chapter 13 - meiosis and sexual life cycles

Chapter 13 - Meiosis and Sexual Life Cycles

High frequency quiz mistakes:Review: Describe the functions of mitosis.

1. Incorrect number of chromosomes used in depiction of mitotic cell division. Question stated that the cell was diploid and the haploid number (n) was 2. Diploid (2n)

Haploid number (n)(number of chromosomes

in a set)2 sets 2X = 4

chromosomes in the

cell



PracticeA. How many chromosomes in a triploid cell with a haploid number of 7?B. A diploid cell has 24 chromosomes. What is the value of n?C. A 4n cell has 40 chromosomes. What is the ploidy of this cell and how many chromosomes would you expect to find in its gametes?

21

12

Tetraploid, 20



PracticeD. Draw a triploid nucleus of a cell with n = 2.



2. What are the two possible chromosome combinations found in human male sperm?

Ans: One of each homologous pair

1 through 22 and X 1 through 22 and Y

- Are the gametes haploid or diploid?

Chapter 13 - Meiosis and Sexual Life Cycles Review: Describe the functions of mitosis.

This brings us to our next adventure…how are the gametes made?

Meiotic Cell Division (the other cell division)

Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles

A. Asexual ReproductionB. Sexual Reproduction

AIM: Compare asexual to sexual reproduction

Reproduction


A. Asexual Reproduction1. One parent

3. Single-cell organisms (certain protists [ex. amoeba] and fungi [ex. yeast] and all prokaryotes [bacteria])

2. Genetically identical offspring (called clones) if we ignore mutations, which they rely on to evolve.


A. Prokaryotic cell cycle (binary fission) B. eukaryotic cell cycle (“mitosis”)

4. Plants (vegetative propagation) and animals

Made possible by begin able to have numerous offspring very quickly as positive mutations are rare, but if you have millions of offspring in a few days one likely has such a mutation.What is a positive mutation in general?

One that give the organism (vehicle) a better ability to survive and reproduce in current environment.


B. Sexual Reproduction- Two parents

- Highly variable offspring due to mixing of DNA of two parents

- Gametes produced (fertilization)

- Most eukaryotes


Tend to have fewer offspring over longer periods of time making mutations not a reliable means of generating diversity……better to shuffle the DNA between two organisms.

Fig. 8.13


Somatic cells

Gametes

NEW AIM: Describe how gametes are formed.

Chapter 13 - Meiosis and Sexual Life Cycles AIM: Describe how gametes are formed.

Meiotic cell division

3. Called reduction division

1. Formation of gametes in animals

2. Formation of spores in plants and fungi

- The number of chromosomes is cut in half (typically from diploid [2n] to haploid [n])

- Therefore essential for sexual reproduction


Compare gametes to somatic cells in terms of chromosome number.(Soma- means body; somatic cell = body cell)

Gametes are haploid (have one set of 23 chromosomes in humans), while somatic cells are diploid (have two sets or 46 chromosomes in humans)

Fig. 8.13


Somatic cells

Gametes

Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cyclesAIM: Describe the eukaryotic cell cycle.

Review the DNA/chromosomes in a human nucleus…


Humans have 23 pairs of chromosomes for a total of 46. This image shows the 46 chromosomes from the nucleus of a single human male cell.

This display of chromosomes is called a Karyotype.

You can see that each chromosome has a very similar (homologous) matching pair with the exception of the sex chromosomes (X and Y). Females would have a homologous pair of X’s. Males have an X and a Y (not homologous).


Humans have 23 pairs of chromosomes for a total of 46.

These are homologous (similar) chromosomesIf the gene for hemoglobin were on one of these (green bar), then it is on the other as well in the same location (locus).

Therefore we have how many of each gene?Ans: at least two of every gene, except for the genes on the X and Y chromosomes in males since these chromosomes are not homologous.


Humans have 23 pairs of chromosomes for a total of 46. Every nucleus has 22 pairs of autosomes (chromsomes 1 through 22)


Humans have 23 pairs of chromosomes for a total of 46. …and one pair of sex chromosomes


Humans have 23 pairs of chromosomes for a total of 46.

