Biology BASIS 8 : Cell Cycle

Preview

Why do cells divide instead of growing larger?

• DNA Overload—the larger a cell grows, more demand is placed on the DNA

• Ratio of SA to Volume—if the cell grows too large, it will be unable to move nutrients and wastes across the cell

Chromatin—form of DNA in resting cells

• “resting” = not dividing

• chromatin cannot be seen in the nucleus

Chromosome—condense form of DNA

• prevents tangling during cell division

• visible during cell division

Diploid (2n)—2 sets of chromosomes

• most cells in our body (all 46 chromosomes)

Haploid (n)—1 set of chromosomes

• reproductive cells (only 23 chromosomes)

Haploid Nuclei

• one of each chromosome type

• in humans, haploid cells have 23 chromosomes

• haploid cells are sex cells (gametes) that fuse together during sexual reproduction—sperm and egg

• Haploid = “half” of a full set of chromosomes

Diploid Nucleus

• two of each chromosome type (2 copies of each gene)

• in humans, diploid cells have 46 chromosomes

• haploid sperm and egg fuse in fertilization to produce a diploid zygote, which then divides by mitosis to form an embryo

• diploid cells are body cells

Somatic Cells

• somatic cells are diploid

• regular type of body cell found in almost every part of the body (non-sex cells)

• human: 46 chromosomes (2n)

• ex: skin, muscle

Gametes

• gametes are haploid

• present in certain parts of the body

• human: 23 chromosomes (n)

• ex: sex cells—sperm & egg

Mitosis

the cell cycle—series of events that cells go through as they grow & divide

during the cell cycle, the cell

• grows

• prepares for division

• divides to form 2 daughter cells

Prokaryotic Cell Cycle

• regular pattern of growth, DNA replication, cell division

• happens rapidly under ideal conditions

• binary fission

  • asexual reproduction

Eukaryotic Cell Cycle

• G1 (1st gap phase)

• S Phase (Synthesis Phase)

• G2 (2nd gap phase)

• M Phase (Mitotic Phase)

Interphase is the period of growth that occurs between cell divisions

Mitotic Phase is the period where the cell divides

• consists of mitosis and cytokinesis

  • mitosis—division of cell nucleus

  • cytokinesis—division of cell cytoplasm

Interphase = in-between: between cell division

• accounts for 90% of the time

• nucleus is well-defined and bounded by the nuclear envelope

• outside the nucleus are two centrioles

  • their function is to organize the microtubules into a spindle

  • they will begin to move apart as spindle microtubules grow out of them

G1 Phase

• at the start of g1, the cell is a newly divided daughter cell

  • increases in size

  • synthesizes new proteins & organelles needed for the newly formed daughter cells

  • at the end, cells can decide to continue or move into g0

G0 Phase

• “opting out”

• after cell growth, some cells go into g0 and will not divide again

• most human cells in g0 phase

• some can be pulled back into the cell cycle if a traumatic event occurs

S Phase

• new dna is synthesized when the chromosomes are replicated

• chromosomes are duplicated

  • 1 copy of the DNA is made for each daughter cell

• at the end, it contains twice as much DNA as it did in the beginning

G2 Phase

• usually shortest of 3 interphase phases

• many of the organelles & molecules required for cell division are produced

Mitosis

1. prophase

2. metaphase

3. anaphase

4. telophase

1. Prophase

early prophase, 30 minutes (early & late combined)

• DNA condenses and becomes visible

• nucleolus disappears

• centrioles separate and move to opposite ends of the cell

• begin to produce microtubules to form a spindle

  • spindle—protein that will separate chromatids and help to pull the cell

late prohase

• nuclear envelope breaks down

• kinetochore proteins attach to the centromeres of the sister chromatids

• centrioles form the spindle in animal cells

2. Metaphase

usually the shortest, 20 minutes

• chromosomes line up at the center of the cell at the metaphase plate

• spindle fibers connect the centromere of each chromosome

• kinetochore microtubules push & pull chromosomes

3. Anaphase

lasts 3 minutes

• kinetochore microtubules shorten, pulling sister chromatids apart

• nonkinetochore microtubules lengthen, helping with cytokinesis

4. Telophase

lasts about 10 minutes

• nuclear membranes begin to form

• chromosomes become diffused again, returning to chromatin

• spindle begins to break apart

• nucleoli return

• cell begins to pinch in, this is called a cleavage furrow (end at cytokinesis)

