3.2.2 - Cell cycle

Mitosis

• Parent cell divides to produce two genetically identical daughter cells

• Needed for growth of multicellular organisms + repairing damaged tissues

Cell cycle

• Contains interphase - cell growth + DNA replication

  • Interphase subdivided into 3 growth stages: G₁, S, G₂

• Then mitosis

Gap phase 1

Cell grows; new organelles + proteins made

Synthesis

Cell replicates DNA, ready for mitosis

Gap phase 2

Cell keeps growing; proteins needed for cell division are made; DNA is checked for errors - cell dies if DNA incorrect

Interphase

(before mitosis in cell cycle)

• Cell carries out normal functions + prepares to divide

• Cell DNA unravelled + replicated

• Organelles replicated

• ATP content increased

Important parts of chromosome in mitosis

• As mitosis begins, chromosomes are made of two strands joined in middle by centromere

• Separate strands called chromatids

  • There are 2 strands because each chromosome has made identical copy of itself during interphase

• When mitosis ends, chromatids end up as one-strand chromosomes in daughter cells

Prophase

• Chromosomes condense - get shorter + fatter

• Tiny protein bundles called centrioles start moving to opposite ends of cell

  • Forms network of protein fibres called spindle

• Nuclear envelope breaks down → chromosomes lie free in cytoplasm

Metaphase

• Chromosomes line up along middle of cell + become attached to spindle by centromere

Anaphase

• Centromeres divide → sister chromatids separate

• Spindles contract → pulls chromatids to opposite poles of spindle, centromere first

  • Makes chromatids appear v-shaped

Telophase

• Chromatids reach opposite poles on spindle

• Chromatids uncoil → become long + thin

  • Now called chromosomes again

• Nuclear envelope reforms around group of chromosomes → two nuclei

• Cytokinesis finishes

• Now two genetically identical daughter cells

• Each cell starts interphase

Cytokinesis

Division of cytoplasm, producing two new cells

Starts in anaphase, finishes in prophase

Cancer

• Mitosis + cell cycle are controlled by genes

• Normally, cells stop dividing after enough new cells are made

  • But if there’s mutation in gene that controls cell division, cells can grow out of control

• Cells keep dividing, forming tumour

• Cancer = tumour that invades surrounding tissue

General cancer treatment

Some treatments are designed to control rate of division in tumour cells by disrupting cell cycle → kills tumour cells

Treatments don’t distinguish tumour cells from normal cells (also kill normal body cells that are dividing) → tumour cells divide more frequently than normal ones, so treatments are more likely to kill tumour cells

Cancer treatment targeting G1

• Some chemical drugs (chemotherapy) prevent synthesis of enzymes needed for DNA replication

• If these aren’t produced, cell is unable to enter synthesis phase (S) → disrupts cell cycle → forces cell to kill itself

Cancer treatment targeting S phase

• Radiation and some drugs damage DNA

• DNA is checked for damage several times in cell cycle

• If severe damage is detected, cell kills itself → preventing further tumour growth

Binary fission stage 1

• Circular DNA + plasmid(s) replicate

• Main DNA loop is replicated once, but plasmids can be replicated many times

Binary fission stage 2

• Cell gets bigger and DNA loops move to opposite ‘poles’ of cell

Binary fission stage 3

• Cytoplasm begins to divide (and new cell walls begin to form)

Binary fission stage 4

• Cytoplasm divides → two daughter cells produced

• Each daughter cell has one copy of circular DNA, but variable number of plasmid(s)

Binary fission

1. Circular DNA + plasmid(s) replicate

• Main DNA loop is replicated once, but plasmids can be replicated many times

2. Cell gets bigger and DNA loops move to opposite ‘poles’ of cell

3. Cytoplasm begins to divide (and new cell walls begin to form)

4. Cytoplasm divides → two daughter cells produced

   Each daughter cell has one copy of circular DNA, but variable number of plasmid(s)

Virus binding to host

• Use attachment proteins to bind to complementary receptor proteins on surface of host cells

• Diff viruses have diff attachment proteins, therefore require diff receptor proteins on host cells → some viruses can only infect one type of cell

Virus cell division

Because not alive, don’t undergo cell division

Virus replication

• Inject DNA/RNA into host cell

• Hijacked cell uses its ‘machinery’ to replicate viral particles