4.2 Sexual Reproduction4.2 Sexual Reproduction

AgendaAgenda

• Asexual vs Sexual reproduction• Haploid and Diploid cells • Meiosis: outcomes & differences from mitosis• How gametes are formed in animals• Processes of producing genetic variation• Errors that occur during meiosis • Genetic disorders

• Related to abnormal chromosome number

• Prenatal genetic testing

Asexual vs Sexual

Asexual Reproduction Asexual Reproduction – mitosis • Requires only one parent• Produce genetically identical offspring• Carried out by bacteria, some plants and somatic

cells

Sexual ReproductionSexual Reproduction – meiosis & fertilization• Requires two parents• Produces genetically distinct offspring• Carried out by most animals and sex cells

(gametes)

Key TermsKey TermsGametes: haploid reproductive cells (i.e. egg and sperm) produced by meiosis.Haploids: cells that contain single, unpaired chromosomes; half as much as the parent cellFertilization: the fusion of two gametes.Zygote: the diploid cell created by fertilization.Diploid: a cell that contains pairs of homologous chromosomes

All together now…

Two haploid gametes, an egg from the mother and sperm from the father, are fertilized to form a diploid zygote which will grow into a baby.

Sexual Reproduction Sexual Reproduction

Sexual ReproductionSexual Reproduction

Fusion/ fertilization

Offspring

Parents

Sexual ReproductionSexual Reproduction

• In Plants

Meiosis - OutcomesOutcomes

• Genetic reduction… Produces daughter cells with half the number of chromosomes than the parent cell

• Genetic recombination… daughter cells have different allele combinations from one another and their parents.– Increases genetic variation

Meiosis vs Mitosis Meiosis vs Mitosis

Meiosis occurs in two parts: Meiosis I and II– Mitosis is most like meiosis II

• Prophase I: homologous chromosomes align in a process known assynapsis. – Genetic material is

exchangedResembles prophase in mitosis

Mitosis vs Meiosis Mitosis vs Meiosis

• Metaphase I: homologous chromosomes line up along the equator of the cell – In mitosis, chromosomes line up in a single file

Metaphase IMetaphase II (like mitosis)

VS

Meiosis vs Mitosis Meiosis vs Mitosis

• Anaphase I: homologous chromosomes separate, centromeres remain intact– this is where the daughter cells become haploid

Meiosis vs Mitosis Meiosis vs Mitosis

In mitosis, each of the 46 chromosomes are treated separately. Homologous chromosomes exist in somatic cells, but they don’t align with each other as they do in meiosis I

Chromosomes line up at the equator in meiosis II similar to mitosis, however in meiosis II, haploid cells are being divided.

Meiosis Mitosis

Prophase I

not identical

Prophase

How are GAMETES formed?How are GAMETES formed?

• The products of meiosis are haploid gametes• The process of sperm production in males is

known as spermatogenesis.• Egg production in females is called oogenesis.

Spermatogenesis Spermatogenesis • Initial diploid cell

(spermatogonium) undergoes mitosis beginning puberty, then meiosis forming 4 haploid cells

• Following meiosis II, cell continue to develop into mature sperm:– The nucleus is organized into a

“head”region– Many mitochondria in the

midsection– Flagellum for locomotion

Oogenesis Oogenesis • Oogonia reproduce by mitosis, begin

meiosis but stop at Prophase I.• Meiosis I continues for one cell each

month beginning at puberty producing an unequal division of the cytoplasm

• The cell that receives most of the cytoplasm (larger haploid) contains many nutrients ideal for zygote growth and continues to Meiosis II

• The smaller cell is referred to a polar body and will degenerate

• Meiosis II produces a mature egg (ovum) and another polar body

Multiple BirthsMultiple Births

Fraternal Twins• Occurs when two eggs are released and both

are fertilized

Identical Twins• Occurs when a zygote divides into two

separate bodies in the first few days of development

Homework

• Handout• Pg. 172 #7-9• Pg. 181 1,2,4-6,8

AgendaAgenda

• Asexual vs Sexual reproduction• Haploid and Diploid cells • Meiosis: outcomes & differences from mitosis• How gametes are formed in animals• Processes of producing genetic variation• Errors that occur during meiosis • Genetic disorders

• Related to abnormal chromosome number

• Prenatal genetic testing

4.2 Part 2: Genetic Variation4.2 Part 2: Genetic Variation

Genetic VariationGenetic Variation

We’ve seen that mitosis produces genetically identical offspring, while the process of meiosis introduces variation. BUT HOW????

1. By exchanging genetic material between maternal and paternal chromosomes during synapsis… Crossing Over

2. By creating gametes that carry different combinations of maternal and paternal chromosomes… Independent Assortment

Crossing OverCrossing Over

• Non-sister chromatids of homologous chromosomes exchange segments of DNA during synapsis (Prophase I)

• Results in a combination of maternal and paternal genes on individual chromosomes

• Crossing over can occur at several points along non-sister chromatids

Independent AssortmentIndependent Assortment• How one set of homologous chromosomes orients itself

along the equator of a cell (Metaphase I) is independent from how another pair is oriented

• This results in the ability to produce 2n combinations of genetically distinct gametes from a single diploid celln = # of chromosome pairs (humans: 223 = 8 388 608)

Errors During MeiosisErrors During Meiosis

Independent Assortment and Crossing Over produces genetic variation, but in the process, can also result in :

• Structural abnormalities (of chromosomes)• the production of abnormal gametes (non-

disjunction)

If gametes containing these errors are fertilized, they will produce a zygote (and therefore offspring) with the same chromosome abnormalities in every cell.

Structural AbnormalitiesStructural Abnormalities

During crossing over, chemical bonds holding DNA together are broken and reformed. •Errors result in incorrect reformation.

Types of Errors: 1.Deletion– a piece of a chromosome is missing2.Duplication– a section of a chromosome is repeated3.Inversion – a region of a chromosome is inverted4.Translocation – a segment of a chromosome is reattached in the wrong place

Non-DisjunctionNon-Disjunction

= the failure of chromosomes to separate properly during meiosis (Anaphase)

• Results in gametes with to many or too few chromosomes

When it occurs in Meiosis I

When it occurs in Meiosis II

HomeworkHomework

• On a sheet of paper name and describe one genetic disorder for each type of meiosis error– You can use your textbook or the internet – Submit by tomorrow