*One of each pair comes from the mother, and the other comes from the father.

What chromosomes would have been found in the sperm that fertilized the ovum that was to be this person?


Humans have 23 pairs of chromosomes for a total of 46. *Therefore, this is the DNA that would have been packaged in the nucleus of the… (sperm or egg?)…father’s sperm that penetrated and fertilized the ovum to form a zygote because only males have Y chromosomes.


Humans have 23 pairs of chromosomes for a total of 46. *And this is the DNA that would have been packaged in the nucleus of the mother’s ovum (has an X).This could also be in a sperm, but the previous slide could not be in an ovum. Explain.Ans: Males are XY. Therefore sperm can have an X or Y (they determine sex. Females are XX and therefore the ovum can only have an X.


Humans have 23 pairs of chromosomes for a total of 46. - One of each of chromosomes 1 through 22 and a sex chromosome as shown to the right is considered to be one complete set or n. The word for this is haploid.

hap- = one-ploid = set


Humans have 23 pairs of chromosomes for a total of 46. If I said that

human cells are 2n that means each cell has how many sets?

2


Humans have 23 pairs of chromosomes for a total of 46. If I said that

human cells are 2n that means each cell has how many sets?

2

The word for this is diploid.

di- = 2-ploid = sets

Humans are diploid organisms. What are our gametes, ha- or diploid?Ans: haploid. When they fuse during fertilization the resulting zygote is diploid.


What is “n”?n is a complete set of chromosomes (1 of each chromosome)n is called the haploid number…or number of chromosomes in a complete set.


What is the value of “n” in humans?23, because we have 23 chromosomes in one set


How and where are the gametes formed in humans?

Gametogenesis - formation of gametes (two types)1. spermatogenesis

-Formation of sperm, occurs in testes (male gonads)2. Oogenesis- Formation of ovum, occurs in ovaries (female gonads)

This is all meiosis (gametogenesis, spermatogenesis, Oogenesis)


The variety of sexual life cycles


Meiotic cell division This figure shows a simple diploid cell with one homologous chromosome pair (n=1).

What will happen first?

Red one is maternal (coming from mother) and blue one is paternal (coming from father).

Ploidy? n?


Meiotic cell division DNA will be replicated.

Then what?


Meiotic cell division Homologous pairs will be pulled apart.

And lastly?


Meiotic cell division Sister chromatids will be pulled apart.

Each resulting cell is haploid with one of each homologous pair!

If you want to add a second, third or even 23 pairs, they all behave the same way.

Meiosis I (1st division)

Meiosis II (2nd division)

Diploid (2n) Diploid (2n)

Haploid (n)

Haploid (n)


Meiotic cell division Overview

(Gamete/spore formation)


MEIOSIS (meiotic cell division)

(Gamete/spore formation)



Mitotic cell division (asexual reproduction) would have evolved first before sexually reproducing organisms, which require meiosis, came onto the scene… Reminder: Evolution builds on old ideas (Ex. RNA world hypothesis)You might hypothesize then that meiotic cell division should be very similar to…Mitotic Cell Division.

General overview to start…

(Gamete/spore formation, reduction division)



The proteins of the cell are simply programmed to pull paired up chromosomes apart (life is simple, just a lot of simple things happening at once making it appear overly complex)…

Why is it logical to pair up homologous chromosomes?

1. Interphase (G1, S, G2)- Similar to cell cycle (mitotic cell division) interphase. Use those notes.


Meiotic cell division (details):

2. Meiosis – two rounds of cell divisiona. Meiosis I – first round (PI, MI, AI, TI)

d. Meiosis II – second round (PII, MII, AII, TII)

b. cytokinesis

e. cytokinesis IPMATPMAT

c. Interaphase II (interkinesis) - only in certain species (do not need to include this on test)

Interphase (G1, S, G2)- Similar to cell cycle (mitotic cell division) interphase. Use those notes.


