THE HUMAN GENOME AND HEREDITY

Nutrition and Gene Expression

Jan 22, 2015

This graphic is a simple cartoonthe genome (46 chromosomes).Most of your cells contain thiscomplete set of chromosomes.(note FEMALE/MALE difference).

If your progeny got exactlythese genes, Then your progeny wouldbe essentially your twin(except of course, muchyounger than you!).

But that’s NOT what happens.

Your children will have46 chromosomes, but 23 of thosechromosomes will be fromthe maternal set, and 23 of thosewill be from the paternal set.

23 are selectedfor the ovum

23 are selectedfor the sperm

The new embryo has a full setof 46 chromosomes

MATERNAL SET PATERNAL SET

SIMPLE MITOSIS: Duplicationof Chromosome 2.

This happens as a regular partof each cell division, since eachcell needs a complete copyof all the DNA in the cell.

For a homework assignment, you willneed 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

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

TWO EXACT COPIESOF ORIGINAL DNA

BUT ONE COPY CAN HAVE THEWRONG DNA BASE!

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

ATGCTAATGTGTCTAT ATACG

TACGATTACACAGATATATGC

DEFECTIVE COPY

A WHOLE REGION CAN BE DELETED

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

ATGCTAATGTGCCTAT ATACG

TACGATTACACGGATATATGC

ATGCTAATGTGCTATACG

TACGATTACACGATATGC

This copy has lost 3 bases from each strand

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.

MEIOSIS

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.

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.

OvumDNA

SpermDNA

BOTH ARE HAPLOID

DIPLOID EMBRYO

Several steps

The following slides expand on the steps of

meiosis. It is of interesting because it

allows the MIXING of genetic material.

1CHROMOSOMES, SET 1 MEIOSIS:The process is shown onlyfor the process in the ovum,it’s in the same in the sperm.

We will illustrate this only forchromosome 1.

When the DNA is duplicatedfor the ova, the female hasa chromosome from her mother (a) and her father (b) that can be used.

NOTICE THE COLOR DIFFERENCE BETWEENMATERNA AND PATERNAL.

a b ba

ba

STEP 1: Each copy ofchromosome 1is duplicated.

a b ba

The chromosomeshave been duplicated.Two of them MOVEwithin the cell, so theare directly adjacent.

a b ba

STEP 2:One of the copiesfrom maternal, andone from paternal,pair up!

STEP 3Part of the DNA sequenceis exchanged betweenthe two that are paired.(this is called: CROSSING OVER).

a b ba

RESULT: For 2 of the4 chromosomes, therehas been PARTIALDNA exchange.

This shows one exchange,there may be 2-3 crossoversror each meiotic pair.

a b/a ba/b

a ba/b b/a

POSSIBLE DNA STRUCTURES FOR CHROMOSOME 1 IN THE OVUM

From mother: no changes

From father: no changes

IN THIS DIAGRAM, MOTHER and FATHER REFER TO THECHROMOSOME YOU GOT FROM EACH PARENT.

NOTE: THE OVA AND SPERM ARE HAPLOID, ANDHAVE ONLY 1 COPY OF EACH CHROMOSOME.

Mixture/maternaland paternal

Mixture/paternaland maternal

a ba/b b/a

WHEN THE OVA AREFORMED, ONE COPYGOES TO EACH OVUM.

a ba/b b/a

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?