MEIOSISChapter 11

REVIEW

Cell has issues when it grows larger in size Not enough DNA Nutrients and wastes cannot pass the cell

membrane

Cell solves these problems through the process of cell division

Mitosis – the portion of cell division where the nucleus divides

MITOSIS Almost every cell of the body uses mitosis to

divide the nucleus Somatic cells – cells that are not sex

cells/gametes Exs) liver cell, bone cell, brain cell etc.

AS LONG AS IT IS NOT SPERM OR EGGS IT USES MITOSIS

Cell grows (G1), synthesizes DNA (S), makes molecules and organelles (G2) and then is ready for cell division (mitosis and cytokinesis)

STAGES OF MITOSIS

1. Prophase – chromatin condenses into chromosomes and nuclear envelope breaks down, centrioles move and spindle fibers form

2. Metaphase – chromosomes line up in middle of cell and spindle fibers attach to centromere

PROPHASE

STAGES OF MITOSIS

3. Anaphase – spindle fibers pull at centromere and separate sister chromatids pulling them to opposite ends of the cell

4. Telophase – chromosomes break down into chromatin and nuclear envelope reforms

THE END OF MITOSIS

After telophase, the cells cytoplasm splits by the process of cytokinesis

As a result we are left with…2 IDENTICAL DAUGHTER CELLS

THE NEW STUFF… There are many studies into

the process that makes each one of us different

Gregor Mendel – a monk born in 1822 who did many studies in the field of genetics

Genetics – the study of heredity

Why is it that we have traits (eye color, hair color, etc.) similar to our parents, yet we are not all alike?

GENETICS

Mendel recognized that the offspring of “parents” were similar and began to investigate why this happens

He came to the conclusion that the parent organisms must pass on traits in their genetic material

These traits are located on their genes (DNA)

MENDEL’S PREDICTIONS

Mendel was correct about the passing of traits and the idea of genes

He wasn’t sure how these events happened but knew1. An organism must inherit a single copy of every

gene from both its “parents”2. When and organism produces its own cells to

pass to offspring, there are 2 sets that must separate from each other so that each cell contains just 1 set of genes

STOP! WHAT DOES THIS MEAN?!?!

Gametes – the sex cells of an organism Sperm and eggs Mitosis deals with non-gametes (somatic cells)

Remember, in mitosis we result in genetically identical cells

WE DO NOT WANT GENETICALLY IDENTICAL OFFSPRING!!!

TRANSLATION OF MENDEL’S IDEA #2

When and organism produces its own cells to pass to offspring, there are 2 sets that must separate from each other so that each cell contains just 1 set of genes

Every cell of the human body contains a specific number of chromosomes (46)

In order for offspring to maintain that number of 46 and not end up with duplicate (92), the parent gamete (sex cell) must half their number of chromosomes

End result > 23 (mom) + 23 (dad) = 46 offspring chromosomes

TRANSLATION OF MENDEL’S IDEA #1

An organism must inherit a single copy of every gene from both its “parents”

Remember, each of the 46 chromosomes needs to halved by the parents (to make 23)

Parents will only contribute each of those 23 genes one time to their offspring (parents do not want to give multiple copies of the

same gene)So, again 23 + 23 = 46 chromosomes total

CHROMOSOMES AND GAMETES

Remember…

Chromosomes – the structures in the cell that carry the genetic material (genes) from the parent cell to the daughter cell

Gametes – sex cells or germ cells (sperm and egg)

CHROMOSOMES There are many chromosomes in the body that

carry information for many different “things”

Examples – eye color, hair color, height, 2nd toe length, etc… (everything that makes you, you!)

When 2 cells come together from 2 parents, the matching chromosomes must come in contact

These matching chromosomes are called homologus Homologous Chromosomes – same chromosome

types between mother and father cells

CHROMOSOME NUMBERS Haploid – a cell that contains a single set of

chromosomes Remember “hap” sounds like half Is usually represented as N

Diploid – a cell that contains both sets of homologous chromosomes Remember “di” means 2 (kinda like “bi” – bicycle) Is usually represented as 2N

Since the somatic cells of the body are diploid we need the sex cells to be haploid so offspring do not have more chromosomes than necessary

WHAT DO DIPLOID/HAPLOID NUMBERS MEAN?

