Unit 10

Cell division When a cell divides, or splits in half, in order to create new cells

Reproduction when an organism creates new offspring 

Organism 

Unicellular: 

when they reproduce, they perform cell division. The mother cell divides into two daughter cells 

Multicellular: 

Asexual; involves one parent, and produces clones with no genetic difference 

Sexual: involves two parents, produces offspring with genetic differences 


Performs cell division for 

  • Repair 

  • Growth 

  • Maintenance

Budding: when a unicellular organism has a growth appearing in one side that develops into a new individual and falls off; exp: Hydra 



Yeast

When conditions are favourable, yeast will reproduce by budding 

When conditions are not favourable, yeast will produce sexually  



Why? If they aren’t doing well and the condition is bad, they don’t want to clone themselves because they know their offspring will have the same bad condition. If they reproduce sexually, there becomes a chance that the mix of genetics can create a version that will survive the condition


Fragmentation: when a piece of an animal is cut off, and it will grow to become a new individual, exp, star fish 


Vegetative propagation: a survival mechanism where a broken piece of plant will grow to become its own individual 


Chromosomes

A molecule of DNA (double helix) 


Prokaryotic, 

Each prok cell will have one circular DNA molecule 

Eukaryotic,

Each eukaryotic will have several linear DNA double helices, each one wrapped around groups of proteins → histones to create chromosomes 

When one chromosome is duplicated, it becomes a set of sister chromatids held together at the centromere, the centre point. 

When it is duplicated, the information is no longer accessible 


Forms of DNA depending on cell activity 

When a cell is NOT dividing 

DNA exists in the form of chromatin, the chromosomes are spread out, and the information is easily accessible 


When a cell IS dividing 

DNA exists in the form of chromosomes that are condolences, wrapped up tightly. Information is no longer accessible 



Cell cycle 

The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromosomes and then segregate the copies precisely into two genetically identical daughter cells.


Steps of the cell cycle 

Interphase 

  1. G1 phase, first gap 

  2. S Phase, synthase phase 

  3. G2 phase, second gap 

Celldivision→ mitotic phase 

  1. Prophase 

  2. Prometa phase

  3. Metaphase 

  4. Anaphase 

  5. Telophase 

  6. Cytokinesis 



Functions in each step:

First Gap

  • Brings in lots of nutrients 

  • Generates lots of energy 

  • Organelles/proteins & other structures will duplicate 

  • Cells grow in size 


Synthase phase 

  • DNA is duplicated 


Second Gap

  • Everything in the first gap will continue if it is not finished 

  • Checks and corrects any error found in DNA 

    • If the error cannot be fixed, the cell enters apoptosis; which is cell suicide 


Mitosis Phase

Pro phase

  • DNA condenses from chromatin into duplicated chromosomes 

  • The nuclear envelope disintegrates, allowing access to the information 

  • Centrioles begin to move away from each other to opposite poles

  • Spindle fibres, microtubules, begin to form. (from the cytoskeleton) 

Prometa Phase 

  • Nucleus disappears 

  • Centrioles continue to move to the opposite pole

  • Kinetachrome fibres from each centrosome connect to the centrosome of each duplicated chromosome and begin to move them to the equator of the cell

Meta Phase 

  • Duplicated chromosomes are lined up at the equatorial plane 

  • Centrioles have reached opposite poles 


Ana Phase

  • Kinetochore fibres shorten, pulling sister chromatids apart & moving the chromosomes away from each other 

  • Non-kinetochore fibres lengthen, stretching the cell


Tela Phase

  • The appearance of the cleave furrow indicates the start of cytokinesis 

  • Spindle fibres disintegrate 

  • Formation of nuclear envelope around each set of chromosomes 

  • DNA unwinds from chromosomes and into chromatin 


Mitosis 

  • Duplication of the nucleus 

Cytokinesis 

The division of the cytoplasm of the cell

  • Animal cells

    • rely on a belt of proteins known as the contractile ring, which is made from actin

  • Plant cells

    • does not rely on a contractile ring because the cell is way too rigid.

    • Golgi apparati at each end of the cell will produce transport vesicles that fill with pectins that move to the centre of the cell

    • at the centre of the cell, the transport vesicles fuse with each other to create the cell plate

      • the cell plate lines up in the middle of the cell to become the new cell wall

    • the cell plate continues to grow and reach the cell membrane to become the cell membranes of the two new cells, the cells will be attached by the middle lamella that is filled with pectin

    • cellulose is secreted by each new cell to create its own cell wall

Drawing of the cells;


Factors that control the cell cycle (environment)

  • space is required around cell for them to divide; density-dependence

    • in tissues, cells form layers and not clumps; cells do not grow on top of each other

  • a rigid surface is needed for attachment before cells can divide; anchorage dependence

  • signal molecules called growth factors inform cells that they need to enter the cell cycle

Controlling the cell cycle checkpoints

  • G1 checkpoint, specialised proteins (cyclins) determine the appropriate conditions, for cells to enter the cell cycle

  • G2 checkpoint, proofreading enzymes, checking to make sure DNA has no errors, cell has all organelles duplicated

  • M checkpoint, proteins will make sure that the duplicated chromosomes are all on the equatorial plane and attached at the centromere by the kinetochore fibres.

Proteins involved in the cell cycle

  • Cyclin; is produced when the growth factor informs cells they need to enter the cell cycle

    • these regulate when the cell moves from one stage to the next

  • cyclin-dependent kinases; enzymes that are activated by cyclins that cause the cell cycle to advance

Genes controlling the cell cycle

  • proto-oncogenes; normally inactive, but are turned on by growth factors to produce cyclins

    • cancer cells have mutated versions of these genes, so a mutation in this gene leads to having a cancerous cell

      • a cancerous cell doesn’t mean you have cancer, it just means they are stuck in the cell cycle

  • tumour-suppressor gene; normally an active, inhibitor, keeps the cell out of the cell cycle

    • produces proteins called p53, p21, & retinoblastoma (RB)

      • p53 stops transcription and stops the cell cycle from moving forward. Due to low level oxygen, damages, or errors. It tries to fix it, if it cannot, then it goes through apoptosis

      • if p53 is malfunctioning, it will continue to divide with the errors that can create cancerous cells

Cancerous Cells

  • Tumour: a clump of constantly dividing cells,

    • Benign, localised in one part of the body

    • Malignant, spreading to multiple parts of the body by a process known as metastasis, this is called cancer

      • To spread, cancerous cells need access to the bloodstream

  • Most common cancers;

    • male; colon cancer

    • female; breast

    • unisex; lung

Prokaryotic cell dividing

Binary Fission;

  • Binary: two-parts

  • Fission: Splits into to (do not confuse with fusion)

The process used by bacteria to divide because they do not have a nucleus, asexual reproduction

Steps:

  • The cell enlarges, and the circular chromosomes duplicate

  • Each circular chromosome will attach to the cell membrane at opposite ends, and the cell continues to enlarge, separating the two circular chromosomes

  • the cell membrane and cell wall are pulled inwards to create a septum that splits the cell into two new daughter cells that are identical to each other (clones)

    • the cell wall will weaken due to enzymes allowing it to fold on itself