Alternation of Generation

 so let's talk about alternation of

generations which is something that is

unique to two plants and some protists

so with alternation of generations it's

very important that you remember how

mitosis and meiosis works so here's a

quick review mitosis is a type of cell

division in which one cell divides and

produces two genetically identical

daughter cells

so this is what growth and repair is all

about now meiosis is different in that

one cell will divide and it will produce

a cell or cells but the daughter cells

have half the number of chromosomes now

it's not just any half the number of

chromosomes though in fact we have take

a diploid cell and go through this

process of meiosis and we wind up with

cells or a cell that is haploid diploid

cells have two copies of each chromosome

whereas haploid cells have only one copy

of each chromosome so alternation of

generations uses both mitosis and

meiosis mitosis one cell divides

produces daughter cells that are

genetically identical meiosis when the

cell divides the resulting cell is

haploid so let's look at alternation of

generations in more detail and see where

the different types of cell division

occur let's start out with it's a life

cycle so where we started is really

arbitrary but we'll just start with with

the sporophyte generation most plants

have a dominant sporophyte generation

so

if you look out your window and you see

a bunch of trees and grass then you're

looking at the sporophyte generation of

plants so think about an oak tree for

example that oak tree represents the

sporophyte generation and that

sporophyte generation is diploid meaning

that

as two copies of each kind of chromosome

now in the in the sporophyte generation

there are special structures where

meiosis takes place and when meiosis

takes place we have a diploid cell that

divides by this process of meiosis and

produces haploid spores now

what this looks like depends upon the

kind of plant that we're dealing with

but just in general remember that

sporophyte s-- produce spores by meiosis

these spores are haploid and so this

haploid spore will then divide by the

process of mitosis and that results in

the gametophyte generation and the

gametophyte generation of course is

haploid now sometimes this is confusing

because this is the first time that

you've probably heard of haploid cells

dividing by mitosis and producing more

haploid cells but remember with mitosis

one cell divides and produces daughter

cells that are genetically identical

so this haploid spore will divide and

produce two cells then those two will

divide and then those will divide and

those will divide and we'll get a

gametophyte now in some plants this

gametophyte is going to be microscopic

and you would never notice it most

probably in other plants such as mosses

the gametophyte generation may be

dominant but for most plants the

gametophyte generation is something that

that you're not going to be aware of

now this haploid gametophyte as the name

suggests produces gametes sperm and egg

if you think about pollen pollen

represents the male gametophyte for

plants such as trees pine trees oak

trees etc so the sperm and the egg are

both haploid dear

in fertilization the sperm will

fertilize the egg and we get a diploid

zygote this diploid zygote

will then divide by the process of

mitosis and we get a sporophyte

generation so this is why it's called

alternation of generations and if I drew

a line between the across you can see I

can actually make the line work you can

see that we alternate between the

sporophyte generation and the

gametophyte generation