Long, linear structures consisting of DNA and histone proteins
Define homologous chromosome
A pair of chromosomes containing the same genes in the same positions (loci), each derived originally from a different parent’s gamete at fertilisation
Define chromatid
One of two threads of a chromosome, formed after DNA replication
Define centromere
Holds together 2 identical sister chromatids
Define chromatin
DNA + proteins together form this substance
The nucleus of all eukaryotic cells contains a set number of chromosome, depending on the species for example humans have 23 homologous pairs ( each pair consists of a maternal and paternal chromosome).
What is the diploid number of chromosome?
46- the total number of chromosomes in a normal body cell
What happens before the start of cell division?
The DNA in the chromosomes replicates with each chromosome becoming two threads called chromatids, held together at the centromeres. They are not visible as individual chromosomes at this stage ( referred to as chromatin)
What happens in g1 of the cell cycle?
Cell prepares for DNA replication- cells grow, respire and new proteins and organelles are produced
What happens in the S phase?
DNA replication
What happens in G2?
Short gap before mitosis- cell grows and prepares for mitosis
What happens during mitosis?
During mitosis, a parent cell divides into two identical daughter cells, each containing an exact, complete copy of DNA of the parent cell, they are genetically identical.
Which phases correspond to interphase?
G1, S, G2
What also occurs during G2?
Checking of the DNA sequence
The duration of interphase gives us information about the rate of cell division
cells with a shorter interphase divide more often e.g cancer cells or skin cells, whereas other cells e.g liver can remain in interphase for months or years before dividing
in multicellular organism, many specialised cells don‘t retain the ability to divide e.g neurons and muscle cells, and are said to reside outside cell cycle ( G0 phase)
What are the 4 stages of mitosis
prophase
metaphase
anaphase
telophase/cytokinesis
What is mitosis for?
For growth of tissue and replacing tissue during repair
it also allows asexual reproduction producing offspring which are genetically identical to the parent
What is the only way that variation can arise when a cell divides by mitosis?
Mutation
What happens during prophase?
Chromosomes condense (due to coiling of DNA) and become visible, the nuclear membrane breaks down. The centromere holds the 2 sister chromatids together
what happens during metaphase?
Chromosomes align in the equator of the cell and attach to spindle fibres by their centromeres
What happens during anaphase?
Centromeres divide, spindle fibres contract pulling apart the chromatids of each chromosome, sister chromatids go to opposite poles of cell
What happens during telophase?
The chromosomes uncoil and become thread like again so they are no longer visible. A new nuclear envelope forms around each set of daughter chromosomes
What marks the end of mitosis?
Cytokinesis- following after mitosis the cytoplasm divides so there will be two new nuclei.
Cytokinesis
Division of cytoplasm usually follows mitosis fairly quickly
Cells without a cell wall pinch themselves in two and a membrane forms in the middle of the cell. Eventually the cell membrane from one side of the cell and two new daughter cells separate.
Cells with a cell wall can’t pinch themselves in two so instead a new cell wall forms in the middle of the cell
What is mitotic index?
A ratio showing the number of cells undergoing mitosis in proportion to the total number of cells
Mitotic index formula?
(Number of cells in mitosis / Total number of cells) x 100
The number of each cells at each stage of mitosis is proportional to time spent in that stage. So if the duration of the cell cycle is known the duration of a particular stage can be estimated
How do prokaryotic cells divide?
Binary fission.
Circular DNA replicates including plasmids
Cell membrane begins to grow between the two main DNA loops and begins to pinch forwards dividing the cytoplasm in two
a new cell forms between the two DNA loops forming two daughter cells each with a copy of the main DNA loop and a variable number of copies of plasmids
How do viruses divide?
Don’t undergo cell division
They replicate by attaching to a host cell using the attachment proteins on their surface
Then they inject their nucleic acid into the host cell which is then used to produce viral components (protein) that are then assembled into new viruses
Describe cancer
mitosis controlled
cancer results from mutations in genes that control cell division causing rapid uncontrolled growth and division of cells
Results in formation of abnormal cells called a tumour. Cancerous tumour cells don’t respond to signals from nerves and hormones as normal cells would and do no undergo programmed cell death( apoptosis) which would normally occur when irreparable damage is detected
How is cancer treated?
Involves blocking some part of the cell cycle to control rate of cell division.
Drugs like cisplatin prevent DNA from replicating
Prevent spindle formation (vinca alkaloids) or disrupt another stage of mitosis