Life Science

Chapter 4- Cell Reproduction

I. Cell Growth and Division: cells in an organism are going through continuous changes; growing, reproducing, exchanging chemicals, repairing, and dying

    1. Cell Cycle: life cycle of a cell- 4 stages that occurs in all living cells
    1. Birth of a cell
    2. Growth/development (Interphase stage)
    3. Reproduction (Process of cell division=Mitosis)
    4. Death
    1. Interphase- a period of cellular growth with:
    1. DNA synthesis
    2. Preparation for cell division
    3. Most of the cells life is in this phase
    1. @ 20 hours for a typical cell cycle with about 18 hrs spent in interphase
    2. in Monerans (bacteria) may be as short as 15-20 mins

 

 
    1. Some specialized cells no longer divide- stay in interphase ex. Nerve, brain cells
    1. mitosis- a period of cell reproduction in the cell cycle that consists of interphase and 4 other phases:

Interphase- the growth period and preparation for division: with the nucleus visible, chromosomes are not visible, but they are duplicating (DNA synthesis)

Phase 1-Prophase:

-Chromosomes condense and become visible

-Nucleus and nuclear membrane disappear

-twp centrioles appear and move to opposite "poles" of cell

Phase 2- Metaphase:

-double stranded chromosomes line up around center (equator) of cell

-each centromere becomes attached to a spindle fiber

(centromere is an area on the double stranded chromosome is attached together)

Phase 3 Anaphase:

-double stranded chromosomes divides at the centromere

-the strands of each chromosome are pulled to opposite ends (poles) by spindle fibers

Phase 4 Telophase:

-centrioles and spindle fibers start to disappear

-chromosomes stretch out and are no longer seen

-nuclear membrane forms around chromosomes (2) groups

-nucleolus appears within each nucleus

-cytoplasm pinches in and two new cells formed from old one.

-EACH CELL IS IDENTICAL TO THE "OLD" CELL

Plant Cell mitosis differs from animal cell mitosis in the following manner:

    1. plant cells do not have centrioles, but do have spindle fibers
    2. plant cells have rigid cell walls- cytoplasm does not pinch in; a cell plate forms between 2 new nuclei and cell walls and cell membranes separating both cells form along cell plate

 

 

II. Asexual (without sex) Reproduction:

    1. Reproduction- how an organism produces other organisms like itself (offsprng)
    2. Asexual: without sex, offspring are produced from 1 parent and are exactly like the parent- Why? Because they have the same chromosomes!

Methods: ( they all are a result of mitosis)
    1. fission- divides in two through mitosis, each new organism is identical

      2. Examples: paramecium, amoeba, bacteria

    2. Budding: a new organism grows from the body of the old organism (may then break off) Examples: hydra, some worms, many plants
    3. Regeneration- a repair or replacement of damaged or worn out parts, an organism may develop from a piece of another organism. Examples: starfish, and some worms.

III. Sexual Reproduction-

    1. Process- manufacture of sex cells by parents (meiosis)
    1. Sex cells- egg cell produced by a female and sperm cell produced by male
    2. Each sex cell has ˝ the number of chromosomes of the parent (identical chromosomes but only have one copy not two)
    3. Fertilization: joining of sex cells (2) from compatible organisms (same type of orgs)
    4. Fertilized egg is called a zygote.
    5. Zygote receives ˝ of its chromosomes from each parent (1 copy of each chromosome from each parent)
    6. Meiosis- is a type of reduction division- it reduces the number (2n) of chromosomes in the parent cell to ˝ (n) the number in the sex cells.
    7. Zygote will grow into same type of organism has its parents, but will have characteristics from both parents.
    1. Fertilization:
    1. Internal- sperm is deposited by male inside female (some exceptions-seahorse) egg is fertilized

a. female carries fertilized egg for some period of time (chicken-until shell hardens around egg) -most mammals until offspring is full term
    1. b. provides some degree of protection to new organism
    2. c. Examples: some sharks, reptiles, mammals, many flowers, birds
    1. External- sperm is deposited by male over eggs outside the female’s body; eggs mature outside the female’s body. Occurs primarily in water.
    1. little protection for eggs except guarding and cared of eggs by parents of some species
    2. Examples: most insects, amphibians, and fish

IV. DNA (deoxyribosenucleic acid)- controls all characteristics of an organism (all its life processes)

1.Code of life- found in nucleus of almost every cell.

2.Chromosomes are composed of genes (sequence of DNA that codes for one protein)

3.DNA- is the same in all cells of an organism.

4.DNA- is different from organism to organism even if they are the same species (only exception are identical twins)

Model of DNA:
    1. spiral staircase with "rails" made of sugar and phosphate
    2. each step of the staircase is made of nitrogen bases

      3. (4types Adenine with Thymine and Guanine with Cytosine)

    3. If A and T or G and C molecules split (during duplication), another molecule of the pair will join the unpaired one, this is how it replicates.

 

RNA: (ribonucleic acid)

    1. similar to DNA (only has one strand or rail and has uracil instead of thymine)
    2. necessary for protein production
    3. 2 types : Transfer RNA and Messenger RNA

Mutations:

  1. code of chromosomes (genes) changed or disrupted by: chemicals, radiation, and or a virus.
  2. Change may or may not be evident; characteristics may or may not change, most are harmful,

very few are helpful.

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