Download 2) Overview of the human genome

THE HUMAN GENOME AND HEREDITY
Nutrition and Gene Expression
Jan 22, 2015
This graphic is a simple cartoon
the genome (46 chromosomes).
Most of your cells contain this
complete set of chromosomes.
(note FEMALE/MALE difference).
If your progeny got exactly
these genes,
Then your progeny would
be essentially your twin
(except of course, much
younger than you!).
But that’s NOT what happens.
Your children will have
46 chromosomes, but 23 of those
chromosomes will be from
the maternal set, and 23 of those
will be from the paternal set.
MATERNAL SET
PATERNAL SET
23 are selected
for the ovum
23 are selected
for the sperm
The new embryo has a full set
of 46 chromosomes
SIMPLE MITOSIS: Duplication
of Chromosome 2.
This happens as a regular part
of each cell division, since each
cell needs a complete copy
of all the DNA in the cell.
For a homework assignment, you will
need to fill in the missing steps.
SEVERAL STEPS
DNA REPLICATION: If every gene was copied
perfectly, the genes in the gametes would be
identical to the parental genes.
This is what is supposed to happen
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
TWO EXACT COPIES
OF ORIGINAL DNA
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
BUT ONE COPY CAN HAVE THE
WRONG DNA BASE!
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
TACGATTACACAGATATATGC
DEFECTIVE COPY
ATGCTAATGTGTCTAT ATACG
A WHOLE REGION CAN BE DELETED
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
TACGATTACACGGATATATGC
ATGCTAATGTGCCTAT ATACG
This copy has lost
3 bases from each strand
TACGATTACACGATATGC
ATGCTAATGTGCTATACG
MEIOSIS
The duplication of the chromosomes during
formation of gametes (eggs and sperm) is
called MEIOSIS.
This is different in fundamental ways from
mitosis. The mechanism of meiosis explains
major features of the way genes are passed
on to the offspring.
Mutations that occur here can be very significant
as will be drawn on the board.
THESE SLIDES HAVE COMPLEX NOTATION.
FURTHER EXLANATION WILL BE PROVIDED
DURING LECTURE.
We use different codes, for example:
#1 = chromosome 1 (forms a pair, in most cells)
M = maternal chromosomes, P = paternal
a,b = each of the two maternal chromosomes
that can be provided to the gamete
DURING MEIOSIS, GAMETES ARE FORMED THAT
ONLY HAVE ONE COPY OF EACH CHROMOSOME.
THE EMBRYO OF COURSE HAS TWO COPIES.
Ovum
DNA
Sperm
DNA
BOTH ARE
HAPLOID
Several steps
DIPLOID EMBRYO
THE PROCESS IS VERY COMPLEX, AND IS OFTEN
MISUNDERSTOOD.
I WILL SHOW YOU GRAPHICALLY, IN THE
FOLLOWING SLIDES, THE STEPS THAT ACTUALLY
OCCUR IN THE FORMATION OF GAMETES.
The following slides expand on the steps of
meiosis. It is of interesting because it
allows the MIXING of genetic material.
MEIOSIS:
The process is shown only
for the process in the ovum,
it’s in the same in the sperm.
1
CHROMOSOMES,
SET 1
a
b
We will illustrate this only for
chromosome 1.
When the DNA is duplicated
for the ova, the female has
a chromosome from her
mother (a) and her father (b)
that can be used.
NOTICE THE COLOR
DIFFERENCE BETWEEN
MATERNA AND PATERNAL.
STEP 1: Each copy of
chromosome 1
is duplicated.
a
a
b
b
The chromosomes
have been duplicated.
Two of them MOVE
within the cell, so the
are directly adjacent.
STEP 2:
One of the copies
from maternal, and
one from paternal,
pair up!
a
a
a
b
a
b
b
b
STEP 3
Part of the DNA sequence
is exchanged between
the two that are paired.
(this is called:
CROSSING OVER).
RESULT: For 2 of the
4 chromosomes, there
has been PARTIAL
DNA exchange.
This shows one exchange,
there may be 2-3 crossovers
ror each meiotic pair.
a
a
a
b
a/b b/a
b
b
POSSIBLE DNA STRUCTURES FOR CHROMOSOME 1 IN THE OVUM
a
From mother:
no changes
a/b
Mixture/maternal
and paternal
b/a
Mixture/paternal
and maternal
b
From father:
no changes
IN THIS DIAGRAM, MOTHER and FATHER REFER TO THE
CHROMOSOME YOU GOT FROM EACH PARENT.
NOTE: THE OVA AND SPERM ARE HAPLOID, AND
HAVE ONLY 1 COPY OF EACH CHROMOSOME.
a
a/b
b/a
b
WHEN THE OVA ARE
FORMED, ONE COPY
GOES TO EACH OVUM.
a
a/b
b/a
b
WHAT DOES THIS MEAN? It means that
50% of the time, the chromosome #1 that
you provide your child has been
REARRANGED so it has a mixture
of your PATERNAL #1 DNA and your
MATERNAL#1 DNA.
You could also provide a chromosome #1
that is identical, to what you got from your
father or from your mother.
The subject of genetic recombination is
complicated, and often is misunderstood.
We will revisit this topic for additional
discussion in further lectures.
THE NEXT TOPIC: Since you get 2 copies of
most genes (one from each parent), what
consequences can occur?