Prophase I (longest phase, up to 90% of meiosis)

-Most complex phase of meiosis


-Occupies 90% of meiotic cell division-Chromosomes condense-Synapsis occurs = homologous chromosomes come together in pairs resulting in the formation of tetrads.

MEIOSIS I: Separate the homologous pairs

Prophase I


-Crossing over occurs – homologous chromosomes exchange equivalent segements i. This “shuffles” the genes

so that the same genes are not always together on the chromosome ii. This site of crossing over is called the chiasma


Prophase I-nucleoli disappear


-centrosomes move to poles-Spindle begins to form

-Nuclear envelope breaks down-MTs attach to kinetochores at centromeres

-Free MTs interact with each other to elongate cell like in mitosis

-one pole is attached to one homologous pair while the other pole attaches to the other homologous pair



Metaphase I-tetrads align on metaphase plate (brought there by kinetochore motor proteins using ATP for fuel)



Anaphase I- Homologous pairs (homologs) separate as kinetochore proteins walk along spindle fibers toward opposite poles- sisters stay together attached by centromere



Telophase I- Homologs arrive at poles- Each pole now has a haploid (n) set: remember that sister chromatids are considered a single chromosomeCytokinesis- Overlaps telophase I

- Results in two haploid (n) cells although the amount of DNA is similar to the starting diploid cell


- Similar to cytokinesis in mitotic phase


Haploid after

cytokinesis – each

cell has one of each

homologous pair

diploid diploid diploid diploid

Meiosis II

- Similar to the mitotic phase – separate the sisters- Meiosis II starts with two haploid cells and forms four haploid cells with half the amount of DNA**** Chromosomes do not replicate. Only the centrosomes replicate during interphase II/prophase II


haploid

separate the sisters


Genetic variation/diversity of offspring

Why go through all of this trouble (use lots of energy) to makes gametes, cross-over, find a mate, try and fertilize an egg to reproduce sexually?

(sexually reproducing organisms do this to mix/shuffle their DNA resulting in very different offspring genetically so that as the environment changes, there will inevitably be certain gene combinations that will survive it.)Ex. The flu pandemic of 1918 may have killed 50 million people, but it didn’t kill everyone. Different gene combinations means different biochemistry and therefore different susceptibility to disease or other environmental changes. Another example is HIV…


Genetic variation/diversity of offspring

Why go through all of this trouble (use lots of energy) to makes gametes, find a mate, try and fertilize an egg…basically to reproduce sexually?

Evolutionary Trade-offs: realize that it is a trade-off since it does require more energy to do this and time to find a mate, etc…There is almost always a trade-off. Ex) We stand upright on two legs using skeletons naturally selected to walk on all fours. Some of the trade offs are back problems, foot problems, hemorrhoids, and many others…

Irish Potato Famine


(another example of the importance of genetic diversity)

- Period of mass starvation and disease in Ireland between 1845 and 1852~1,000,000 died and another 1,000,000 fled

- Potato blight, a disease that destroys potato plants caused by the fungus Phytophthora infestans, ripped through and destroyed all the Irish crops because all the plants were identical and therefore if one is susceptible to the disease, they all are…

1/3rd of Irish population depended on potatoes as major source of food. Potatoes do not reproduce well from seed (sexual reproduction) and were asexually propagated (vegetative propagation). Therefore, all the potato plants in Ireland were essentially genetically identical…clones.

Never depend on one crop, especially one crop of genetically identical plants.

How do sexually reproducing organisms generate this kind of

diversity

?

Chapter 13 - Meiosis and Sexual Life Cycles NEW AIM: How do sexually reproducing organisms generate diversity?When a human male and female conceive a child, there are greater than 64 trillion possible outcomes (>64 trillion different possible genetic combinations in the offspring)

1. INDEPENDENT ORIENTATION OF CHROMOSOMES

1. INDEPENDENT ORIENTATION OF CHROMOSOMESBecause the tetrads independently and randomly orient themselves on the metaphase plate during metaphase I, many different chromosomal combinations can arise:

How many different possible gametes can be generated by

a human?

