chapter overview reproductioncontents.kocw.net/kocw/document/2014/hanyang/bertbinas/15.pdf · •...

Chapter overview • Reproduction

• Reproduction of cells vs. reproduction of organisms

• Reproduction and proliferation

• Non-sexual reproduction

• Mitosis and the cell cycle

• Cancer

• Sexual reproduction

• Meiosis and sex

• Chromosomal abnormalities in humans

• The biological advantages of diploidy and sex

SEXUAL REPRODUCTION AND MEIOSIS

(continued)

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Gametes and the Life Cycle of a Sexual Organism

• The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next.

• In most higher organisms, most of the life cycle is diploid

• In most higher organisms, reproduction is sexual


• Meiosis http://www.cellsalive.com/meiosis.htm http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://www.johnkyrk.com/meiosis.html

• Two key characteristics of meiosis: – 1. Reduction of chromosome number: Diploid (2n) Haploid (1n) (gametes) – 2. Crossing over (will be discussed later)


In humans, meiosis starts before birth and stops in Prophase I until puberty (“meiotic arrest”)


Meiosis II is an ordinary mitosis!


• The key difference between mitosis and meiosis:

-Homologous chromosomes (each chromosome consisting of two sister chromatids) pair at the begin of meiosis tetrades

- Homologous chromosomes do not pair in mitosis

- Therefore, the mitotic spindle treats each chromosome in meiosis the same way as it treats a chromatid in mitosis.

Meiosis and Genetic Variation

• Meiosis is a major mechanism contributing to genetic diversity

A a

• What is “genetic diversity”? Alleles are slightly different versions of the

same gene. In a diploid cell, there can be up to 2

allelelic versions of a given gene Within a species (= a population of

organisms that can mate with each other), there can be many allelelic versions of the same gene

• Meiosis-related mechanisms causing genetic diversity: – Independent assortment of chromosomes – Random fertilization – Crossing over

b B

Gene 1

Gene 2

Allelic versions of gene 1

Allelic versions of gene 2


Independent Assortment of Chromosomes

• Is a consequence of the mechanism of meiosis just discussed

• Every chromosome pair orients independently of the others during meiosis.

– 2 homologous pairs of chromosomes 22 combinations

– Humans: 23 homologous pairs of chromosomes 223 (= 8 x 106) combinations

Every human sperm cell represents one of 8 million possible combinations of the father’s chromosomes

Every human egg represents one of 8 million possible combinations of the mother’s chromosomes

Random Fertilization

• The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote

– A human egg cell represents 1 of 8 million possibilities;

– sperm cell represents 1of 8 million other possibilities

Number of possible different zygotes:

8 x 106 X 8 x 106 = 6.4 x 1013

Every human baby represents one of > sixty thousand billion possible combinations of its parents’ chromosomes


Crossing over = genetic recombination

• Is an exchange of chromosome segments that probably originated from an ancient repair mechanism

• In crossing over, – Homologous chromosomes

exchange genetic information at prophase I of meiosis

– Genetic recombination occurs. – Resulting chromosomes are called

recombinant – in practice, there are at least 10

crossing-over per chromosome (10 chiasmata)

Crossing over further dramatically increases the number of possible combinations of genes in offspring of sexual species

Three real-life examples of crossing over

CHROMOSOMAL ABNORMALITIES

When Meiosis Goes Awry

• Meiosis is very error-prone

– The most frequent error is nondisjunction (= homologous chromosomes are not separated during anaphase ) gametes with abnormal chromosome numbers (aneuploidy)

–10%–25% of fertilized human oocytes are aneuploid

–Resulting embryos usually die (with some exceptions: see later)


• Two types of non-disjunction • Fertilization after non-disjunction in the oocyte

What causes Nondisjunction? • Factor number one: age of the pregnant woman

•Meiosis in women begins before birth and lasts till ovulation (decades) • It appears the longer the meiotic arrest the greater the risk

• Factor number 2: environmental chemicals •Bisphenol A (BPA) is the monomer used to manufacture plastics such as food and beverage cans and dental sealents.

• In mice, BPA increased the number of abnormal meioses from 1 to 40% •BPA is also known to have estrogen-like effects connection with factor 1?

