Meiosis

Sexual Reproduction

• Chromosomes are duplicated in germ cells

• Germ cells undergo meiosis and cytoplasmic division

• Cellular descendents of germ cells become gametes

• Gametes meet at fertilization

Asexual Reproduction

• Single parent produces offspring

• All offspring are genetically identical to one another and to parent

Sexual Reproduction

• Involves

– Meiosis

– Gamete production

– Fertilization

• Produces genetic variation among offspring

Question 2

2. Sexual reproduction involves three (3) processes, identify them.

Fertilization

http://www.wappingersschools.org/RCK/staff/teacherhp/johnson/visualvocab/page4.html

Fertilization Video

• http://www.youtube.com/watch?v=t_eNmc_gHyE

Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome

• One chromosome in each pair from

mother, other from father

• Paternal and maternal chromosomes carry

different alleles

Sister Chromatids

Prior to “S” of interphase, each chromosome consists of a single “chromatid”.

A pair of sister chromatids

http://www.vcbio.science.ru.nl/eng/virtuallessons/cellcycle/chromosome/

Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits

• This variation in traits is the basis for evolutionary change

Variation in Offspring

http://www.seapix.com/jagphoto/ladybugs.htm

Gamete Formation

• Gametes are sex cells (sperm, eggs)

• Arise from germ cells

testes

ovaries

anther ovary

Figure 10.2aPage 164

Chromosome Number

• Sum total of chromosomes in a cell

• Germ cells are diploid (2n)

• Gametes are haploid (n)

• Meiosis halves chromosome number

Meiosis: Two Divisions

• Two consecutive nuclear divisions

– Meiosis I

– Meiosis II

• DNA is not duplicated between divisions

• Four haploid nuclei form

Meiosis Video

• http://www.youtube.com/watch?v=D1_-mQS_FZ0

Meiosis I

Each homologue in the cell pairs with its partner,

then the partners separate

In-text figurePage 165

Meiosis II

• The two sister chromatids of each duplicated chromosome are separated from each other

one chromosome (duplicated)

two chromosomes (unduplicated)

In-text figurePage 165

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

Figure 10.4 Page 167

Prophase I

• Each duplicated chromosome pairs with homologue

• Homologues swap segments

• Each chromosome becomes attached to spindle

Figure 10.4 Page 167

Metaphase I

• Chromosomes are pushed and pulled into the middle of cell

• The spindle is fully formed

Figure 10.4 Page 167

Anaphase I

• Homologous

chromosomes

segregate

• The sister

chromatids remain

attached

Figure 10.4 Page 167

Telophase I

• The chromosomes

arrive at opposite

poles

• Usually followed by

cytoplasmic

division

Figure 10.4 Page 167

Prophase II

• Microtubules attach to the kinetochores of the duplicated chromosomes

Figure 10.4 Page 167

Metaphase II

• Duplicated chromosomes line up at the spindle equator, midway between the poles

Figure 10.4 Page 167

Anaphase II

• Sister chromatids separate to become independent chromosomes

Figure 10.4 Page 167

Telophase II

• The chromosomes arrive at opposite ends of the cell

• A nuclear envelope forms around each set of chromosomes

• Four haploid cells

Figure 10.4 Page 167

Crossing Over

•Each chromosome

becomes zippered to its

homologue

•All four chromatids are

closely aligned

•Nonsister chromosomes

exchange segmentsFigure 10.5Page 168

Effect of Crossing Over

• After crossing over, each chromosome

contains both maternal and paternal

segments

• Creates new allele combinations in

offspring

Random Alignment

• During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes

• Initial contacts between microtubules and chromosomes are random

Random Alignment

• Either the maternal or paternal member of a homologous pair can end up at either pole

• The chromosomes in a gamete are a mix of chromosomes from the two parents

Possible Chromosome Combinations

As a result of random alignment, the number of possible combinations of chromosomes

in a gamete is:

2n

(n is number of chromosome types)

Possible Chromosome

Combinations

or

or

or

1 2 3

Figure 10.6Page 169

Plant Life Cycle

multicelled sporophyte

multicelled gametophytes

zygote

gametes spores

meiosisfertilization

mitosis

mitosis

Diploid

Haploid

Figure 10.7Page 170

Animal Life Cycle

multicelled body

gametes

zygote

meiosisfertilization

mitosis

Diploid

Haploid

Figure 10.7Page 170

Oogenesis

GrowthMeiosis I,

Cytoplasmic DivisionMeiosis II,

Cytoplasmic Division

ovum (haploid)

primary oocyte (diploid)

oogonium (diploid)

secondary oocyte

(haploid)

first polar body

(haploid)

three polar bodies

(haploid)

Figure 10.8Page 171

Spermatogenesis

GrowthMeiosis I,

Cytoplasmic DivisionMeiosis II,

Cytoplasmic Division

cell differentiation, sperm formation

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

spermato-gonium (diploid )

sperm (mature, haploid male

gametes)

Figure 10.9Page 171

Oogenesis

• http://www.youtube.com/watch?v=kkrY1DTeqvs&feature=related

Spermatogenesis

• http://www.youtube.com/watch?v=POpbN6RHOO0&feature=related

Cancer rise and sperm quality fall 'due to chemicals'

• BBC March 2011 “Sperm quality significantly deteriorated and testicular cancers increased over recent years, a Finnish study says…. research suggests environmental reasons, particularly exposure to industrial chemicals, may be behind both trends.”

