BIO CELL REPRODUCTION

Cellular Reproduction

How does 1 cell become a multicellular organism?

Cell Size Limitations

• Cells don’t continually grow larger, they have a size limit

• The cell size is influenced by several factors (one being the Surface Area to Volume ratio)

• Surface area of the cell: area covered by the plasma membrane

• Volume of the cell: the space taken up by the inner contents of the cell

• Easy to stress out instruction giver/nucleus, stressful if its barely surviving 

• Cells should always be at homeostasis,

  • If not, the organelles such as nucleus and ribosomes and DNA, lose their normal function and could stop working completely

• The bigger the cell gets, the ratio of surface area to volume decreases

• Cell with a higher ratio of surface area to volume= remaining small, can sustain themselves more efficiently

Chromatin and Chromosomes

• 1 strand of DNA= 1 sequence of DNA

• 2 strands of DNA make up a double helix

• Histones: 8 proteins organized in a structure organized like tennis balls

• Nucleosome: DNA wrapped twice around histones

• Chromatin: a sequence of nucleosomes supercoiled into a whirlpool

• Chromosomes: condensed structures that contain the DNA that are visible during cell division

• The phosphate groups in DNA create a negative charge, which attracts the DNA to the positively charged histone proteins and form a nucleosome

• The nucleosomes group together into chromatin fibers, which supercoil to make up the chromosome.

Advantages of small cell size

• Smaller cells can

  • Transport substances easily

    • Diffusion is inefficient over longer distances

    • The cytoskeleton transportation network becomes less efficient for a cell for the distance to travel becomes too large

  • Cellular Communication is more efficient in smaller cells

    • Instructions insufficiency

• Small cells still contain many structures especially DNA

  • 1 strand of DNA = 2.2m → DNA is highly folded

    • Into chromatin and X-shaped chromosome to fit in the nucleus of eukaryotes

    • Into chromatin and ring-shaped chromosomes to fit in the cytoplasm of prokaryotes.

Cell Cycle

• Cell Cycle: Cells reproduce by a cycle of growing and dividing 

  • Each round of the cell cycle makes one cell becomes two cells allowing the body to grow and heal injuries

  • The duration of the cell cycle varies between cell types (8 min–1 year) for most actively dividing animal cells it takes 12-24hours

• There are 3 main phases of the cell cycles

  • Interphase

    • Cell grows into a mature, functional cell

    • Duplicates the DNA in its nucleus

    • Prepares for division

    • Interphase is divided into 3 stages

      • Gap 1 (G₁)

        • Starts immediately after a cell divides

        • Phase of growing, carrying out normal cellular functions, and preparing to replicate DNA

        • At the end of G₁, cells (eg. muscle) and nerve cells exit the cycle and do not divide again (G0)

      • Synthesis (S)

        • Phase of copying DNA to prepare for divison

      • Gap 2 (G₂)

        • Preparing for the division of nucleus by synthesizing all the actors (eg. histones) of cell divison

        • When the cell takes inventory and controls its readiness for mitosis

  • The M Phase

    • Mitosis

      • Stage of the cell cycle during which the cell’s nucleus and nuclear material divide

      • The mother cell splits into two genetically identical daughter cells

      • So, in multicellular organisms, mitosis

        • Increases the number of cells allowing the growth of the organism to its adult size

        • Allows organisms to replace damaged cells

      • In other organisms, mitosis

        • Allows the asexual reproduction

        • Helps maintain chromosome

    • Mitosis takes place through 4 phases

      • Prophase: first and the longest stage

        • Chromatin condensed into chromosomes

        • Each chromosome is a single structure containing one part of the original genetic material and one part of the genetic material that was replicated in S

        • Sister chromatids are structures that contain identical copies of DNA. They are attached at the centrometre.

        • As prophase continues, the nucleolus disappears, the microtubules rearrange to form the spindle fibers, centrioles(organelle located at the cytoplasm near the nuclear envelope/membrane of nuclues), and aster fibers form a spindle apparatus

        • Near the end of prophase, the apparatus stick to each chromosome, linking each of the sister chromatids to a pole of the cell, and the Nuclear envelope disappears (means the end of the prophase)

      • Metaphase:second stage of mitosis

        • Sister chromatids are pulled along the spindle apparatus toward the center of the cell, the equator, where they line up

        • Note: metaphase is one of the shortest stages of mitosis which successful completion ensure the equal separation of DNA

• Anaphase: The third stage of mitosis

  • Sister chromatids are pulled apart

  • Microtubules of the spindle apparatus begin to shorten

  • Sister chromatids separate, separating the replicated DNA, and resulting into 2 genetically identical chromosomes

  • At the end of Anaphase, chromosomes move toward the poles of the cell

• Telophase: last stage of mitosis

  • Chromosomes arrive at poles and begin to uncoil/relax

  • Two new nuclear envelops begin to gorm around each set of chromosomes and the nucleoli reappear

  • The proteins of the spindle apparatus are recycled by the cell to build parts of the cytoskeleton

• Cytokinesis

  • Cell’s cytoplasm divides, resulting in two cells with identical nuclei

  • In animal cells, microfilaments constrict/pinch off the plasma membrane and cytoplasm ant the furrow (where the cell membrane is being pinched) to form two cells

  • In plant cells, instead of pinching in half, a new structure called the cell plate forms between the two daughter nuclei

  • Cell walls then form on either sides of the cell plate separating the 2 daughter cells

• Differentiated cells are mature cells that acquired the function and exited the cycle

  • They are in their final cell of functioning, you cannot replace them

  • Example: Neuron cells

Cell Cycle 

• Cells have specific instructions for carrying out and completing the cell cycle without accumulating mistakes no stopping

• The complexity of these regulations require many cell cycle regulations

  • CDKs-Cyclins

    • The cell cycle is driven by a combination of 2 classes of proteins, cyclins and their specific enzymes cyclin-dependent kinases (CDKs)

    • They initiate the various activities taking place in the cell cycle, as each phase has its specific couple of cyclin-CDK signaling its start

    • Different cyclin-CDK control different activities during different stages of the cell cycle

  • Quality control checkpoints

    • They are checkpoints that monitor the cycle progression and can stop it if something goes wrong

    • G1 checkpoint monitors DNA damages and stops the cycle before it enters the S phase in case unrepaired DNA damages were found

    • S phase checkpoint monitors the quality of DNA replication and checks for mutations or mistakes ← important

    • G2 checkpoint monitors the readiness of the cell for mitosis

    • Spindle checkpoints detect failures of the spindle in any step of mitosis and stop the cycle ← important