Chapter 13 - Meiosis and Sexual Life Cycles AIM: How do sexually reproducing organisms generate diversity?

In the example to the left, the cell is diploid with n=2. The outcome of meiotic cell division is 4 different possible combinations.

n Possible combinations

2 4

3 8

4 16

5 32

DIPLOIDS


a human?


How many chromosomal combinations can be made in the gametes of a diploid cell (2n= 6), or n = 3?Let’s call the chromosomes 1m, 1d, 2m, 2d, 3m and 3d. Just make all the possible gamete combinations (remember that each gamete will get on of each homologous pair):1m, 2m, 3m1m, 2m, 3d1m, 2d, 3d1m, 2d, 3m

1d, 2m, 3m1d, 2m, 3d1d, 2d, 3d1d, 2d, 3m

8 possible combinations in the gametes


a human?


We can keep playing this game for n=4, n=5, etc… until we get to humans, n=23:

n Possible combinations

2 4

3 8

4 16

5 32

23 8,388,608

DIPLOIDSn Possible combinations (2n)

2 2n = 4

3 2n = 8

4 2n = 16

5 2n = 32

23 2n = 8,388,608

The number of POSSIBLE chromosomal combinations in the gametes of a diploid organism is 2n where n = haploid number.


How many possible sperm and ovum combinations?

There are 223 different sperm combinations and 223 different ovum combinations.

That makes 223 x 223 = 246 different combinations

2. Random fertilization

246 = ~64 trillion possible combinations

You are 1 in 64 trillion


(well…not exactly)

What are we ignoring?Crossing-over

Chapter 13 - Meiosis and Sexual Life Cycles AIM: How are homologous chromosomes related?

3. Crossing Over

A Gene coding for a specific RNA/polypeptide can have different versions (slightly different DNA sequences) possibly resulting in an RNA/polypeptide/protein with a very similar functionality, but not identical. These different gene versions are called alleles.

Alleles

AllelesC and c are different versions of the same gene.

They are alleles. C codes for a protein that will somehow be involved in making mouse fur black while c will make almost the same protein, but will be involved in making mouse fur white. If I showed you a picture of C and c you would not be able to tell them apart without looking VERY closely.


3. Crossing Over

Without crossing-over, C and E would always travel together in the gametes just as c and e would always be together. If genes are located on the same chromosome we say they are LINKED.


3. Crossing Over


How can these different genes (C and E; c and e) be unlinked so that C is with e and E is with c?

3. Crossing Over


3. Crossing OverDuring prophase I, just after synapsis, crossing over will occur. During crossing-over there is a reciprocal (equal both ways) chromosomal exchange between homologous chromosomes

Why must it be an equal exchange?

Each gamete must get one set of chromosomes will all the necessary genes. An unequal exchange would result in some gametes getting too many genes and some getting too few


3. Crossing Over

Reminder: Crossing over occurs at sites called chiasma and there can be more than one crossing-over per homologous pair…


3. Crossing Over

This figure shows three crossing over events:


Crossing over begins after synapsis (formation of the tetrad) during prophase I.

3. Crossing Over


Enzyme will cut the DNA at the same locus of each homologous pair…

3. Crossing Over


Other proteins will reconnect the chromosome fragments to the opposite homolog…

3. Crossing Over


and meiosis continues as normal...

3. Crossing Over


3. Crossing Over

The resulting chromosomes have special names (doesn’t everything?)

Parental type chromosome (looks like parent)

Parental type chromosome (looks like parent)

Recombinant chromosome

Recombinant chromosome

*A recombinant chromosome is one that has been broken and recombined with another chromosome hence recombinant. Crossing over is also known as homologous recombination – recombining homologous chromosomes.


Identify the sources of genetic variability in sexually reproducing organisms:

Is that it? Are we finally finished?

1. Independent orientation2. Random fertilization3. Crossing-Over

(not quite…)4. Mutations

Mutations are changes in the DNA that can be caused by electromagnetic radiation (light: UV, x-rays, gamma rays), mutagenic chemicals, viruses, nuclear radiation, mistakes during replication, and others. However, such changes are only relevant to ones offspring if they occur where?