Diploid Barbie2N = 46

chromosomes

Diploid Ken2N = 46

chromosomes

EqualsDiploid Baby

2N = 46 chromosomes

Haploid Barbie Cell

N = 23 chromosomes

Haploid Ken CellN = 23

chromosomes+

Sperm cell

Egg cell

HOW ARE HAPLOID GAMETES PRODUCED? Meiosis

a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

Involves 2 distinct divisions1. Meiosis 12. Meiosis 2

Starts with 1 diploid cell and results in 4 haploid daughter cells that are GENETICALLY DIFFERENT from the parent cell

MEIOSIS 1

Prior to meiosis 1, each chromosome is replicated Same as S phase of interphase prior to mitosis

Meiosis 1 starts with the cell beginning to divide very similarly to mitosis Unlike mitosis, meiosis 1 has homologous pairing

in each step

Homologous Pairing – the same chromosome types from mom and dad come together (eye color, hair color, etc.)

PROPHASE 1

Homologous chromosomes pair and form a tetrad Since 1 chromosome is made of 2 chromatids,

there are 4 chromatids in a tetrad

While in their tetrad, homologous chromosomes are able to trade/swap information in a process called crossing over this process results in the exchange of traits

(alleles) between the same chromosomes therefore creating new trait combinations

One reason why you are different from your parents!

REMAINDER OF MEIOSIS 1

Meiosis utilizes the remainder of the cycles as mitosis did separating homologous chromosomes instead of sister chromatids

Metaphase 1 – spindle fibers attach to homologous chromosomes

Anaphase 1 – spindle fibers separate homologous chromosomes

REMAINDER OF MEIOSIS 1

Telophase 1/Cytokinesis – the nuclear envelope reforms around chromosomes and the cytoplasm splits

As a result of Meiosis 1, 2 daughter cells are produced tat have half the number of genetically different chromosomes

MEIOSIS 2 After Meiosis 1, the 2 cells produced proceed

to meiosis 2 (there is no chromosome replication)

Each of the daughter cells move through Meiosis 2 much in the same way Mitosis occurs

Prophase 2 – the centriole move to opposite ends of the cell, the nuclear envelope breaks down

Metaphase 2 – chromosomes line up in the middle of the cell and spindle fibers attach to the centromere

MEIOSIS 2

Anaphase 2 – the spindle fibers pull on the centromere and split the sister chromatids

Telophase 2 & Cytokinesis – the nuclear envelope forms and the cell cytoplasm splits

4 genetically DIFFERENT cells result

GAMETE FORMATION

The male gamete that is produced is called a sperm (spermatocyte) There are 4 sperm cells that are produced as a

result of meiosis

The female gamete that is produced is called an egg (oocyte) There is 1 egg cell produced as a result of

meiosis The 3 other cells produced are called polar

bodies Polar Bodies are not used in reproduction and are

considered to be the trash bags of the egg, but can be useful in genetic testing

COMPARING MITOSIS AND MEIOSIS Sloppy Copy

Mitosis/Meiosis Picture Fold paper down middle Draw Mitosis on left side starting with

interphase and ending with cytokinesis beginning with 4 chromosomes (X’s)

Draw Meiosis on right side starting with interphase and ending with cytokinesis 2 beginning with 4 chromosomes (X’s)

Mitosis/Meiosis Compare/Contrast Graphic Organizer 3 ways they are similar 3 ways they are different

THE WORK OF GREGOR MENDEL

Every living thing has a set of characteristics inherited from its parent(s)

Genetics – study of heredity

The father of genetics was Gregor Mendel Was an Austrian monk that

studies the passing of traits in pea plants

GREGOR MENDEL’S PEA PLANTS

He was put in charge of the gardens at the monastery

He knew how the process of fertilization occurred When the male and female cells join during

sexual reproduction In plants fertilization happens because of

pollination

CREATING PLANTS

Pea plants are able to self-pollinate The sperm in the pollen can fertilize the egg cell

of the same plant As a result, a plant can be created from only 1

“parent” and therefore have the same characteristics of that 1 parent

Since plants were self-pollinating, they would also be considered true-breeding They would produce offspring identical to

themselves Ex) tall plants would make more tall plants Ex) green seeded plants would make more green

seeded plants

MENDEL’S GETS STARTED

Even though true-breeding plants are good, Mendel was interested in what would happen if different traits were “crossed”

Mendel manipulated flowers so they could not self-pollinate and then began to cross breed plants with different characteristics Ex) cross a tall plant with a short plant Ex) cross a yellow seed plant with a green seed

plant

GENES AND DOMINANCE

Mendel studied contrasting pea plant traits Trait – a specific characteristic (such as color or

height) that varies from one individual to another

Mendel looked at the offspring that he created by crossing parents with different traits These offspring were called hybrids

P – symbol for the parental generation

F1 – symbol for the 1st generation

WHAT DID THE 1ST GENERATION LOOK LIKE?

