FRST 210 lecture 11- sexual reproduction I

Sexual reproduction

• Requires fertilization for reproduction

• Involves cell division via meiosis

• Results in new, genetically different plant

Why is sexual reproduction a strong indication of evolutionary advantage?

• It has evolved repeatedly in evolutionary history

How many daughter cells are produced in mitosis? Are homologs paired during cell division? Do chiasmata form between homologous chromosomes? What is the ploidy after cell division? Are the daughter cells genetically identical? (Mother cell with 2N=24 chromosomes)

• Two

• No

• No

• 24 (2n)

• Yes

How many daughter cells are produced in meiosis? Are homologs paired during cell division? Do chiasmata form between homologous chromosomes? What is the ploidy/chromosome count after cell division? Are the daughter cells genetically identical? (Mother cell with 2n=24 chromosomes)

• Four

• Yes

• Yes

• 12 (n)

• no

Homologous chromosomes

• Same type of hcromosome

Chiasmata

• C(Crossing over points)

• Genetic diversity is created in meiosis through this process (genetic recombination) and independent assortment

Major features of meiotic cell division

• Chromosomes are replicated and homologous chromosomes (= same type of chromosome) are paired together

• Chiasmata forms and portions of DNA are exchanged between homologous pairs

• Homologous pairs are separated and assort independently into the daughter cells

Features of sexual reproduction

• Sexually reproducing organisms have a haploid (n) phase and a diploid (2n) phase in their life cycles

• Ploidy reflects the number of each type of chromosome

• Meiosis occurs in diploid cells to create haploid cells

• The haploid and diploid phases in organisms can be either unicellular or multicellular → it can be helpful to remember the extent and the nature of haploid and diploid phases

Ploidy

• Reflects the number of each type of chromosome

Zygotic meiosis

• The haploid phase is unicelluar or multicelluar

• Haploid individuals produce haploid gametes (eggs or sperm)

• Fertilization create a single-celled zygote (=diploid)

• Characteristic of most fungi, some green algae, and many protozoa

Gametic meiosis

• The diploid phase is unicelluar or multicellular

• Diploid individuals produce haploid gametes (eggs or sperm)

• Fertilization creates a zygote and a new individual (=diploid)

• The diploid individual undergoes meiosis to create haploid cells → these develop into gametes, not independent organismss

Sporic meiosis

• The life cycle involves an alternation of generations

• Sporophytes and gametophytes can look similar or toatally different

• Gametophytes create gametes via mitosis (not meiosis)

Gametophyte

• Haploid, have one set hof chromosomes

• Produces the zygote (gamete producing)

Sporophytes

• Diploid, have two set of chromosomes

• They are the dominant generation, undergo meiosis

• Produces spores which gametes arise, dominant form in vascular plants

• Contains both gametophyte and megagametophytes

Sporic meiosis

• Diploid phase is multicelluar

• Diploid individuals (sporophytes) produce haploid spores

• Spores germinate to create new multicelluar haploid individuals (gametophytes)

What happens in sporic meiosis?

• Life cycle involves an alternation of generations

• Sporophyte and gameotophytes can look similar or toally different

• Gametophytes create gametes via mitosis (not meiosis)

New sporophute indviduals are created from fertilization via gametes produced by the gametophyte individuals

Alternation of generations

• Typical in Kingdom Plantae

• Means that the sporophyte (diploid) and gametophyte (haploid) phases (generations) alternate

• Sporophyte phase tends to be more complex and dominant compared to the gametophyte phase

Life cycle in ferns (and fer n allies)

Ferns and fern allies (= horsetails, lycophytes, sikemosses, etc.) have gametophytes and sporophytes that are:

• Independent

• Self-supporting (i.e. photosynthetic)

• Homosporous

• Sporophytes are the domminant generation in ferns (and fern allies) and produce sporangia

• Develop on specialized leves called sporophylls

• Site of meiosis → produces haploid sppores

Homospory

• Producing a single type of spore that develops into a bisexual gametophyte

What is the term when sporangia are clustered together?

• Sorus, plural name is sori)

Are most ferns homosporous or heterosorous?

• They are homosporous, as a single type of spore is released and germinates to reate an independent gametophyte

What does the bisexual gametophyte created from homospory develop?

• Archegonia (egg-producing)

• Antheridia (serm-producing)

What does fertilization of egg and sperm create in fens?

• Fertilizzation creates a new diploid sporophyte that develops on the gametophyte

• The gametohyte does not survive this process and is crushed by the new sporophyte

Differences of homospory and heterospory

• Most ferns and fern allies are homosporous (sporangia undergo meiosis to create one type of spore)

• Some species of ferns and clubmosses evolved two types of spores, so only some of these species are heterosporous, and each type of spore creates a different gametophyte.

• Heterospory is considered evolutionarily advantangeous as it has evolved repeatedly in lants.

Four steps of fern life cycle

• Spore dispersal

• Mature gametophyte develops, and sperm and egg come together and fertilize

• New sporophyte grows on the gametohyte

• Mature sporophyte develops and creates sporangium, and releases spores again.

What do megasporangium create?

• Megasporangium produces megaspores

Megasporangia→ megaspore→megagametophyte (egg producing)

What do microsporangium create?

• Microsporangium produces microspores

Microsporangium→microspore→microgametophyte (sperm producing)

What do microgametophytes develop to encase sperM?

• Antheridia

What do megagametophytes develop to encase eggs?

• Archegonia

Mitosis in somatic cells

• One cell division, resulting in two daughter cells

• Chromosome number per nucleus maintained (e.g., for a diploid cell)

• Interphase (Duplicates chromosomes, not apart of mitosis)

• Prophase (Chromosomes are visible condensing

• Metaphase (Chromatids are lined up at the middle of the cell)

• Anaphase (Chromatids are pulled away by spindles, moved to opposite sides of the cells)

• Telophase (New nuceli are forming to make two new cells)

Meiosis in cells in the sexual cycle

General notes:

• Two cell divisions, resulting in four products of meiosis

• Chromosome number halved in the products of meiosis

• Full synapsis of homologs at prophase I

• At least one chiasma

• Promotes variation among the products of meiosis

• Cell undergoing meiosis is diploid

• Cell becomes an haploid cell

Processes:

• Prophase I(Chromosomes are visible condensing and match up with their homologous pairs (around the same size, and contain the same types of genes in the same locations, and chromosomes cross over, resulting in recombinant chromosomes)

• Metaphase I (Chromosomes are in the middle, but are in the middle

• Anaphase I (Chromosomes in pairs are pulled away

• Telophase I (New nuclei are forming to make two new cells, and undergo cytokinesis to make new cells)

Part 2 (Same :

• Prophase II (Chromosomes condense in both cells, and the nuclear envelope is broken)

• Metaphase II (The chromosomes are in the middle of the cell, and the chromosomes are in a single line)

• Anaphase II (The chromatids are pulled away from each other)

• Telophase II (The chromosomes are split by cytokinesis to create four non-identical cells)