mitosis and the cell cycle

What are the 2 types of cell division in eukaryotes?

meiosis and mitosis

what happens in mitosis?

a parent cell divides to produce 2 genetically identical daughter cells

• each with an exact copy of DNA of the parent cell

what is mitosis needed for?

the growth of multicellular organisms and for repairing damaged tissues

what does the cell cycle consist of?

a period of cell growth and dna replication called interphase

• after interphase is mitosis

• interphase is divided into 3 separate growth stages- G1, G2 and S

what happens in G1?

the cell grows and new organelles and proteins are made here

what happens in S?

cell replicates its DNA, ready to divide by mitosis

what happens in G2?

cell keeps growing and proteins needed for cell division are made

chromosomes structure?

• as mitosis begins, the chromosomes are made of 2 strands joined in the middle by a centromere

• the separate strands are called chromatids

why are there 2 strands of chromosomes?

because each chromosome has already made an identical copy of itself during interphase

• when mitosis is over, the chromatids end up as one strand chromosomes in daughter cells

what happens in interphase?

the cell carries out normal functions, but also prepares to divide

• the cell’s DNA is unravelled and replicated

• the organelles are also replicated

• more ATP is produced to be used in cell division

what happens in prophase?

• the nuclear envelope breaks down and the nucleolus disappears

• chromosomes are left floating in the cytoplasm

• the chromosomes coil more tightly and become shorter and fatter. they can be see under a light microscope

• small protein bundles called centrioles move to the opposite poles of the cell

• microtubules form the mitosis spindle between the centrioles

what happens in metaphase?

• the chromosomes line up along the mid-line of the cell called the equator

• in metaphase, the chromosomes are maximally condensed

• they are attached to the spindle by the centromere

what happens in anaphase?

• the chromosomes break down into 2 chromatids

• the sister chromatids separate at the centromere

• the spindles contract and pull the chromatids to each pole of the cell

what happens in telophase?

• the chromatids reach the opposite poles and begin to unravel (de-condense) becoming chromosomes again

• nuclear envelopes form around the chromosomes, so now there are 2 nuclei

• the cytoplasm splits and 2 daughter cells are formed *(cytokinesis)

• the daughter cells are genetically identical to one another

• the cell cycle starts again

how to calculate mitotic index?

(number of cells in mitosis/total number of cells in the sample) x100

what is cancer?

a tumour that invades surrounding tissue

what are some treatments for cancer designed to do?

• control the rate of cell division in tumour cells by disrupting the cell cycle

• this kills tumour cells

• however, these treatments cannot distinguish between normal cells and cancer cells, which is why some normal cells are also targeted.

why are treatments more likely to kill cancer cells?

because normal cells don’t divide as frequently

what part of the cell cycle does chemotherapy target

the g1 phase

• it prevents the synthesis of proteins needed for DNA replication.

what happens if the cells needed for DNA replication are not produced?

• the cell will be unable to enter the synthesis phase, disrupting the cell cycle and forcing the cell to kill itself

what part of the cell cycle does radiation target?

the synthesis (s)

what does radiation do in s phase?

at many points in the cell cycle (including just before and during S phase), the DNA in the cell is checked for damage

• if severe damage is detected (like radiation), the cell will kill itself- preventing further tumour growth

how to investigate mitosis (experiment)?

by using stained squashed cells from root tips

root tip: sample preparation?

• use forceps to handle the tips

• root tips must be sprouting

• place into 5M HCl

• after 5 mins, rinse the tips in cold water in a watch glass and then pat to dry on paper towel

root tip: cutting the root tips?

• using a sharp scalpel, cut tips that are 2mm long

• place root tip onto microscope slide (ensure slide is clean to reduce risk of artefacts)

root tip: staining?

• carefully add 2-3 drops of stain and leave for 2 mins

• use a mounted needle to spread the root tips into a thin layer

• place a coverslip over the top of the tips

root tip: squashing?

• squash down by applying force to the cover slip

• this will make the tissue thinner and allow light to pass through it

• don’t smear coverslip sideways, otherwise, you will damage the chromosomes

what is an eyepiece graticule?

a transparent ruler with numbers, but no units

what is a stage micrometer?

a microscope slide with an accurate scale and it’s used to work out the value of divisions on the eyepiece graticule at a particular magnification

what are artefacts?

things that you can see down the microscope that aren’t part of the cell or specimen you are looking at

• e.g- dust, air bubbles, fingerprints

• they are most common in electron micrographs

why are artefacts most common on electron micrographs?

because you need a lot of preparation before you can view them under an electron microscope

what happens in binary fission?

the cell makes copies of its genetic material, before physically splitting into 2 daughter cells

what happens in binary fission?

• the circular DNA and plasmid(s) replicate.

• the main DNA loop is only replicated once, but plasmids can be replicated loads of times

• then, the cell gets bigger and the DNA loops move to opposite poles of the cell

• the cytoplasm begins to divide (and new cell walls start to form)

• the cytoplasm divides and 2 daughter cells are produced

• each daughter cell has one copy of the circular DNA , but can have a variable number of plasmids

how do viruses replicate themselves?

• they use their attachment proteins to bind to complementary receptor proteins on the surface of host cells

• because they are not alive, they cannot undergo cell division.

• instead, they inject their DNA or RNA into the host cells- this hijacked cell then uses its own machinery (enzymes, ribosomes, etc) to do the virus’ dirty work and replicated the viral particles

why do different viruses require different receptor proteins?

because different viruses have different attachment proteins and therefore require different receptor proteins on host cells.

• as a result, some viruses can only infect one type of cell, whereas others can affect many