In the gametes

I guess you aren’t 1 in 64 trillion possible

combinations…


You are 1 in some ridiculously huge number that I can’t

calculate possible combinations!(The real question is, would you still be you if you were any

other combination? And how many changes in your DNA would it take for it not to be you anymore?...)

A particular diploid species of annelid has a chromosome number of 10. How many chromosomal combinations are possible in the gametes assuming no crossing over/mutations?


Review Question:

n = 52n = 25 = 32

A fruit fly (Drosophila melanogaster) somatic cell (diploid) contains 8 chromosomes. How many possible offspring can be generated if crossing over/mutations is ignored?


Review Question:

n = 424 = 16 possible sperm 24 = 16 possible ovum

16 x 16 = 256

Chapter 13 - Meiosis and Sexual Life Cycles AIM: What happens when meiosis goes wrong?

Nothing is perfect, including meiosis. A number of diseases can be caused by errors in meiosis resulting in abnormal numbers of chromosomes.How can one determine if a disease is caused by or if someone has an abnormal chromosome arrangement?


1. Isolate lymphocytes (type of white blood cell) from a simple blood sample, or fetal cells from amniotic fluid / placenta 2. Stimulate cell division in a tissue culture flask 3. Arrest (stop) the cell in metaphase. Why?

- Chromosomes are condensed and visible, and it is a checkpoint, which allows us to stop it here by inhibiting internal signals.

4. Put cells on a cover slide and lyse them (break them open with certain chemicals) and wash away cell fragments leaving behind the chromosomes.5. Stain the chromosomes with special dye

(Process takes about a week)

How can one determine if a disease is caused by or if someone has an abnormal chromosome arrangement?

Fig. 8.19


A second method:

1. Blood sample taken and spun in a centrifuge. - Centrifuges can spin samples at very high speeds (10,000’s of rpms resulting in forces as high as 100,000 times the force of gravity)- The more dense material ends up on the bottom (red blood cells in this case) and less dense material on above this (white blood cells and then fluid or plasma).

Fig. 8.19


2. A hypotonic solution is added to the sample, which will lyse the red blood cells (weaker), leaving the white ones, some of which are in prophase/metaphase swollen, but not popped.

A second method:

Fig. 8.19


3. A drop of the sample containing WBCs is placed on a cover slide, dried and then stained.

A second method:


For both methods, the result is a smear of condensed chromosomes from a single cell called a karyotype.

karyotyping

A digital picture is taken and the chromosomes are first counted. There should be (from a human cell):46

What is the first thing you would do?

Then what?


The chromosomes are paired up and organized according to size and banding patterns (the dye sticks better to A-T rich regions – segments of the DNA with many A-T base pairs):Is this smear (karyotype) from a male or a female cell?Female, it is XX


Is this smear (karyotype) from a male or a female cell?Male, it has a Y chromosome

There are 3 number 21 chromosomes. This is called Trisomy 21

(tri:3, somy:body)

Down Syndrome

Analyze the karyotype:


What causes Down Syndrome (Trisomy 21)?

1. First hypothesize what the chromosome combination present in the gametes of the parents of an individual with Down Syndrome would be.- One gamete could have an extra chromosome 21 (two number 21’s), while the other gamete is normal.2. How could an extra chromosome end up in one of the gametes?

CHROMOSOMAL NONDISJUNCTION in Meiosis II

CHROMOSOMAL NONDISJUNCTION in Meiosis I

Viable Autosomal nondisjunctions: 1) Trisomy 21 (Down syndrome)- most common in viable (survivable) births 2) Trisomy 18 (Edwards syndrome) 3) Trisomy 13 (Patau syndrome) 4) Trisomy 12 (A indicator of Chronic Lymphocytic Leukemia) 5) Trisomy 9 6) Trisomy 8 (Warkany syndrome 2)

Non-Viable Autosomal nondisjunctions: Trisomy 16 - most common trisomy in humans:

- occurs in more than 1% of pregnancies. - Usually results in spontaneous miscarriage in the first

trimester. (Do not memorize these other then trisomy 16 info and Down Syndrome)



What about if non-disjunction occurs in the sex chromosomes?


