bind to the DNA and hold it around the nucleosome

Acetylation and methylation can affect how tightly the DNA is bound and this can influence gene expression

Cytokinesis

NOT PART OF MITOSIS

Animal cells have a ring of contractile actin and myosin and myosin proteins pinches cell membrane together to split the cytoplasm.

Actin + myosin filaments form around the equator of cell. They contract forming a cleavage furrow, and membrane pinches off.

Plants don’t have actin and myosin so use vesicles instead.

Meiosis recap

Meiosis results in the formation of genetically individual cells that are haploid.

Meiosis and variation + crossing over

Random orientation of bivalents, (random assortment), means we do not know which of the maternal or paternal chromosomes will go into which gamete.

Crossing over occur when the chromosomes align on the equator, the end sections of chromosomes can become tangled and so there can be an exchange of genes between chromosomes, meaning that the final version of that chromosome is not the same as the original copy

Equal and unequal cytokinesis

During cytokinesis both cells must receive equal distribution of organelles in order to survive and function.

They must receive one mitochondria

In Meiosis, and oogenesis (the formation of the eggs in the ovaries) in particular one of the cells produced at each stage fails to mature, or often die, due lack of organelles. These cells are known as Polar Bodies. 

Yeast reproduce through Budding. This is another example of uneven cytokinesis. 

Down syndrome and non-disjunction

Downs is a genetic condition caused by an extra chromosome.

Caused by non-disjunction

ND occurs when the spindle fibres fail to separate the centromeres between two sister chromatids in the second division of meiosis

SO both chromatids are pulled into the same gamete

The other gamete does not have full set of chromosomes and fails to survive

Cell proliferation

Term used to describe the process used to make new cells for growth and repair through mitosis. Usually starts with stem cells.

Plants have specialised tissue used for proliferation known as meristem tissue.  

Meristem tissue is distributed all through the plant. 

Plants can grow continually throughout their lives. 

Proliferation and interphase

During interphase the cell is highly metabolically active. Interphase is also the time that Mitochondria and chloroplasts replicate. 

Interphase has 3 sub=phases: 

G1 subcellular components are replicated so that the daughter cell can grow and carry out its normal cells function. 

S DNA Replication (this occurs at the start of both Mitosis and Meiosis)

G2 Checking of DNA Replication and multiplication of Organelles to ensure both daughter cells have mitochondria and ribosomes etc.

Control of the cell cycle using cyclins

Cell regulatory proteins that control cell cycle

The cyclins are in very low levels in the cell and peak at specific stages. 

The cyclins activate enzymes known as Cyclin Dependant Kinases (CDKs) to phosphorylate and activate other enzymes with specific targets in a cascade of activity (similar to the clotting cascade).

Specific CDKs must reach threshold levels in order for transition to the next phase.

What are proto-oncogenes

Healthy genes that have been identified as those with the potential to cause cells to become cancerous.

Sometimes mutations can occur which convert them to oncogenes. This means that the cells become activated when they should not be and more mitosis is stimulated. This is known as a gain of function. It only needs to happen in one gene and more of the gene will be transcribed. 

Tumour suppressor genes are the genes that should cause apoptosis. If they are mutated on both copies this will result in uncontrolled cell division.

Tumours

A benign tumor is one that is slow growing and not invasive

A malignent tumor is one that is fat growing and may be invasive. Removed by surgery, radio + chemo. Chemo is often used to prevent any undetected metastasis

Metastasis occurs when a small gorup of cells break of the tumor and move around the body, so they can settle in other organs in the body and start forming a secondary tumor.

If the primary tumor was a lung cancer all the secondary tumors would need to be treated as lung cancers.

Makes secondary tumors hard to cure so its important to catch tumors erly before metastasis