• result is two nuclei that are exact copies of each other

Cytokinesis (division of cytoplasm)

can begin as early as anaphase

• at the end of mitosis, 2 nuclei have been formed

• each nucleus has identical sets of chromosomes

• process of cytokinesis differs in animal and plant cells

• usually occurs at same time as telophase

  • Cytokinesis in Animal Cells

    • in animal cells, a ring of actin begins to contract to form a cleavage furrow

    • contract until cell is pinched into two separate cells

    • each new cell contains its nucleus, cytoplasm, & organelles

  • Cytokinesis in Plant Cells

    • in plant cells, a cell plate forms midway between the two nuclei to separate the 2 cells

    • cell plate becomes cell wall

Apoptosis—programmed cell death

Internal Regulators

• proteins produced inside cell

• allows the cell cycle to proceed only when certain events have occured in the cell itself

External Regulators

• contact inhibition is an example of an external regulator

• factors that respond to events that occur outside the cell that direct cells to speed up/slow down cell cycle

Cancer

• uncontrolled cellular division

  • cancer cells divide uncontrollably, forming masses of cells called tumors that can damage surrounding tissues

  • not all tumors are cancerous

• metastasis

  • cancer cells may break loose from tumors & spread throughout the body, disrupting normal activities & causing serious medical problems/death

• p53-key: protein in cancer

  • protein senses DNA damage

  • can halt cell cycle progression in G1 if there is DNA damage

  • if the DNA damage = bad enough, p53 can force cell in apoptosis

  • more than half of all human cancers have p53 mutations of some sort

Sexual Reproduction

• diploid cells—2 sets of chromosomes

• haploid cells—1 set of chromosomes

• gametes—sex cells (egg & sperm)-haploid chromosome #

• fertilization—fusion of sperms & eggs

• zygote—fertilized egg & sperm, has diploid chromosome #, one set from each parent

Meiosis

process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes

• meiosis must reduce chromosome # by half (n)

• fertilization then restores the 2n number

  • occurs for gametes only

  • reduces the chromosomes # in reproductive cells by half

  • has two divisions, meiosis I and meiosis II

    • meiosis I

      • interphase I

      • prophase I

      • metaphase I

      • anaphase I

      • telophase I

    • meiosis II

      • prophase II

      • metaphase II

      • anaphase II

      • telophase II

Interphase I

• cell build up energy

• DNA replication (make duplicated chromosomes

• cell doesn’t change structurally

Prophase I

• chromosomes coil and become individual chromosomes, nucleus, & nuclear envelope disappears

• homologous chromosomes come together as pairs by synapsis forming a tetrad

• non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation

• centrioles move to opposite poles between them

  • synapsis—pairing of homologous chromosomes

    • group of 4 chromatids

  • in mitosis, there is no pairing of homologous chromosomes

Metaphase I

• homologous pairs of chromosomes align along the equator of the cell

• 2 chromosomes attach to one spindle fiber by means of the kinetochore of the centromere

  • independent assortment—how they line up differently

• in mitosis, homologous chromosomes do not line up next to each other

Anaphase I

• in meiosis, sister chromatids remain attached at centromere

Telophase I & Cytokinesis

• duplicated chromosomes have reached the poles

• a nuclear envelope & nucleolus re-forms around chromosomes

• each nucleus now has a n # of chromosomes

• cell invaginates, forming a cleavage furrow, extending to 2 separate n cells

Prophase II

• chromosomes coil and become compact

• nuclear envelope and nucleolus, if re-formed, disappears again

• centrioles move to opposite poles, forming spindle fibers between them

Metaphase II

• individual duplicated chromosomes align on the equator (metaphase plate)

• 1 chromosome per spindle fiber attached by means of kinetochore of centromere

• centrioles have reached the poles

Anaphase II

• spindle fibers contract

• duplicated chromosomes split in half

• sister chromatids separate & move to opposite poles

Telophase II

• daughter chromosomes have reached the poles

• two cells invaginate and form 4 daughter haploid cells (gametes)

• uncoil and form chromatin

• nuclear envelope & nucleolus form around chromatin again

• centriole form centrosome

down syndrome caused by extra chromosome in chromosome 21, called trisomy 21