topic 2 - meiosis and sexual reproduction
TRANSCRIPT
![Page 1: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/1.jpg)
Topic 2
Meiosis and Sexual Reproduction
![Page 2: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/2.jpg)
Content
• An evolutionary overview
• Overview of meiosis
• How meiosis puts variation in traits
• From gamete to offsprings.
![Page 3: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/3.jpg)
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
![Page 4: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/4.jpg)
Sexual Reproduction in Plants
• In flowering plants, pollen tubes with male gametes grow
down into the ovary, containing female gametes
![Page 5: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/5.jpg)
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
![Page 6: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/6.jpg)
• 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
![Page 7: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/7.jpg)
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
![Page 8: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/8.jpg)
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
![Page 9: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/9.jpg)
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
![Page 10: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/10.jpg)
Homologous Chromosomes
• One chromosome in a homologous pair is inherited from the
mother, one from the father – different forms are alleles
![Page 11: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/11.jpg)
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
![Page 12: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/12.jpg)
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
![Page 13: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/13.jpg)
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
![Page 14: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/14.jpg)
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
![Page 15: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/15.jpg)
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
![Page 16: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/16.jpg)
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
![Page 17: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/17.jpg)
Stages of Meiosis
![Page 18: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/18.jpg)
Meiosis I: Prophase I
• Homologous chromosomes condense, pair up, and swap
segments
• Spindle microtubules attach to chromosomes as the nuclear
envelope breaks up
![Page 19: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/19.jpg)
Meiosis I: Metaphase I
• The homologous chromosome pairs are aligned midway
between spindle poles
![Page 20: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/20.jpg)
Meiosis I: Anaphase I
• The homologous chromosome separate and begin
heading toward the spindle poles
![Page 21: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/21.jpg)
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
![Page 22: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/22.jpg)
Meiosis II: Prophase II
• The chromosomes condense
• Spindle microtubules attach to each sister chromatid as the
nuclear envelope breaks up
![Page 23: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/23.jpg)
Meiosis II: Metaphase II
• The (still duplicated) chromosomes are aligned midway
between poles of the spindle
![Page 24: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/24.jpg)
Meiosis II: Anaphase II
• All sister chromatids separate
• The now unduplicated chromosomes head to the spindle
poles
![Page 25: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/25.jpg)
Meiosis II: Telophase II
• A cluster of chromosomes reaches each spindle pole
• A new nuclear envelope encloses each cluster, 4 haploid
(n) nuclei form
![Page 26: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/26.jpg)
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
![Page 27: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/27.jpg)
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
![Page 28: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/28.jpg)
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
![Page 29: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/29.jpg)
MITOSIS vs MEIOSIS
![Page 30: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/30.jpg)
Mitosis vs meiosis
![Page 31: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/31.jpg)
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
![Page 32: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/32.jpg)
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
![Page 33: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/33.jpg)
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
![Page 34: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/34.jpg)
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
![Page 35: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/35.jpg)
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
![Page 36: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/36.jpg)
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
![Page 37: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/37.jpg)
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
![Page 38: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/38.jpg)
Multicellular diploid organism (2n)
Gametes (n)
Fertilization
Zygote (2n)
Meiosis
Mitosis
(a) Animals
![Page 39: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/39.jpg)
Unicellular or multicellular haploid
organism (2n)
Gametes (n)
Fertilization
Zygote (2n)
Meiosis
Mitosis Mitosis
(b) Simple eukaryotes
![Page 40: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/40.jpg)
Key Terms
• sperm
• Mature male gamete
• Haploid product of meiosis
• egg
• Mature female gamete, or ovum
• Haploid product of meiosis
![Page 41: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/41.jpg)
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)
![Page 42: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/42.jpg)
![Page 43: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/43.jpg)
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)
![Page 44: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/44.jpg)
![Page 45: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/45.jpg)
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
![Page 46: Topic 2 - Meiosis and Sexual Reproduction](https://reader030.vdocuments.mx/reader030/viewer/2022021502/577cc9cc1a28aba711a4a730/html5/thumbnails/46.jpg)
Fertilization
• Two gametes fuse at fertilization, resulting in a diploid
zygote
• Human sperm surrounding an egg during fertilization