Sexual Reproduction: Genetic Variation

Reproduction Sexuée (Sexual Reproduction)

• Sexual reproduction involves two parents.
• Offspring are genetically different from each other and from their parents.
• Asexual reproduction involves only one parent.
• Offspring from asexual reproduction are genetically identical to the parent (clones)
• Asexual reproduction results in no genetic variability.
• Sexual reproduction generates genetic variability, leading to adaptation to the environment.

Characteristics of Sexual Reproduction

• Genetic recombination occurs, resulting in new combinations of genes.
• Human somatic cells are diploid (2n)
• Human gametes are haploid (n).
• A zygote is diploid (2n).
• Sexual reproduction involves an alternation of haploid and diploid cells.

Alternation between n and 2n in Sexual Reproduction

• A diploid individual undergoes meiosis to produce haploid gametes.
• A haploid egg and sperm fuse during fertilization to form a diploid zygote.
• The zygote undergoes mitosis to develop into a diploid individual.

Méiose (Meiosis)

• Gametes (n) are formed by meiosis from germ cells (2n).
• One stem cell (2n) gives rise to four daughter cells (n).
• Two consecutive cell divisions occur with only one phase of DNA duplication (S phase).
• The daughter cells are distinct from each other and different from the mother cell.

Phases de la Méiose (Meiosis Phases)

• Meiosis consists of two consecutive divisions separated by an interphase without an S phase.
• The first division (Meiosis I) involves genetic recombination and chromosome reduction.
• The second division (Meiosis II) is similar to mitosis.
• Prophase I involves pairing of chromosomes and recombination.
• Metaphase I involves alignment of chromosome pairs.
• Anaphase I involves separation of chromosomes.
• Telophase I is the final stage of the first meiotic division
• Prophase II is first stage of the second meiotic division
• Metaphase II involves alignment of chromosomes.
• Anaphase II involves separation of chromatids.
• Télophase II is the final stage of the second meiotic division

Meiosis I Phases

• During Prophase 1, homologous chromosomes pair up, and crossing-over occurs
• During Metaphase 1, homologous chromosomes move to the equator of the cell
• During Anaphase 1, homologous chromosomes move to the opposite poles of the cell
• During Telophase I and cytokinesis, chromosome gather at the poles of the cells and the cytoplasm divides and the cells are split

Meiosis II Phases

• During Prophase II, a new spidle forms around chromosomes
• During Metaphase II, Metaphase II chromosomes line up at the equator.
• During Anaphase II, Centromeres divide and chromatids move to the opposite poles of the cells
• During Telophase II and cytokinesis, a nuclear envelope forms around each set of chromosomes and the cytoplasm divides and the cells are split

Phases of Meiosis I Detail

• Prophase I includes leptotene, zygotene, pachytene, diplotene, and diakinesis stages.
• Homologous chromosomes pair up during Prophase 1 due to the synaptonemal complex.
• Paired chromosomes form bivalents.
• During the pachytene stage of prophase 1, crossing over occurs between homologous chromosomes.
• Homologous chromosomes meet at the end of prophase I with the help of chiasmas

Metaphase I

• Bivalents (united homologous chromosomes) are aligned on the equatorial plate.

Anaphase I

• Homologous chromosomes separate and move to opposite poles.
• Each chromosome carries fragments of its homologue.

Telophase I

• Chromosomes arrive at the poles.
• A nuclear envelope appears.

Cytokinesis

• Cell divides.

Meiosis II

• The two newly formed daughter cells (n) undergo normal mitosis.
• Each daughter cell divides into two daughter cells (n) which are distinct from each other.

Méiose vs Mitose (Meiosis vs. Mitosis)

• Meiosis involves two consecutive cell divisions with only one phase of duplication (S phase).
• One stem cell (2n) gives rise to four daughter cells (n) in meiosis.
• Daughter cells are distinct from each other and different from the mother cell in meiosis.
• Mitosis is only one cell division.
• A stem cell gives rise to two daughter cells (2n) in mitosis.
• The meiosis starts with 1 cell division (2n) which creates four cells (n)
• The mitosis starts with 1 cell division which creates two cells (2n)

Gamétogenèse humaine (Human Gametogenesis)

• The process of forming gametes (n) from germ cells (2n) is called gametogenesis
• Gametogenesis occurs in the gonads
• Female: ovaries
• Males: testicles
• The gametes come from the gonads: ovaries in women and testicles in men.
• Germ cells (2n) divide by meiosis, giving rise to spermatozoa (spermatogenesis) or eggs (oogenesis).

L'ovogenèse (Oogenesis)

• Oogenesis begins during embryonic development
• Oogonia from primordial germ cells divide by mitosis.
• Primary oocytes undergo the first meiotic division (meiosis I).
• Meiosis I of the primary oocyte is stopped at prophase I during embryonic development.
• Cytoplasm is unevenly distributed in both meiosis I and meiosis II during cytokinesis.
• Only one functional ovum is formed from each primary oocyte, along with two or three polar bodies.
• Meiosis continues with each menstrual cycle, forming secondary oocytes.
• Secondary oocytes are directed from the ovaries to the fallopian tubes.
• Ovogenesis arrested at metaphase II
• If fertilization occurs, meiosis II is completed
• A mature egg and a polar body are obtained
• If there is no fertilization, it is eliminated with menstruation

Sperméogenèse Details

• Spermatogenesis starts at puberty and continues throughout life.
• Though it does progressively diminish with age
• The Spermatogies come from the primordial germinal cells and divide by mitosis
• Primary spermatocytes enter meiosis.
• Each primary sperm (2n) provides 4 mature sperm
• At the end of meiosis, 4 spermatides are obtained
• Spermatozoids lose their tail and only inheret the mitochondria

Différences entre L'ovogenèse and Sperméogenèse (Differences between Ovogenesis and Spermogenesis)

• During ovogenesis, a large quantity of nutritious material accumulates
• Female cells are of differnet sized, males cells are of equal sized
• Ovogenesis only produces one functional cell, whereas spermatogenesis produces four functions cells

Fécondation (Fertilization)

• Ovulation involves the release of a gamete (ovocyte secondaire) from the follicle.
• The ovocyte is expelled from the ovary into the fallopian tube.
• High LH cause local muscle contraction
• The oocyte reaches the Fallopian tubes, where it meets some other cells
• Sperm provides centrosomes
• The secondary oocyte remains arrested at metaphase II
• Fertilization generally occurs mostly in fallopian tubes
• The ovule modifies by making the coating stronger so other sperm cant get in
• For this, granules come to give their enzymatic content
• This fertilisation includes the two nucleoids giving 2n and making a sperm
• The spermatozoon also gives one centriole
• The mother gives the genetic infomation mostly