Topic 2 - Meiosis and Sexual Reproduction

Topic 2

Meiosis and Sexual Reproduction

Content

• An evolutionary overview

• Overview of meiosis

• How meiosis puts variation in traits

• From gamete to offsprings.

Why Sex?

• Sex mixes up the genes of two parents, so offspring of sexual

reproducers have unique combinations of traits

• Diversity offers sexual reproducers as a group a better

chance of surviving environmental change than clones

• sexual reproduction

• Reproductive mode by which offspring arise from two

parents and inherit genes from both

Sexual Reproduction in Plants

• In flowering plants, pollen tubes with male gametes grow

down into the ovary, containing female gametes

Sexual Reproduction and Meiosis

• Asexual reproduction produces clones

• Sexual reproduction mixes up alleles from two parents

• Meiosis, the basis of sexual reproduction, is a nuclear

division mechanism that occurs in reproductive cells of

eukaryotes

• In asexual reproduction, one parent transmits its genes

to offspring

• In sexual reproduction, offspring inherit genes from two

parents who usually differ in some number of alleles

• Differences in alleles are the basis of differences in traits

Introducing Alleles

• Somatic cells of sexually-reproducing multicelled organisms

contain pairs of chromosomes: one from the mother and one

from the father

• 2 chromosomes of every pair carry the same set of genes,

except for the nonidentical sex chromosomes.

• somatic

• Relating to the body

Introducing Alleles (cont.)

• The 2 genes of a pair are often not identical: Maternal and

paternal genes can encode slightly different forms (alleles) of

the same gene’s product

• alleles

• Forms of a gene that encode slightly different versions of

the gene’s product

Introducing Alleles (cont.)

• Most genes have multiple alleles – one reason individuals of a

sexually reproducing species do not look exactly the same

• Offspring of sexual reproducers inherit new combinations of

alleles, which is the basis of new combinations of traits

Homologous Chromosomes

• One chromosome in a homologous pair is inherited from the

mother, one from the father – different forms are alleles

What Meiosis Does

• Sexual reproduction involves fusion of reproductive cells from

two parents

• Meiosis halves the chromosome number in reproductive cells

so offspring have the same number of chromosomes as the

parents

• meiosis

• Nuclear division process that halves the chromosome

number

• Basis of sexual reproduction

Sexual Reproduction

• The process of sexual reproduction begins with meiosis in

germ cells, which produces gametes

• germ cell

• Diploid reproductive cell that gives rise to haploid gametes

by meiosis

• gamete

• Mature, haploid reproductive cell (egg or sperm)

• Gametes usually form inside special male and female

reproductive structures

Maintaining Chromosome Number

• Gametes have a single set of chromosomes, so they are

haploid (n): Their chromosome number is half of the diploid

(2n) number

• Diploid number is restored at fertilization, when two haploid

gametes fuse to form a zygote, the first cell of a new

individual

Key Terms

• haploid

• Having one of each type of chromosome characteristic of

the species

• fertilization

• Fusion of two gametes to form a zygote

• zygote

• Cell formed by fusion of two gametes

• The first cell of a new individual

Diploid cells vs haploid cells

• Diploid cells

• Characteristic number of chromosome pairs per cell

• Homologous chromosomes

• Similar in length, shape, other features, and carry similar attributes

• Haploid cells

• Contain only one member of each homologous chromosome pair

The Process of Meiosis

• Meiosis starts like mitosis, but sorts chromosomes into new

nuclei twice, forming 4 haploid nuclei

• DNA replication occurs prior to meiosis – the nucleus is

diploid (2n) with two sets of chromosomes, one from each

parent

• During meiosis, chromosomes of a diploid nucleus become

distributed into four haploid nuclei

• Consist meiosis I and meiosis II stages

Stages of Meiosis

Meiosis I: Prophase I

• Homologous chromosomes condense, pair up, and swap

segments

• Spindle microtubules attach to chromosomes as the nuclear

envelope breaks up

Meiosis I: Metaphase I

• The homologous chromosome pairs are aligned midway

between spindle poles

Meiosis I: Anaphase I

• The homologous chromosome separate and begin

heading toward the spindle poles

Meiosis I: Telophase I

• Two clusters of chromosomes reach the spindle poles

• A new nuclear envelope forms around each cluster, so 2

haploid (n) nuclei form

Meiosis II: Prophase II

• The chromosomes condense

• Spindle microtubules attach to each sister chromatid as the

nuclear envelope breaks up

Meiosis II: Metaphase II

• The (still duplicated) chromosomes are aligned midway

between poles of the spindle

Meiosis II: Anaphase II

• All sister chromatids separate

• The now unduplicated chromosomes head to the spindle

poles

Meiosis II: Telophase II

• A cluster of chromosomes reaches each spindle pole

• A new nuclear envelope encloses each cluster, 4 haploid

(n) nuclei form

How Meiosis Introduces Variations in Traits

• 2 events in meiosis introduce novel combinations of alleles

into gametes:

• Crossing over in prophase I

• Segregation of chromosomes into gametes

• Along with fertilization, these events contribute to the variation

in combinations of traits among the offspring of sexually

reproducing species

Crossing Over in Prophase I

• In prophase I, chromatids of homologous chromosomes align

along their length and exchange segments (crossing over)

• crossing over

• Process in which homologous chromosomes exchange

corresponding segments during prophase I of meiosis

• Introduces novel combinations of traits among offspring

Segregation of Chromosomes Into Gametes

• When homologous chromosomes separate in meiosis I, one

of each chromosome pair goes to each of the two new nuclei

• For each chromosome pair, the maternal or paternal version

is equally likely to end up in either nucleus

• Each time a human germ cell undergoes meiosis, the 4

gametes that form end up with one of 8,388,608 (or 223)

possible combinations of homologous chromosomes

MITOSIS vs MEIOSIS

Mitosis vs meiosis

Key Concepts

• Recombinations and Shufflings

• During meiosis, homologous chromosomes come together

and swap segments

• Then they are randomly sorted into separate nuclei

• Both processes lead to novel combinations of alleles

among offspring

12.6 Mitosis and Meiosis: An Ancestral Connection?

• Mitosis makes exact copies of chromosomes; meiosis mixes

genes up and halves the chromosome number

• Though their end results differ, the four stages of mitosis and

meiosis II are similar

• Sexual reproduction probably originated by mutations that

affected processes of mitosis

Key Concepts

• Mitosis and Meiosis Compared

• Similarities between mitosis and meiosis suggest that

meiosis may have originated by evolutionary remodeling

of mechanisms that already existed for mitosis and, before

that, for repairing damaged DNA

From Gametes to Offspring

• Gametes (eggs and sperm) are specialized cells that are the

basis of sexual reproduction

• All gametes are haploid, but they differ in other details

• Gamete formation differs among plants and animals

Gamete Formation in Plants

• Two kinds of multicelled bodies form during the life cycle of a

plant: sporophytes and gametophytes

• sporophyte

• Diploid, spore-producing stage of a plant life cycle

• gametophyte

• Haploid, multicelled body in which gametes form during

the life cycle of plants

Sporophyte (2n) (multicellular

diploid organism)

Gametophyte (n) (multicellular

haploid organism)

Fertilization

Zygote (2n)

Meiosis

Mitosis

Spores (n)

Mitosis

Mitosis

Gametes (n)

(c) Plants, some algae, and some fungi

Gamete Formation in Animals

• In animals, a zygote matures as a multicelled body that

produces gametes by meiosis of diploid germ cells

• In male animals, the germ cell develops into a primary

spermatocyte, which undergoes meiosis to form four sperm

• In female animals, a germ cell becomes a primary oocyte,

which undergoes meiosis to form one egg and three polar

bodies, which degenerate

Multicellular diploid organism (2n)

Gametes (n)

Fertilization

Zygote (2n)

Meiosis

Mitosis

(a) Animals

Unicellular or multicellular haploid

organism (2n)

Gametes (n)

Fertilization

Zygote (2n)

Meiosis

Mitosis Mitosis

(b) Simple eukaryotes

Key Terms

• sperm

• Mature male gamete

• Haploid product of meiosis

• egg

• Mature female gamete, or ovum

• Haploid product of meiosis

Sperm Formation in Animals

1. A diploid male germ cell develops into a diploid primary

spermatocyte as it replicates its DNA

2. Meiosis I in the primary spermatocyte results in two haploid

secondary spermatocytes

3. Four haploid spermatids form when secondary spermatocytes

undergo meiosis II

4. Spermatids mature as sperm (haploid male gametes)

Egg Formation in Animals

1. A diploid female germ cell (oogonium) develops into a diploid

primary oocyte as it replicates its DNA

2. Meiosis I in the primary oocyte results in a haploid secondary

oocyte and a haploid polar body

• Unequal cytoplasmic division makes the polar body much

smaller than the oocyte

3. Meiosis II, followed by unequal cytoplasmic division in the

secondary oocyte, results in a polar body and ovum (egg)

Key Concepts

• Sexual Reproduction in the Context of Life Cycles

• Gametes form by different mechanisms in males and

females, but meiosis is part of both processes

• In most plants, spore formation and other events intervene

between meiosis and gamete formation

Fertilization

• Two gametes fuse at fertilization, resulting in a diploid

zygote

• Human sperm surrounding an egg during fertilization