To Mendel’s surprise, the offspring were not a combination of the 2 different traits

One of the traits did not even appear

MENDEL’S CONCLUSIONS

1. Biological inheritance is determined by factors that are passed from one generation to the next

We know that this is because of genes – chemical factors that determine traits

The different forms of a gene is an allele Ex) alleles of height – tall & short

alleles of color – yellow & green

2. Some alleles are dominant and others are recessive (principal of dominance)

Dominant traits will be exhibited whenever present Recessive traits will only be exhibited when it’s the

only trait present

SEGREGATION

Mendel wanted to know if the recessive trait had completely disappeared in the F1 generation

He allowed the F1 generation plants to cross by self-pollination, creating a F2 generation

F1 CROSS RESULTS

Mendel discovered the recessive traits reappeared

Mendel concludes that the dominant allele had only masked the recessive allele and that recessive alleles never disappeared The alleles must separate or segregate from

each other Gametes pick up one allele or another during the

segregation process

HOW DID SEGREGATION WORK?

Mendel concludes: When each of the F1 plant

flowers produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene

Therefore, each F1 plant produces 2 types of gametes – those with the allele for tallness and those with the allele for shortness

MENDEL’S RESULTS

Mendel repeated his experiments many times to see what would happen

He realized that for each cross, he got the same basic results Ex) whenever he crossed Tt for height, ¾ of

offspring were tall and ¼ of offspring were short

Mendel realized that he could apply the principals of probability to explain the results of genetic crosses

GENETICS AND PROBABILITY Probability – the likelihood that a particular

event will occur

Example – Coin Flip The chances that a coin will come up heads is

50% (1/2 or 1:2) The chances that a coin will come up tails is 50%

(1/2 or 1:2)

If you flipped a coin and it came up heads, what are the chances that it will come up heads the next time? The next time? Each event is independent of each other, so each time is

a 50% chance

COIN TOSS ACTIVITY

Hypothesis: If I toss a coin 2 times, I would expect to get _______ Heads and ________ Tails.

Toss your coin 2 times and record results Toss 1 ____ H _____ T Toss 2 ____ H _____

T

Toss your coin 8 more times and record results

Toss 3

Toss 4

Toss 5

Toss 6

Toss 7

Toss 8

Toss 9

Toss 10

ACTUAL COIN TOSS RESULTS

For first 2 tosses: Number Heads ______ Number Tails ______

Total Results Number Heads ______ Number Tails ______

Did the actual results for the entire class come closer to your hypothesis of 50/50 chance for heads and tails?

HOW IS COIN FLIPPING RELEVANT TO GENETICS?

When we are dealing with probabilities of an event, past outcomes DO NOT affect future ones

Each event is completely separate and therefore has just as equal chances for an outcome as the one before

The segregation of alleles is completely random, just like a coin toss You randomly get what you get, every single

time

PUNNETT SQUARES Punnett Square – is a diagram that shows the gene

combinations that can result from a genetic cross

Dominant traits are expressed by capital letters (T) Recessive traits are expressed by lower case letters

(t)

Homozygous – organisms that have identical alleles for a particular trait (TT or tt) Are true breeding (pure-bred) organisms for a trait

Heterozygous – organisms that have different alleles for a particular trait (Tt) Are hybrid organisms for a particular trait

PUNNETT SQUARE RESULTS The outcomes of a punnett square can be

described by both their “letters” and their “appearances”

Phenotype – the physical characteristics of an organism (the appearance) Ex) tall, short, yellow, green

Genotype – the genetic makeup of an organism (the letters) Ex) TT, Tt, tt

Organisms can have the same phenotype even though they have different genotypes TT – tall & Tt – tall

PROBABILITY AND SEGREGATION

Was Mendel’s model for segregation correct when looking at alleles for height? YES! (branching tree example)

Are we able to determine the same results by looking at probabilities for height? YES (punnett square example)

The results are the same no matter how you look at it

PROBABILITIES PREDICT AVERAGES

Probabilities can predict the average outcome of a large number of events (remb. the coin tossing)

They cannot predict the precise outcome of an individual event The predictions are based on chances for each

individual event

When dealing with genetics, the larger the number of offspring, the closer your results will be to the predicted number

INDEPENDENT ASSORTMENT

Mendel said that alleles segregate during the formation of gametes

When Mendel was doing his experiments he wanted to know if alleles segregated on their own (independently) or if one allele can affect another allele

Mendel goes back to the pea plant to find the answer

2 FACTOR CROSSES – F1

Mendel performed his pea plant experiments now looking at 2 different traits at once

Mendel crossed purebred plants round yellow (RRYY) and wrinkled green (rryy) [P] and looked at their offspring [F1]