Normal BPA in drinking water

• There are two situations when an abnormal chromosome number does not lead to death:

•When the chromosome is very small •When the chromosome is a sex chromosome


Down Syndrome: Extra Chromosome 21 • Down Syndrome (= “trisomy 21”)

– Not lethal because chr. 21 is very small by comparison – Most common chromosomal abnormality (1 in 700 children) – Children have facial, mental, and inner organ defects – Incidence increases with age of mother


Abnormal Numbers of Sex Chromosomes • Nondisjunction also affects the sex chromosomes

– Effects of abnormal numbers of sex chromosomes milder than with unusual numbers of autosomes

– Y chromosome is very small and carries very few gene that mostly affect maleness but are not vital

– Mammalian X chromosome is unique because cells can normally operate with only one functioning X chromosome (any other copies of the X become inactivated in each cell)


Breast development

Poor beard growth

Under- developed testes

Man with Klinefelter syndrome (XXY)

• Klinefelter syndrome (XXY) – Not rare! 1 in 500 boys – Male sex organs, small testes, sterile, – breast enlargement, feminine body contours, – Otherwise health not much affected – Treatment: testosterone


Constriction of aorta

Woman with Turner syndrome (XO)

Poor breast development

Web of skin

Under-developed ovaries

• Turner syndrome (XO) – 1 in 2,500 girls – Short stature, a web of skin extending between neck and

shoulder – Normal intelligence – Sex organs do not mature fully – Female sex since the default sex in humans is female.

Y chromosome determines maleness regardless of number of X chromosomes

– Treatment • Growth hormone replacement therapy: injections (at age

8 till puberty) • Hormone replacement therapy: Estrogen and

progesterone tablets • Can give birth (with donor eggs and assisted

reproduction) • Can marry (with hormone treatment)

• Chromosomal abnormalities are only one of the reasons why pregnancies can fail

• In humans 15% of all pregnancies result in miscarriage in the first 3 months

• The other major reason is rejection of the embryo by the mother (immunological reasons, starvation, infection, stress,…) • The embryo is a foreign tissue for the mother • Foreign tissues are rejected by the immune system • Special mechanisms suppress rejection during pregnancy, but these mechanisms

can also frequently fail

BIOLOGICAL ROLE OF SEX

The biological advantages of diploidy and sex

The origin and biological roles of sex are still incompletely understood

… but some important statements can be made

• Some organisms are haploid, some are diploid, some are both

• Bacteria are always haploid

• Lower plants are most of the time haploid

• Most animals are diploid (only our sperm and egg cells are haploid)

• Higher plants are diploid or even polyploid

• Less complex organisms with high proliferation rate are more likely to be haploid

• More complex organisms with relatively lower proliferation rate are more likely to be diploid

• Protection from damaging mutations:

When one of the two alleles of a gene is damaged, the other can still function

• Repair of double-strand mutations:

When one of the two alleles of a gene is damaged, the other can be used as a template for repair

Homologous chromosomes

Intact allele product (protein)

Damaged allele no product

Repair

• Advantages of diploidy

Different variants of the same gene are called alleles

• Some organisms are sexual, some are asexual (and some can be both)

http://en.wikipedia.org/wiki/Sex

•Bacteria have completely different strategies of evolution (lateral transfer of genes rather than sex; extremely high proliferation rates)

•Plants can easily be both sexual and asexual

•Animals are mainly sexual

Diploid stage of life cycle = can be major stage like in humans or minor stage like in lower plants

Haploid stage of life cycle = can be minor stage like in humans or major stage like in lower plants

2 major mechanisms:

• Reshuffling of chromosomes reshuffling of alleles rapid genetic variation

• Crossing over of homologous chromosomes can accelerate and stabilize linkage of useful alleles by bringing them together on the same chromosome

rapid adaptation to new environment (especially in organisms with few offspring and slow generation cycles)

• Advantages of sex

Sex facilitates adaptation of a species by rapidly creating novel combinations of alleles

• In a haploid organism the two good alleles would never come together. But in a diploid organism, the two good alleles may come together (most efficiently through sex) in the same cell

Sex brings good alleles together in the same organism and can fix such successful combinations in the population by crossing over

A

b B

a A

b B

a

B b

a A

B b

a A

Meiosis without crossing over

(2n)

• Furthermore, if the two chromosomes can undergo crossing over, then the two good alleles may become stably linked on the same chromosome. Such an organism has a higher chance to produce offspring with two good alleles.

