Another of the characteristics that all living things possess is the ability to reproduce -- true for organisms consisting of a single cell through those made up of billions of cells.
The biological characteristics of all organisms are determined by molecules of DNA that reside in the nuclei of the cells -- DNA molecules contain "instructions" which determine cell function and all other characteristics.
Reproduction: the production of new organisms or cells from pre-existing ones -- involves the transmission of DNA from parents to offspring
Cells and organisms display two types of reproduction:
1. Asexual reproduction -- only one parent and offspring are genetically identical ("clones") of the parent -- offspring receive exactly the same DNA instructions as parent has.
2. Sexual reproduction -- two parents and offspring have slightly different DNA message from those of the parents and from other offspring.
Underlying each of these reproductive modes is a specific type of cell division: mitosis for asexual reproduction and meiosis for sexual reproduction. We will consider these two processes in a moment.
Let’s first take a superficial look at the nature of the DNA instructions present in all cells.
Molecules of DNA are found associated with proteins in structures called chromosomes -- can think of these as long, thread-like fibers -- chromosomes are found in the cell nucleus.
All species have a characteristic number of chromosomes in all their cells -- this is called the diploid number and is symbolized as the 2N number -- e.g. for humans, 2n = 46; domestic dog, 2N = 78; fruit fly, 2N = 14.
In order for cells to reproduce, chromosomes must undergo replication so that there will be copies of the chromosomes to be passed on to offspring cells.
A chromosome that has replicated then consists of two "sister" chromatids held together by a structure called a centromere.
Figure 8.2:
Cells that are not dividing are said to be in interphase -- majority of the cell's life is spent in interphase -- time during which cells grow -- chromosomes appear as an amorphous mass called chromatin -- just prior to the beginning of mitosis, chromosome replication occurs and chromosomes consist of pairs of duplicate sister chromatids.
1. Prophase -- first phase of mitosis.
a. chromatin condenses into thick, short chromosomes.
b. centrioles begin moving towards opposite ends of nucleus.
c. spindle fibers (microtubules) begin to appear.
d. nuclear envelope begins to disappears.
Figure 8.5 (Prophase):
2. Metaphase -- second phase of mitosis.
a. chromosomes align along equator of spindle apparatus.
b. attached to spindle fibers at centromere.
Figure 8.5 (Metaphase):
3. Anaphase -- third phase.
a. centromeres of each chromosome divide and chromatids now separate -- sister chromatids are now chromosomes.
b. spindle fibers begin new chromosomes towards opposite poles.
Figure 8.5 (Anaphase):
4. Telophase -- final phase of mitosis.
a. nuclear envelope reappears around two new daughter nuclei.
b. chromosomes revert to the diffuse chromatin condition.
Figure 8.5 (Telophase):
New daughter cells now enter interphase, where they will spend the majority of their lives.
Cytokinesis -- division of cytoplasm -- occurring concurrently with mitosis -- takes place differently in plants and animals.
Importance of mitosis: each new daughter cell receives the same number and kind of chromosomes as the mother cell; daughter cell are genetically identical to their mother cell and to each other.
Mitosis serves two major functions:
1. process of growth and repair of multicellular organisms -- all of us began life as a single cell that has undergone and continues to undergo mitotic cell division.
2. process by which organisms reproduce asexually -- offspring are "clones" of their parent.
a. binary fission -- single-celled organisms (e.g. ameba) reproduce simply by splitting into two new cells.
b. budding -- bud on parent organism (e.g. yeast) develops into a new organism through repeated cell divisions.
c. vegetative reproduction -- plants send out stolons (above ground) or rhizomes (underground) which become new individuals -- e.g. strawberries, Bermuda grass, ferns.
Questions about mitosis?
Now, let's look at meiosis -- the process underlying sexual reproduction.
Here's a overview of sexual reproduction:
1. new organism begins its life as a single-cell called a zygote.
2. a zygote formed is from the fusion of specialized sex cells called gametes (e.g. sperm and egg).
3. each of two parents contributes one gamete to the zygote.
4. since the zygote is diploid (2N), the gametes that fuse to form it must have 1/2 the diploid number -- gametes, therefore, are said to be haploid (1N) -- e.g. human zygotes, 2N = 46; human sperm cell (or ova) 1N = 23.
5. haploid gametes are produced in germ tissue (gonads) by a special type of cell division that results in the daughter cells (gametes) having only 1/2 the number of chromosomes of the mother cell -- this type of cell division is called meiosis.
In sexually reproducing organisms, chromosomes exist as homologous pairs -- each chromosome has a "mate" or homologue that carries the same kind of genetic information -- one member of the homologous pair is maternally inherited and the other is paternally inherited -- e.g. humans have a diploid number of 46 chromosomes arranged as 23 homologous pairs.
Let's look at meiosis -- consists of two nuclear division (meiosis I and II) -- in the first division, the chromosome number is reduced by 1/2 (diploid to haploid) -- the second division is actually nothing more than mitosis.
End result of meiosis is 4 haploid daughter cells.
Prior to meiosis, mother cells are in interphase -- like mitosis, DNA replication occurs during interphase just prior to meiosis.
Meiosis I: the reduction division -- diploid mother cell produces 2 haploid cells.
Meiosis II: mitotic division of 2 haploid cells to produce 4 haploid daughter cells.
The important aspects of meiosis occur in Meiosis I.
Prophase I -- each chromosome, consisting of two chromatids, lines up side-by side with its homologue -- this structure is called a tetrad.
Figure 9.4:
Important event occurs: crossing over -- chromatids of homologous chromosomes may randomly exchange segments -- recall that homologous chromosomes have similar, but often slightly different sets of DNA instructions -- importance of crossing over is that it produces new combinations of genes on chromosomes -- "genes shuffled".
Note: after crossing over, chromosomes are no longer identical to those of the parent cell.
Metaphase I: tetrads align along equator.
Figure 9.4:
Anaphase I -- tetrads separate -- each chromosome with its 2 chromatids goes to an opposite pole -- note: this is where the reduction in chromosome number occurs -- tetrads, rather than centromeres in mitosis, separate.
Figure 9.4:
At
the end meiosis I: 2 haploid nuclei.
Meiosis II: 2 haploid nuclei undergo mitosis to produce 4 haploid daughter cells.
Figure 9.4:
In our species, meiosis occurs in spermatogenesis (sperm production) and oogenesis (egg production.
1. spermatogenesis -- 4 daughter cells that can develop into 4 sperms -- tiny, flagellated cells adapted for locomotion.
2. oogenesis -- only one of the daughter cells becomes an ova -- other three are small, nonfunctional polar bodies.
Importance of Meiosis:
1. produces haploid gametes so that the diploid number of the species remains constant generation after generation.
2. source of genetic variation because crossing over brings together new gene combinations on chromosomes.
NEXT TIME: Patterns of Inheritance.