Outline 10: Cell Cycle

Cell Theory

1. All living things are made of one or more cells

   1. Unicellular - made of one cell

   2. Multicellular - made of more than one cell

2. Cells are the basic units of structure and function

3. All cells come from pre-existing cells

Cell Division = Cell Reproduction (Unicellular vs. Multicellular)

1. Unicellular = cell division is the same as reproduction — one cell divides to create a new independent organism

2. Multicellular = cell division allows for growth, development, and repair

Genome (prokaryote vs eukaryote)

• Genome — total genetic information of a cell or organism

  • Prokaryotes — composed of single double-stranded DNA molecule in the form of a loop/circle; located in the nucleoid

  • Eukaryotes — consists of several double stranded linear DNA molecules; located inside the nucleus and wrapped around proteins

Levels of DNA Packaging

1. DNA double helix wraps around histone proteins

2. DNA + histone proteins = nucleosome

3. Nucleosome coils to thicker fiber

4. Coiled nucleosome = chromatin

Chromatin vs Chromosome

• Chromatin

  • “loose”, uncoiled form of DNA 

  • Found when cell is not dividing 

• Chromosome

  • one complete molecule of DNA

  • When cell is ready to divide, the chromatin coils up tightly to form visible chromosomes

    • Tightly packaged form keeps DNA organized and protected during cell division

Chromosome Anatomy: Chromatid

Single DNA molecule of two strands of duplicated DNA

Chromosome Anatomy: Sister Chromatids

two identical DNA molecules that make up a replicated chromosome

Chromosome Anatomy: Centromere

region where sister chromatids are bonded together

Ploidy

the number of sets of chromosomes in a cell, or in the cells of an organism.

Types of Ploidy: Haploid (n)

• a cell that contains one set of chromosomes — just one copy of each type

• Gametes (sex cells) are haploid 

• EXAMPLE: Humans: n=23 

Types of Ploidy: Diploid (2n)

• a cell has two complete sets of chromosomes — one from each parent 

• These chromosomes come in homologous pairs

  • they are the same size and shape

  • they carry the same type of genes

Can both haploid and diploid cells go through mitosis?

Yes!

Cell Cycle

ordered series of events involving cell growth and cell division that produces two new daughter cells 

Cell Cycle: Prokaryotes vs Eukaryotes

• Prokaryotes: binary fission

  • Simpler process

• Eukaryotes: mitosis

Cell Cycle: Interphase

• period between cell division when the cell is active but not dividing; preparing for cell division

• Makes up 90% of the total cell cycle 

• Has three stages: G1, S, G2

Interphase: G1

• the cell grows, develops, and carries out its normal daily functions

• organelles duplicate, and the cell increases in size

• many cells (like nerve cells) spends most or even all of their life in G1 and do not divide again

Interphase: S

• The cell replicates its DNA, making an exact copy of its genetic material

• each chromosome now consists of two identical sister chromatids joined at the centromere

• Ploidy does not change!!

Interphase: G2

• the cell continues to grow and prepares for mitosis

• there are high metabolic activity, including production of proteins and organelles needed for division

• DNA is still in chromatin form

• G2 is usually shorter than G1 or S

M Phase: What stages does it include and how long does it last?

1. Mitosis — division of the nucleus; contains 4 stages

2. Cytokinesis — division of the cytoplasm

3. Lasting less than 10% of the total cell cycle

Mitosis: Prophase

• Nuclear envelope begins to break down

• Centromeres(microtubule-organizing centers) move to opposite poles 

• Spindle fibers start to form outward from centromeres, pushing them farther apart

• Chromosome condenses and becomes visible

• Kinetochores form on chromatids; serves as an attachment points for spindle fibers 

• Spindle fibers attach to kinetochores

Mitosis: Metaphase

• Chromosomes are lined up at the metaphase plate

• Each sister chromatid is attached to spindle fiber originating from opposite poles of the cell

Mitosis: Anaphase

• the sister chromatids separate at the centromere

• they are pulled to opposite poles of the cell by the kinetochores and spindle fiber

• once separated, each chromatid is now considered an individual chromosome

Mitosis: Telophase

• Chromosomes arrive at opposite poles and begin to decondense into chromatin

• nuclear envelope material surrounds each set of chromosomes

• two identical nuclei are now present — identical to the parent nucleus

• the spindle fibers break down

Cytokinesis

• The physical splitting of of the cell into two daughter cells

• Usually begins during telophase and completes after mitosis ends

Cytokinesis: Animals

• A cleavage furrow forms when the cell membrane pinches inward

• furrow deepens until the parent cell completely divides into two identical daughter cells

Cytokinesis: Plants

• A cell plate forms in the center of the cell at the location of the metaphase plate

• the cell plate develops into a new cell membrane and eventually cell wall, separating the two daughter cells