Key Genital Measurement Linked to Male Fertility

http://tinyurl.com/477s9wl

March 08, 2011| Reuters “When it comes to male fertility, it turns out that size does matter…..

Key Genital Measurement Linked to Male Fertility

• March 08, 2011| Reuters “When it comes to male fertility, it turns out that size does matter….. The dimension in question is not penis or testicle size, but a measurement known as anogenital distance, or AGD…Men whose AGD is shorter than the median length -- around 2 inches (52 mm) -- have seven times the chance of being sub-fertile…scientists tested for the presence of phthalates in the pregnant women's urine. They found that women who had high levels of phthalates in their urine during pregnancy gave birth to sons who were 10 times more likely to have shorter than expected AGDs…Phthalates are a group of chemicals widely used in industrial and personal care products, including fragrances, shampoos,

soaps, plastics, paints and some pesticides.”

Elite cyclists 'risk infertility'

• BBC 29 June 2009 “Professional cyclists should consider freezing their sperm before embarking on their careers, say researchers.”

Daily sex 'best for good sperm'

• BBC 30 June 2009 “Having sex every day improves sperm quality and could boost the chances of getting pregnant, research suggests.”

The Sperm Race

• BBC “Over the past 50 years, sperm counts around the world have fallen from an average of 113 million sperm per ml to between 66 and 76 million. Scientists have suggested that male infertility is the leading cause of infertile couples.”

The Sperm Race

• "antioxidants and micronutrients are important for semen quality"

• “How we changed their diets

The smoothies were just one of the additions the men in our study made to their diets. They also increased their nutrient intake and cut down considerably on fatty foods and alcohol, also increasing the amount of exercise they did.”

The Sperm Race

• Blueberry Bliss• 1 mango,1 handful blueberries, Juice ½

lime, and100ml apple juice • Peachy Treat• 1 ½ peaches, ½ mango, ½ orange,

and100ml orange juice

• http://www.bbc.co.uk/sn/humanbody/truthaboutfood/sexy/spermrace.shtml

Fertilization

• Male and female gametes unite and nuclei

fuse

• Fusion of two haploid nuclei produces

diploid nucleus in the zygote

• Which two gametes unite is random

– Adds to variation among offspring

Fertilization Restores Diploid State

Sperm, haploid

Ova, haploid

Zygote, diploid

Factors Contributing to Variation among Offspring

• Crossing over during prophase I

• Random alignment of chromosomes

at metaphase I

• Random combination of gametes at

fertilization

Mitosis• Functions

– Asexual reproduction– Growth, repair

• Occurs in somatic cells

• Produces clones

Mitosis & Meiosis Compared

Meiosis• Function

– Sexual reproduction

• Occurs in germ cells

• Produces variable offspring

Prophase vs. Prophase I

• Prophase (Mitosis)

– Homologous pairs do not interact with each

other

• Prophase I (Meiosis)

– Homologous pairs become zippered together

and crossing over occurs

Anaphase, Anaphase I, and Anaphase II

• Anaphase I (Meiosis)

– Homologous chromosomes separate from

each other

• Anaphase/Anaphase II (Mitosis/Meiosis)

– Sister chromatids of a chromosome separate

from each other

Results of Mitosis and Meiosis

• Mitosis

– Two diploid cells produced

– Each identical to parent

• Meiosis

– Four haploid cells produced

– Differ from parent and one another

So if Sexual Reproduction is Advantageous,

• How do you explain the all female species of the Whip-tailed lizard, the parthenogenetic Cnemidophorus uniparens ?

• (That’s correct, this is a unisex species that undergoes “virgin birth”!)

Most products of crossbreeding, such as the mule, are sterile…

• “New Mexico Whiptail, is an all-female species that is actually a mixture (hybrid) of … the Western Whiptail, which lives in the desert, and the Little Striped Whiptail, a denizen of grasslands.”

http://www.amnh.org/exhibitions/expeditions/treasure_fossil/Treasures/Unisexual_Whiptail_Lizards/lizards.html?50

New Mexico Whiptail is Triploid

These lizards have 3 sets of chromosomes (3n) = 69

Studies have shown that just prior to meiosis I, this number is doubled to 138.

The primary oocytes then go through what appears to be a normal meiosis I an II thus maintaining 3n = 69. Then the ova matures and begins mitosis and produces all female embryos.

http://www3.interscience.wiley.com/cgi-bin/abstract/109918083/ABSTRACT?CRETRY=1&SRETRY=0

How does this species remain vigorous without the normal genetic

variants?

Source of Additional Information

• Wikipedia (free online encyclopedia)

• http://en.wikipedia.org/wiki/Meiosis

• This site goes a little beyond the scope of this lecture. You are responsible for mastering the material in this lecture only.