Reproduction - Sexual Reproduction

Lecture #12

Sexual Reproduction

• one of the major ways that organisms have evolved to more predictably incorporate new genetic diversity into their population

• major defining characteristic of Eukaryotes

• involves the production of gametes with require fertilization to form zygotes

eggs and sperm

what are the more common names for gametes?

for sexual reproduction to work, we need a way to ensure that our resulting zygote has the same number of chromosomes as we did as the parent

Remember: for sexual reproduction to work, we need a way to ensure that our resulting zygote has the same number of chromosomes as we did as the parent

Ploidy 

• the number of complete sets of chromosomes in a healthy cell

Diploid

• 2 copies of each chromosome

Human Ploidy : n = 23

• humans have 24 types of chromosomes

  • of which humans have 23 pairs

• n = 23 & 2n = 46

Homologous Chromosomes

• Definition: chromosomes that are evolutionarily and structurally the same (but not identical) 

  • evolutionarily derived from the same ancestral structure

• they code for the same genes but may have different alleles for that gene

• humans have at least 22 pairs of these chromosomes

  • you get 1 of each from your dad, and 1 of each from your mom

  • these 22 ______ chromosomes are referred to as autosomes bc they are not involved with sex determination

autosomes

• the 22 homologous chromosomes that are NOT involved with sex determination

allele

• any of the alternative versions of a gene that may produce distinguishable differences in an individuals phenotype

two sex chromosomes

• X & X = biological female in mammals

  • X & X chromosomes are homologous (the same)

  • Homozygous sex

• X & Y = biological male in mammals

  • X & Y are NOT homologous (diff in origin and structure)

  • Heterozygous sex

Biological sex

• humans have 2 sex chromosomes X & Y 

  • makes 2 karyotypic conditions:

    • XX: female, homozygous sex

    • XY: male, heterozygous sex

• _______ is more complicated than simply XX or XY chromosomes

  • Birds:

    • ZW: female, heterozygous sex

    • ZZ: male, homozygous sex

  • Many species don’t have sex chromosomes at all

    • a crocodiles sex is determined by the temperature of the environment around the egg

Biological sex in humans in more than just XX vs XY, there is an entire spectrum of sex chromosome conditions, each with a very different biological outcome 

• Remember: Biological sex in humans in more than just XX vs XY, there is an entire spectrum of sex chromosome conditions, each with a very different biological outcome 

Sex & the chromosome problem

• in order to create zygotes and offspring which match the parents’ ploidy, our gametes need to have fewer chromosome than the rest of our cells 

  • we need specifically 1 of each chromosome

    • human gametes need to be Haploid

Haploid

• humans gametes are _____ because they can only have 1 copy of each chromosome

Meiosis

• Definition: a specialized type of cell division that allows for sexual reproduction to occur

• two important things occur during _____:

  • Genetic diversity is generated

  • Chromosomal reduction takes place

Genetic diversity is generated & Chromosomal reduction takes place

what two important things occur during meiosis?

Genetic diversity is generated

• crossing over (recombination)

• independent assortment

• random combination of gametes

  • last aspect occurs after meiosis

Chromosomal reduction

• Diploid parent cell is reduced to haploid daughter cell

  • egg or sperm cell

• by reducing ploidy in egg and sperm cells:

  • when these haploid cells combine into a new embryo —> the fused embryo will have the same level of ploidy as the parent organism (diploid)

Meiosis

• included 1 interphase & 2 cycles of division: 

  • interphase

  • meiosis I

  • meiosis II

• genetic recombination occurs:

  • crossing over (recombination)

  • independent assortment

• mother cell genetically distinct from daughter cells

• 4 daughter cells are created from the 1 mother cell

• daughter cells are haploid gametes

  • eggs and sperm

interphase, meiosis I & meiosis II 

what are the 3 phases of meiosis?

Meiosis - Interphase

• G1 phase: cell grows

• S phase: DNA duplicates

• G2 phase: Centrosome duplicates

G1 (cell grows), S (DNA duplicates), G2 (centrosome duplicates) 

what are the 3 phases of Meiosis - Interphase?

Meiosis - Meiosis I

• where the majority of genetic variation is created

  • crossing over (recombination)

  • independent assortment

• cell goes from 2n → n

• 5 stages of Meiosis I:

  • prophase I

  • metaphase I

  • anaphase I

  • telophase I & cytokinesis

prophase I, metaphase I, anaphase I, telophase I & cytokinesis

what are the 5 stages of meiosis - meiosis I? 

Meiosis I - Prophase I

• nuclear membrane dissolves

• centrosomes start migration to opposite ends of the cell (centrosomes split apart)

• chromatin condenses to form sister chromatids

• homologous chromosomes align with each other to form a tetrad 

• crossing over (recombination): gene swapping between homologous non-sister chromatids 

Meiosis I - Metaphase I 

• centrosomes move to opposite ends of cell

• Tetrads line up along the metaphase plate 

• independent assortment: homologous pairs of chromosomes orient randomly

Meiosis I - Anaphase I

• Meiotic spindles pull apart the tetrads

  • homologous chromosome tetrads separate

  • sister chromatids still attached by a centromere

Meiosis I - Telophase I 

• genetic material moves to opposite ends of the cell

• cleavage furrow forms

• nuclear envelope starts to reform

• intact sister chromatids present in daughter cells

  • sister chromatids are not identical

Meiosis I - Cytokinesis I

• single cell splits into two daughter cells 

  • involves the cleavage furrow

• two daughter cells are haploid and genetically different from the parent cell

  • result of crossing over and independent assortment

Meiosis II 

• similar to mitosis, but with only ½ the number of chromosomes to separate 

Meiosis II - Prophase II

• DNA chromatin condenses into structures consisting of two sister chromatids

  • attached in the center by a centromere

  • sister chromatids are not identical

• the centrosomes move to the opposite ends of the cell (being to produce microtubules)

• nuclear envelope breaks down and the nucleolus disappears

Meiosis II - Metaphase II

• microtubules on either end of the cell attach to the centromeres

• pulling force in opposite directions powered by motor proteins 

  • sister chromatids lined up along the metaphase plate dividing line along the cell

  • sister chromatids are not identical

meiosis II - Anaphase II 

• motor proteins pull sister chromatids apart at the centromere

• the split chromatids begin moving towards the opposite ends of the cell 

  • split chromatids are not identical 

meiosis II - Telophase II

• start to undo the cellular changes that took place during prophase II:

  • nuclear membrane re-forms

  • the chromatids unwind and form chromatin

• split starts to form down the center of the cell along the metaphase plate

• daughter cells formed are both : haploid & genetically unique from each other

meiosis II - Cytokinesis II 

• daughter cells produced at the end of meiosis I now split into two other daughter cells of their own 

• initial diploid parent cell gives rise to 4 unique haploid gamete cells

Fertilization

• return to diploidy

• random:

  • ratio btw sperm and eggs can be >500,000,000:1

  • this results in what is effectively a random fertilization by one of those 500,000,000 totally unique sperm

Meiosis is a vital source of genetic diversity in sexually reproducing organisms 

Remember: Meiosis is a vital source of genetic diversity in sexually reproducing organisms