SEXUAL REPRODUCTION

ALL CELLS COME FROM PRE-EXISTING CELLS
(however, sometimes one cell comes from two!)

ASEXUAL REPRODUCTION: involves one parent; daughter cells are identical to the parent cell

SEXUAL REPRODUCTION: involves two parent cells; each parent gives some of its traits (characteristics) to the offspring

MITOSIS vs. MEIOSIS

MITOSIS produces two cells with the same diploid (two sets) number of chromosomes MEIOSIS produces four cells with only one set of chromosomes (called monoploid or haploid); these cells are called gametes:

SPERM (MALE) EGG or OVUM (FEMALE)


* in sexual reproduction, a gamete (sperm or egg) from each parent fuses (combines--called FERTILIZATION)
* this produces a fertilized egg cell which is called a ZYGOTE

Remember, different organisms have a different number of chromosomes that make up one set...(n):

bull frogs have 13 per set (n)
fruit flies have 4 per set (n)
HUMANS HAVE 23 PER SET (n)

...however, in our body cells, we have a total of 46 chromosomes (2n)...WHY?

BECAUSE WE GET HALF OF OUR CHROMOSOMES FROM EACH PARENT!

* humans have a diploid number of chromosomes (two sets)
* one chromosome from each set comes from each of our parents
* similar chromosomes from each parent are called HOMOLOGOUS CHROMOSOMES
* these chromosomes have instructions for the same characteristics

TIME FOR THE SMELLY SHOE DEMO!

Meiosis and Sexual Reproduction
here is a little history of the discovery of this type of cell division
review the haploid (monoplod) and diploid cells in organisms
watch all the basics of meiosis
what exactly are homologous chromosomes?

I. PROCESS OF MEIOSIS-
* in mitosis, there is replcation of the DNA, then a division of the chromosomes/cytoplasm

* in meiosis, there is a replication of the DNA, then the chromosomes get shuffled up, the the chromosomes/cytoplasm divide, then they divide a second time

* because the cell only copies the chromosomes once but divides twice, it is sometimes called REDUCTION DIVISION


A) INTERPHASE
* single-stranded chromosomes are replicated (duplicated) to from double-stranded chromosomes

B) FIRST MEIOTIC DIVISION

PROPHASE I:

* homologous double-stranded chromosomes line up, forming a TETRAD (that is four chromatids lined up in a row); this is called SYNAPSIS
* while they are lined up, ends of the chromosomes can switch places; this process of shuffling up the genetic information is called CROSSING-OVER


METAPHASE I:

* as in mitosis, the tetrads line up in the equatorial plane (middle) of the spindle in the cell

ANAPHASE I:

* the homologous chromosomes of the tetrad separate from each other and move to different ends of the cell
* this is called DISJUNCTION

TELOPHASE I:

* the first meiotic division ends as the cytoplasm divides into two daughter cells

C) SECOND MEIOTIC DIVISION
* the second mitotic division is much like mitosis; no further replication of chromosomes occur

PROPHASE II:

* the spindle forms for the second time
* the chromosomes move toward the center of the spindle

METAPHASE II:

* the chromosomes become attached to the spindle in the center of the spindle
* the chromosomes are STILL DOUBLE-STRANDED

ANAPHASE II:

* the centromeres divide and the sister chromatids separate
* now, the SINGLE-STRANDED chromosomes move toward opposite ends of the spindle

TELOPHASE II:

* both daughter cells from the first mitotic division divide forming FOUR monoploid cells, having only one set of chromosomes

MITOSIS vs. MEIOSIS

MITOSIS: 1) growth and asexual reproduction; 2) occurs in all cells (my-toes-ees); 3) daughter cells are identical to parent (same # of chromosomes)

MEIOSIS: 1) sexual reproduction; 2) occurs in sex cells (my-oh-sees); 3) daughter cells are different than parent 1/2 # of chromosomes)

Meiosis and Sexual Reproduction
watch all the basics of meiosis
a side-by-side comparison of these different cell divisions from PBS
made for Regents biology, this is a must see animation/ description of meiosis
review the stages of meiosis in this interactive
the process of crossing-over, explained
see how crossing over affects chromosomes
good review of meiotic cell division; nice vocabulary review
check out these two interactives where you have to place the cells in the correct order of meiosis
this time, John Kyrk uses his fantastic animations on meiosis
a narrated animation of meiosis; includes practice questions
an on-line text book; click on all four animations to see how meiosis and mitosis occurs

II. GAMETOGENESIS
(GAMETE = sex cells) (GENESIS = creation)
* the production of sex cells
* occurs in specialized organs called gonads

TESTES (males) OVARIES (females)

* some organisms have one or the other
* others have both male and female gonads and are called hermaphrodites

A) SPERMATOGENESIS
(SPERM = sperm) (GENESIS = creation)

* the production of sperm
* a primary sex cell in the testes undergoes meiosis (replication and division) and produces 4 functional monoploid sperm cells

B) OOGENESIS
(OO = egg) (GENESIS = creation)

* the production of egg cells
* a primary sex cell in the ovary undergoes meiosis and forms ONLY 1 EGG
* eggs are larger than sperm due to unequal cytoplasmic division (yolk!)
* besides 1 functional ovum (egg), 3 non-functional polar bodies are formed

EGG (OVUM) vs. SPERM

EGG: 1) size depends on organism; 2) round in shape; 3) non-motile (cannot move); 4) has yolk to nourish the developing embryo

SPERM: 1) microscopic; 2) made up of a head and a tail and middle piece; 3) motile; 4) middle section has lots of mitochondria for energy to swim

 

Meiosis and Sexual Reproduction
an egg, the biggest cell for every sexually reproducing species...and they are fun to throw around!

III. FERTILIZATION
* the union between a monoploid (n) sperm nucleus with a monoploid (n) egg nucleus
* forms a diploid (2n) ZYGOTE which contains one set of chromosomes from each parent

A) EXTERNAL FERTILIZATION

* gametes fuse outside the female parent (externally)
* in this process, the female produces a large number of eggs because...

1) the eggs can be crushed
2) the eggs can be eaten
3) the eggs can dry out
4) the eggs might not survive

* occurs in aquatic vertebrates such as fish and amphibians

B) INTERNAL FERTILIZATION

* gametes fuse within the female parent in her moist reproductive tract (internally)
* females produce fewer eggs because they are protected inside the mother
* occurs in most terrestrial vertebrate animals C) PARTHENOGENESIS
* the development of an unfertilized egg without fusion with a sperm
* occurs in insects (drone or worker bees)



SO...IN MEIOSIS...a human diploid cell has chromosomes replicate and divide, then shuffles up its chromosomes, then another division without duplication of chromosomes, which yield human monoploid sex cells

* in meiosis, the homologous chromosomes are randomly assorted for each new monoploid gamete
* this results in forming new combinations of characteristics, which leads to VARIATION in the species when fertilization occurs

Meiosis and Sexual Reproduction