CELL REPRODUCTION

General Biology

Genetics is explained using a historical approach with genetic problems that stress practical aspects. The cell cycle, human genetics, cancer concepts, and biotechnology have been updated.

Chapter 8 Cellular Reproduction: Cells from Cells

The function of cell division is followed by details of the cell cycle. Stages of mitosis are reviewed and prokaryote cell division is contrasted. The causes of cancer are discussed in light of the cell cycle.

I.  Background Vocabulary of nucleus and its contents

Chromatin – the collective term for the cell’s DNA and associated proteins found in the nucleus when a cell is not dividing.  Chromatin consists of all of the chromosomes in the nucleus (one can’t see chromosomes in a nucleus until the nucleus is ready to divide).

Chromosome - (see fig. 9.2).  a DNA molecule and associated proteins, or if the chromosome is replicated, two identical DNA molecules and proteins held together at a region called the centromere.  In replicated (=duplicated) chromosomes, each of the two identical DNA molecules and proteins held together is called a sister chromatid.  The structure of chromosomes is not visible when the nucleus is not dividing.

Chromosome number – every kind of organism has a particular number of chromosomes.  This number is found in the nuclei of all its cells (except in special reproductive cells).  Ex. humans 46; ferns 100’s; houseflies 12; hydra 32; mosses 12

Karyotype - a pictorial display of an organism’s chromosomes arranged by size and shape.  See fig. 13.1 for example of human karyotype.

Haploid and Diploid – through karyotype preparations, cytologists discovered that a nucleus may contain one or two sets of chromosomes.  Human nuclei all have two sets of chromosomes, that is they are diploid [“have double the number as compared to haploid”]; only human gamete nuclei have a single set of chromosomes and are thus called haploid [half as many as the diploid number].  The concept of set is indeed quite important.   Reflecting this importance, not only were new words created (haploid and diploid) but also symbols were adopted to express this all important concept:

            n = one set, where “n” is for set and the “one” is implied; also = haploid.

            2n = two sets, where “n” is for set; also = diploid.

            A “set” of chromosomes is one of each kind of chromosome.  Fortunately, chromosomes have specific appearances based on overall length and centromere position.  Thus, for humans, cytologist discovered only 23 uniquely shaped chromosomes, that is 23 chromosomes in a set.  Also, it was discovered that for every uniquely shaped chromosome there were two chromosomes exactly alike.  The two chromosomes that correspond between sets constitute a homologous pair of chromosomes.

Homologous Chromosomes – two chromosomes with the same morphology (appearance) that carry the same sequence of genes.  Homologous chromosomes are from opposite sets of chromosomes.   

The Cell Cycle

  Cell cycle:  A symbolic way to represent the events within a cell as it prepares to divide; for a developing embryo, most cells are busy repeating the cell cycle.  But keep in mind that for us as adults with fully differentiated cell types, not all cells complete the cell cycle because some specialized cells will rarely divide again (e.g. muscle and nerve cells) or never divide again (red blood cells), especially those cells that function only after their death, e.g. cells that line our mouth are dead, cells that make up our hair are dead, in plants water conducting xylem cells are dead and dead cells can’t do the cell cycle. 

            The cell cycle consists of two major periods:  Interphase (a name for the period in which a cell is not dividing; the cell is otherwise functioning metabolically, doing the things a cell needs to do to stay alive, ATP production for example. So, interphase is not a “resting stage” per se.  It can be considered a resting stage only from the point of view that the cell is “resting” between divisions.  During this pause or “rest” between divisions one particular metabolic event will occur if the cell is to divide again (see below).  The second major period of the cell cycle is cell division.

            The important thing to know about the cell cycle is this:  it accounts for a very important event required for any future cell divisions.  This event is intangible otherwise as it can’t be seen by human eyes.  This event is indicated on the cell cycle as “S” for synthesis.  The “S” phase of interphase is when DNA is replicated (duplicated).  This is when chromosomes gain their sister chromatids through a duplication process to be covered later.  It is important to lose the misconception that sister chromatids “pair up.”  The original chromosome essentially splits itself down the middle lengthwise and becomes two chromatids held together at the centromere.   

Reproduction:

_____________________________

_____________________________

Cell Division plays a role in:

_____________________________

_____________________________

I. Passing on Genes from Cell to Cell

Before a parent cell divides, it _________________ its chromosomes.  The resulting _____________________ are genetically identical.

II. The Reproduction of Organisms

In __________________________, single-celled organisms reproduce by simple cell division.

Some _______________________ organisms can into pieces then grow into new individuals.

Sexual reproduction is different:

__________________________________________________________

__________________________________________________________

CELL CYCLE AND MITOSIS

Almost all of the genes of a eukaryotic cell:

__________________________________________________________

III. Eukaryotic Chromosomes

Each eukaryotic chromosome contains one very long _________ molecule, typically bearing thousands of ____________.

The number of eukaryotic chromosomes in a cell ________________________________________________________.

Chromosomes are made of _________________, a combination of DNA and protein molecules.  They are not visible in a cell until _______________ occurs.

The DNA in a cell is packed into an elaborate multilevel system of _________________ and ___________________.

Before a cell divides, it duplicates all of its __________________________, resulting in two copies called ___________________.

IV. The Cell Cycle

Eukaryotic cells that divide undergo an orderly sequence of events called the _______________________.