All offspring were heterozygous round yellow (RrYy) We are not able to see if the alleles

segregate independently with these results…

2 FACTOR CROSS – F2 Mendel crossed 2 of the F1 offspring

to create a F2 generation

Mendel wanted to know if dominant/recessive alleles stay together (RY/ry)

Mendel looked at the F2 results and saw… 9/16 were round yellow 3/16 were round green 3/16 were wrinkled yellow 1/16 was wrinkled green

Remember our P generation

genotypes…RRYY – round yellowrryy – wrinkled green

F2 RESULTS

Mendel saw in the F2 generation the presence of offspring that did not exist in any parent Round green and wrinkled yellow

This fact means that alleles are capable of segregate independently

Independent Assortment – genes for different traits are able to separate on their own and do not influence each others inheritance the alleles for seed shape and color did not

influence each other since they were capable of separation

PRINCIPAL OF INDEPENDENT ASSORTMENT

This states… Genes for different traits can segregate

independently during the formation of gametes

Along with crossing over, independent assortment is the other process that accounts for genetic variation

SUMMARY OF MENDEL’S PRINCIPALS The inheritance of biological characteristics is

determined by individual units known as genes. Genes are passes from parents to offspring

In cases in which 2 or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

In most sexually reproducing organisms, each adult has 2 copies of each gene – one from each parent. These genes are segregated from each other when gametes are formed

The alleles for different genes usually segregate independently of one another.

BEYOND DOMINANT AND RECESSIVE

There are some exceptions to some of Mendel’s principals

Since many genes have more than 2 alleles and many traits are controlled by more than 1 gene, genetics can be very complicated

Genetics deals with more than just dominance and recessiveness

PATTERNS OF INHERITANCE (HOW GENES ARE INHERITED)

Incomplete Dominance – when one allele is not completely dominant and another completely recessive The offspring will be a mixture of 2

phenotypes Ex) some flowers with red and white

parents will result with a pink phenotype

Codominance – both alleles contribute to the phenotype of an organism The offspring will show more than 1

phenotype Ex) a white chicken and a black chicken

have offspring that are black and white “speckled”

PATTERNS OF INHERITANCE (HOW GENES ARE INHERITED)

Multiple Alleles – when genes have more than one allele possibility The individual will not have more than

2 allele, there are just more than 2 possibilities in the population

Ex) coat color in rabbits

Polygenic Traits – traits that are controlled by 2 or more genes Traits can be produced by the

interaction of genes Ex) fly eye color, skin color, coat color

in labs

APPLYING MENDEL’S PRINCIPALS There are many scientists that tried to test

Mendel’s principals

Thomas Hunt Morgan – a scientist from the early 1900’s that performed genetic tests on fruit flies

Why Fruit Flies? Small organism (easy to care for and doesn’t

take up much room) Reproduces quickly Inexpensive to get and keep

Fruit flies will prove to be very important to the study of genetics!!

GENETIC AND THE ENVIRONMENT

Characteristics of organisms are not determined by genetics alone

The interaction of organisms and the environment can play a role in genetics Height of plants determined by the sun Evolution of organisms based on

surroundings The development of disease based on

contact with different substances in environment

“genes provide a plan, how plan unfolds depends on environment”

GENE LINKAGE

We have been able to look at how genes located on different chromosomes assort independently

What happens to genes on same chromosomes Do they assort? Must they be inherited together?

These questions were tested by Thomas Hunt Morgan with his fruit fly research

GENE LINKAGE IN DROSOPHILA MELANOGASTER Morgan was able to identify more

than 50 genes

He saw that many genes were inherited together Ex) many times reddish-orange

eyes and small wings were almost always inherited together and were rarely separated

Does this mean independent assortment doesn’t occur?

Through much research, Morgan discovered 4 linkage groups

GENE LINKAGE IN DROSOPHILA MELANOGASTER When Morgan crossed the linkage

groups, he realized… that the linkage groups assorted

independently all of the genes that were involved with the

linkage group were inherited together

This information leads to 2 conclusions1. Each chromosome is actually a group of

linked genes2. It is the chromosome that assorts

independently, not the gene

Mendel didn’t see gene linkage because the genes he was looking at were on different chromosomes

GENE MAPS Just because genes are found on the

same chromosome, does not mean that they are linked forever Crossing-over can separated genes on

same chromosomes producing new alleles

Researchers of Morgan’s looked at the rate at which linked genes were separated and used their findings to produce gene maps

Gene map – shows the relative locations of each gene on a chromosome Crossing-over rates were used to construct

the drosophila and human genome map