Meiosis with crossing over

(1n)

Asexual reproduction: B gets lost before it can combine with A; may

arise later again

Sexual reproduction: Beneficial mutation B

can easily combine with beneficial mutation A

Time lag of asexual evolution (at similar reproductive rate)

Com

posi

tion

of p

opul

atio

n C

ompo

sitio

n of

pop

ulat

ion

http://en.wikipedia.org/wiki/Sex

No crossing over

With crossing over

Crossing over speeds up evolution by fixing combinations of new good alleles

Hence, sex may amplify the advantages of diploidy

But sex has disadvantages

• Need to find a partner (self-fertilization is a strategy that can bridge periods of loneliness – especially in plants)

• Sexual competition (male against male and female against female) often favors traits that reduce chance of survival

• The twofold cost of sex: slower reproduction rate

The advantage of sex is lost when there is no need for genetic variation

In a stable environment, the phenomenon of sex disappears

Sexual reproduction

Asexual reproduction

Assuming same number of offspring (here: 2)

per parent http://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction#Two-

fold_cost_of_sex

http://china.org.cn/photos/2010-02/24/content_19467238_5.htm

For example, yeast cells undergo meiosis in starvation

http://en.wikipedia.org/wiki/Yeast

Stress such as starvation

Examples of animals that can alternate between asexual and sexual reproduction

Aphids •Asexual reproduction in warm part of year •Sexual reproduction before winter

An aphid giving birth to a baby aphid

Daphnia •Asexual reproduction in good nutrition and summer •Sexual reproduction before winter and in crowded situation

Examples of animals that can alternate between asexual and sexual reproduction

Biological role • Sex builds on the phenomenon of diploidy and provides additional benefits: • Rapid adaptation of the species to a changing environment by creating new

combinations of old alleles and fixing good combinations of new alleles (Note: Self-fertilization is a workaround for situations when mates are hard to find).

• In multicellular organisms, sex led to the specialization into germ and somatic cells (and hence to death) and specialization into very different females and males.

• Sex has also a cost, therefore asexual reproduction prevails in constant environments. There are strategies such as self-fertilization or switches between sexual and non-sexual stages that for a limited time can reduce the cost of being sexual.

Summary – Sexual reproduction Nature of sex • Generation of new combinations of alleles by reshuffling alleles of a population. • Hence sexual reproduction typically involves more than 1 parent (exception: self-

fertilization).

Mechanism • Cycles of meiosis (usually with crossing overs) and fertilization. • Meiosis creates novel haploid sets of chromosomes. Fertilization combines them into

diploid sets.

요약- 유성생식 성의 특징

• 개체군의 형질 재조합으로 인한 새로운 세대 생성

• 유성생식은 일반적으로 1명 이상의 개체가 필요하다 (예외: 자가수정).

기작

• 감수분열 (보통 염색체 교차도 함께)과 수정.

• 감수분열은 염색체의 새로운 반수체 (N)를 생산한다. 수정은 이들을 이배체(2N)로 조합한다. (대게 이들은 새로운 조합이다).

생물학적 역할

• 성은 이배체 현상을 만들 뿐만 아니라 추가적인 이득이 있다:

• 오래된 대립형질의 새로운 조합을 만들어내고 좋은 조합의 새로운 대립형질을 고침으로서 변화한 환경에 종이 빠르게 적응할 수 있게 한다 (주석: 자가수정은 배우자를 찾기 어려운 상황에 대한 차선책이다).

• 다세포 유기체에서, 성은 생식세포와 체세포의 특성화, 그리고 여성과 남성의 차이에 대한 특성화를 만들어 낸다.

• 성 또한 비용이 듦으로, 무성생식은 변함없는 환경에서 유리하다. 자가수정 또는 유성과 무성생식 전환과 같은 전략을 통해 제한된 시간 동안 생식에 드는 비용을 줄일 수 있다.

가장 중요한 단어들

• Fertilization, meiosis, haploid, diploid, aneuploid

• Crossing over, recombination, gene, allele

• Somatic cells, germ cells (= gametes), eggs (= oocytes), sperm cells

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