The cell cycle consists of two distinct phases:

1. ______________________

2. ______________________

Mitosis:

_____________________________________________________

_____________________________________________________

Mitosis consists of four distinct phases:

1. ____________

2. ____________

3. ____________

4. ____________

Cytokinesis:

__________________________________________

__________________________________________

__________________________________________

Click on http://www.cellsalive.com/mitosis.htm for animation on mitosis

For an online lesson on mitosis, click on http://www.biologylessons.sdsu.edu/ta/classes/lab8/mitosis.html

For a view of animal mitosis, click on: http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/cell_division/wf_review_fs.html

Click on: http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html for a cell cycle and mitosis tutorial

To view plant mitosis, click on: http://micro.magnet.fsu.edu/micro/gallery/mitosis/mitosis.html

V. Cancer Cells: Growing Out of Control

Normal plant and animal cells have a cell cycle ______________________.

A. What is Cancer?

Cancer is a disease of the _________________.  Cancer cells do not respond normally to the _______________________.

Cancer cells can form ________________.  Tumors are ______________________________________________.  If a tumor is ________________ it can spread to other parts of the body.

B. Cancer Treatment

Cancer treatment can involve:

1. __________________________________________________________________

2. __________________________________________________________________

Cancer cells are often grown in _____________________ for study.

C. Cancer Prevention and Survival

Cancer prevention includes changes in lifestyle:

1.  ______________________________________________

2.  ______________________________________________

3.  ______________________________________________

4.  ______________________________________________

5.  ______________________________________________

6.  ______________________________________________

Meiosis, the Basis of Sexual Reproduction

Sexual reproduction depends on:

1.  _________________________

2.  _________________________

I. Homologous Chromosomes

Different organisms of the same species have the same number and type of _____________________________.

A somatic cell:

1.  ______________________________

2.  ______________________________

A ____________________ is an orderly arrangement of chromosomes.  ____________________ chromosomes are matching pairs of chromosomes.

Humans have:

1.  ___________________________________

2.  ___________________________________

II. Gametes and the Life Cycle of a Sexual Organism

The life cycle of a multicellular organism is the sequence of stages leading from the _________ of one generation to the ____________ of the next generation.

Humans are diploid organisms:

1. ______________________________________

2. ______________________________________

III. The Process of Meiosis

Meiosis occurs at different times in the life cycle of plants, animals, and fungi, but its phases are the same. In humans, meiosis is a part of spermatogenesis and oogenesis. The contribution of meiosis to the evolutionary process is discussed.

  Meiosis.  Meiosis is one of the necessary events needed to complete sexual reproduction.  As a type of nuclear division (or karyokinesis) meiosis is a bit more complex than mitosis.  So, one might ask what’s the benefit of a more complex division process (meiosis) over that of a simpler division process (mitosis)?   Meiosis does have beyond the events of mitosis such occurrences as synapsis (the physical pairing of homologous chromosomes during prophase I)  and separation of homologues (the two chromosomes of a synapsed pair separate during anaphase I and end up in separate cells by the end of meiosis I).  You might answer that meiosis is a two rather than one division process, that four rather than 2 daughter cells are formed, and that chromosome numbers are reduced by one half in the newly formed daughter cells.  These are all true but such facts do not answer the question of what’s the benefit of meiosis over that of mitosis?  Both mitosis and meiosis play important roles.  Mitosis allows for growth in multicellular organisms, repair and replacement of damaged tissues, and asexual reproduction.  To understand the benefits of meiosis it is necessary to think of meiosis as part of sexual reproduction.  To put the question another way, what is the benefit of sexual reproduction (a meiosis requiring event) over that of asexual reproduction (a mitosis requiring event)?  You need only to complete a Punnett square for the genetic cross between identical parents, say Aa breeds with Aa (each parent is identical, no variation).   The results of this cross should yield variation among the offspring.   Thus, sexual reproduction via meiosis and fertilization introduces genetic variation. 

   MEIOSIS halves the chromosome number.

            Fertilization is another event required for sexual reproduction.  Fertilization doubles the chromosome number in the offspring.  But offspring have the same chromosome number as their parents.  Therefore, the parents, through meiosis, reduced the chromosome number by one-halve in their cells capable of fertilization.  Following fertilization the chromosome number is restored to the diploid number (i.e., two sets of chromosomes, 2n).  Meiosis halves the parental diploid chromosome number by simply separating the sets of chromosomes in specialized cells in gonads (animals) or sporangia (plants; sporangia are sacs that produce spores, spores are like seeds in that they grow into a new individuals).  The process of separating the sets of chromosomes is called meiosis.  It requires a diploid cell to undergo two divisions thus producing four daughter cells each with a single set of chromosomes (i.e., haploid or n).      

In meiosis:

1. _______________________________________

2. _______________________________________

3. _______________________________________

IV. Review: Comparing Mitosis and Meiosis

All the events unique to meiosis occur during __________________.

V. The Origins of Genetic Variation

Offspring of sexual reproduction are genetically ______________________ from their parents and from _______________________.

A. Independent Assortment of Chromosomes

In independent assortment, every __________________ orients itself ____________________ of the others during ______________.

B. Random Fertilization

The human egg cell is fertilized randomly by one _______________, leading to genetic variety in the __________________.

C. Crossing Over

In crossing over:

1. _______________________________________

2. _______________________________________

VI. When Meiosis Goes Awry

What happens when errors occur in meiosis?

A. How Accidents During Meiosis Can Alter Chromosome Number

In nondisjunction:

1. ________________________________________________________

2. ________________________________________________________

The result of nondisjunction __________________________________________________.

B. Down Syndrome: An Extra Chromosome 21

Down Syndrome:

1. ______________________________________

2. ______________________________________

The incidence of Down syndrome increases with the __________________________________________.

C. Abnormal Numbers of Sex Chromosomes

Nondisjunction also affects the _______________________________.

Evolution Connection: New Species from Errors in Cell Division

Errors in __________________ may have been instrumental in the ___________________ of many species.

Polyploids:

1. ___________________________________

2. ___________________________________

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