Klinefelter’s Syndrome (XXY)- Sterile male typically- Female characteristics


XXXXYKlinefelter’s syndrome: can be any multiple of X with a single Y.


- Shorter stature (height)Turner Syndrome

- Enlarged hands and feet- Underdeveloped ovaries (infertile)- Other problems (high blood pressure, heart problems, etc…)


(normal for the most part)


What is the total number of chromosomes you would expect to find in a woman with Turner syndrome? 45, missing one X


Other mishaps can occur during meiosis resulting in chromosomal abnormalities that may cause disease. These abnormalities include…

1. Deletions2. Duplications3. Inversions4. Translocations

Chapter 13 - Meiosis and Sexual Life Cycles AIM: What are some other ways chromosomes can be altered?

There is a “mistake” made during meiosis resulting in the loss of a chromosome segment.


Diseases caused by deletions include:1. Cri du chat “cry of the cat”

- Low birth weight, cognitive (information processing) delays, motor (movement) and speech problems

- Cat-like cry

- Chromosome 5 deletion- Life span is normal (your book is incorrect)


Diseases caused by deletions include:2. Williams syndrome

- Deletion of 26 genes in chromosome 7- Elf like facial appearance- Unusually cheerful- Unpredictable negative outbursts- Mental retardation- Shortened life span due to narrowed arteries


Diseases caused by deletions include:3. Duchenne’s muscular

dystrophyi. Dystrophin gene- Largest gene in genome- 2.4 million base pairs!!!!!!!!!- Takes 16 hours to transcribe (to make a mRNA)!!!!!!!- Protein is 3500 amino acids!- X linked gene (means it is on the X-chromosome)- Function: connects muscle cell cytoskeleton to ECM (anchors cell to ECM)


Diseases caused by deletions include:3. Duchenne’s muscular

dystrophyii. Symptoms begin at 2 to 6 years oldiii. Develop muscle weakness and eventually failure

v. Survival beyond 20 years old is rarevi. Affects 1 in 3500 MALES

iv. Wheelchair by age 12

Why are carrier females prevalent, but afflicted females rare?


Duplications can accompany deletions if the deleted section is inserted into the homologous chromosome during crossing over.This can have a major role in evolution. Can you predict why?


The duplicated genes are now free to mutate (change) without causing a loss of critical proteins since the original genes are there. The genes can eventually produce proteins that might become new tools for the cell…giving it new functions.**Many of our genes arose by duplication events followed by mutations resulting in many different proteins having similar structure (divergent evolution).


Above are three enzymes (proteins) that catalyze three different reactions coded for by three different genes. Do you think those genes arose independent of each other? Highly, highly, highly unlikely. These three genes likely arose by gene duplication

from a single original gene.


Inversion results when a segment of a chromosome gets flipped or inverted.

- Less likely to be harmful compared to deletion and duplications- All genes still present


Reciprocal translocations can occur between NONHOMOLOGOUS chromosomes.Normal if it happens in somatic cells – all genes still present.If occurs in meiosis it can lead to Down syndrome if a piece of chromosome 21 is tranlocated onto another chromosome.

Reciprocal translocation


Congenital disorders

Non-Congenital disorders

- Disorders present at birth (all the ones we have spoken about thus far)

- Disorders that occur or arise after birth

- Caused by genetic changes in sperm and/or ovum or environment in uterus.

- Ex) cancer, heart disease, etc…


Cancer (non-congenital) caused by a reciprocal translocation between chromosomes 9 and 22.

The translocation activates a cell division gene.


Reminder -Review Question

Genetic changes are ONLY passed to offspring (inherited) if they occur…

IN THE GAMETES

chapter 13 - meiosis and sexual life cycles

Documents

n cell

diploid cell

sexual cyclesa

triploid cell

sexual life cycles review

sexual reproductiontend

sexual reproductionaim

sexual cyclesb