HAZEL SIVE: At this point
after your class exercise,
you should be familiar
with these three
aspects of mutation.
You should be able to
take a piece of DNA,
conceptually turn it into RNA,
and conceptually translate it
into protein.
You should understand what
happens to the protein sequence
when the DNA is changed
by a point mutation
or by an insertion
or deletion mutation.
And so with that
in mind, let us go
on to our second
topic today, which
is called allele segregation.
OK, first thing we need to think
about is what an allele means.
And so let us write
down on the board
the first line, which
is that an allele is
a alternate form of a gene.
Alleles can be called variants.
And they may be designated
relative to some reference DNA
sequence, some reference gene,
that is called wild type.
Although this is
not done in humans,
it's done in various
other organism.
So variants-- and let's say
may be relative to wild type.
That may be your reference.
Alleles are there because
of different DNA sequences.
That's the point.
So an allele is due to
DNA sequence variations
or differences.
We may not know
them all the time.
And we'll get to
that in a moment.
But we can designate
how a gene's alleles
look by this kind of notation.
So for example, let's
designate the apple gene.
And it might have
alleles apple big B--
and maybe that gives
you a green apple--
or apple little b.
And maybe that gives
you a yellow apple.
How do these genes
relate to the chromosomes
that we talked
about way back when,
these collections of genes that
form huge arrays in the cell
and are how the cell's
genes are organized?
They relate like this.
In a diploid cell,
each of these alleles
would be on the
matching chromosome,
would be on one of the
matching chromosomes.
So in diploid cells, two n
cells, each chromosome, chr,
pair has the same or different
alleles of particular genes.
That is a little background
to then look and see
what we mean by alleles
in a visual sense.
One can look at
flower color and see
that flower color in a
particular floral species
varies enormously.
And these variations
in flower color
are due to different
alleles of one or more genes
encoding flower color pigments.
Eye color-- it's the same idea.
Eye color comes from
different alleles
of the same genes
that contribute
to the color of our eyes.
And labradors-- I have a
black labrador at home.
But there's a whole array
of different colors.
Actually, there's really
four colors of labrador.
And their color fur depends
on particular alleles
again of the pigment
that makes fur color.
So this is the notion
behind alleles.
And it is hugely important in
understanding how life works.
Let us continue thinking about
the chromosomes and the alleles
on the chromosomes.
And we'll get a bit more
visual here rather than
just writing it on the board.
So I want to talk here
about cell division
and allele segregation.
During the process of mitosis,
division of the body cells,
the outcome cells,
the daughter cells,
have got exactly the same set
of alleles as the parent cell.
There are some variations
on this occasionally
that can lead to disease.
But by and large,
during mitosis,
the alleles that start
off in the parent cell
are the ones that land
up in the daughter cell.
So in the body cells,
the daughter cell
has alleles equal to the
parent, identical to the parent.
And that actually
is just another way
of saying that mitosis
gives identical daughter
cells to the parent.
We talked about that previously.
We also talked about
meiosis, the process
that gives rise to the germ
cells, the egg and sperm.
These are also called gametes.
And that's a term you'll
need to know today, gametes.
During meiosis, things
get a little strange.
And you remember from
our previous discussion
that you land up
with four cells.
From every starting
diploid cell,
you land up with
four haploid cells.
During the process
of meiosis, alleles
can really sort themselves out.
That is true even for
just one chromosome.
But when you've got lots of
chromosomes as in our cells,
we have 23 pairs
of chromosomes, you
can get all sorts of sorting
out of alleles between the germ
cells so that they
don't look anything
like the parental cell.
So I will write here
that in the germ cells
in the gametes, the alleles--
"segregate" is the term--
it means sort out such
that the gametes do not
have the same alleles as the
parent allele set, let us say.
So you need to bear this in mind
as we go through some visuals
because words do not show
it as well as visuals.
Here to get us back into the
frame of molecular biology
is the notion of alleles that
there would be the apple B
gene that would have the
particular protein sequence
on the top of the slide.
And the apple small b allele
would have the protein sequence
on the bottom of the slide.
They're both apple
genes, but they've
got different DNA sequences and
maybe some different function.
Let's look at allele
segregation during mitosis.
This is also called somatic
inheritance, body cell
inheritance.
We're starting off
with a cell that
has a pair of homologous
chromosomes, just one pair,
including the apple gene
with its two alleles.
DNA replication occurs, and
you get duplicated chromosomes.
Now, I've shown you on these
chromosomes one of the homologs
has the large B allele.
And the other homolog
has the little b allele.
After DNA replication,
the cell has now
duplicated its chromosomes.
And it has two big B and
two little b alleles.
But those segregate
to the daughter cells
so that each daughter
cell looks just
like the parent cell with a
big B and a little b allele.
That is mitosis
slightly restated
from the way we did
before where now we're
following the alleles,
the variations
of genes that are on
homologous chromosomes,
matching chromosomes.
Now we're going to
look at meiosis.
And you'll see things
get a little different.
We start again with the
diploid parental cell.
It undergoes DNA replication.
And then meiosis I occurs such
that the replicated homologs
move entirely to each
of the daughter cells.
One daughter cell
has two big Bs.
And the other daughter cell has
two little Bs after meiosis I.
But then there's this
process of meiosis II where
there's a second division.
And you land up with
four haploid cells.
And you can see in my example
that there are two kinds
of gametes that come out here.
Two of the gametes have
got the big B allele.
And two of the gametes have
got the little b allele.
That's different
than the parent cell.
And depending on whether that's
an egg or a sperm and depending
on what the mating of
the organism looks like,
the outcomes of using the
big B or the little b gametes
are going to be different.
We'll explore that
a bit later today.
And then let's get
even more complicated
and throw another
chromosome in the mix
because most organisms don't
just have one chromosome.
They have a number of them.
So I've drawn you
some schematics here
where the parent has two
pairs of chromosomes.
And on each chromosome, there
are two different alleles
of a particular gene.
There again is the apple gene
with its big B and little b
alleles.
And there is the honey gene
with its big H and its little h
alleles.
So that's what we're starting
with in the parent cell.
DNA replication
occurs, and you get
the duplication of the
whole array of chromosomes
and all the alleles.
In meiosis I, there
is four possibilities
for what might happen because
these alleles, the chromosomes,
segregate independently
at this point.
Big B, the duplicated big B,
can segregate with the little h
chromosomes, or it can segregate
with the big H chromosomes.
Little b can segregate with
the little h chromosomes
or with the big H chromosomes.
This is a stochastic.
It's probabilistic.
After meiosis II,
there are four kinds
of gametes that have come out.
I've drawn you four of them
because each is duplicated.
You can either be big B little
h, big B big H, little h little
b, or big H little b.
This is what is meant
by allele segregation--
how these different
gene variants sort out.
And how they sort
out really dictates
what the next generation of the
organism is going to look like.
And this is something
that we have
to bear in mind when we
think about inheritance
and we think about phenotypes
and traits associated
with inheritance.
So I want you to go and do
the class exercise associated
with allele segregation now
so that you can play around
with understanding
allele segregation.
