Reproduction:Sexual Reproduction and Meiosis

Objectives

• Compare and contrast sexual vs. asexual reproduction.

• Summarize and describe the events of meiosis and gamete formation.

• Compare and contrast mitosis vs. meiosis

Terms you must know• Chromosomes -Kinetochore• Homologous -Synapsis• Diploid -Chiasma• Haploid -Disjunction• Meiosis -Gametogenesis• Gametes -Spermatogenesis• Crossing over -Oogenesis

Mitosis and Meiosis

• Recall that mitosis is a part of cell division which produces daughter cells that are genetically identical to the parent cell.

• All of your body cells reproduce by mitotic cell divisions (except sex cells and those that do not divide).

Body cells and sex cells• In mitosis, a human cell with 46

chromosomes divides and produces two daughter cells each containing 46 chromosomes.

• Human body cells contain 23 pairs of chromosomes and are said to be diploid.

• Diploid means 2 sets of chromosomes. If N is the number of chromosomes in one set then 2N is the number of chromosomes in 2 sets

Body cells and sex cells• 1 set of 23 chromosomes comes from the

father and the other set of 23 chromosomes comes from the mother.

• These 2 sets of chromosomes are said to be homologous meaning that each of the 23 chromosomes that came from the father has a corresponding chromosome that came from the mother.

Body cells and sex cells• In contrast to body cells, male and female

gametes (sperm and egg) only contain one set of 23 chromosomes and are said to haploid.

• Haploid: Refers to one set of chromosomes

• Haploid – N

• Diploid – 2N

• Body cells are diploid

• Sex cells are haploid

• Body cells reproduce by mitosis.

• Sex cells reproduce by meiosis.

Questions1. How many chromosomes are in a human

sperm or egg cell?

2. How many chromosomes are in a human body cell?

3. A fruit fly body cell has 8 chromosomes, how many chromosomes in a fruit fly sperm cell?

4. Is the chromosome number in a fruit fly sperm cell haploid or diploid?

Meiosis and gamete formation

• How are haploid (N) gamete cells produced from diploid (2N) cells?

• This is where meiosis comes in.

• During meiosis, the number of chromosomes per cell is cut in half through the separation of homologous chromosomes.

Meiosis and Gamete formation

• Meiosis involves two distinct stages:

• Meiosis 1: The first meiotic cell division.

• Meiosis 2: The second meiotic cell division.

• By the end of meiosis 2, the diploid cell that entered meiosis has become four haploid cells.

1 diploid cell

4 haploid cells

Meiosis 1• Interphase: Before meiosis 1 begins, the

cell grows and each chromosome is duplicated.

• There are two rounds of cell division in meiosis beginning with meiosis 1.

• Meiosis one has four phases– Prophase 1– Metaphase 1– Anaphase 1– Telophase 1

Prophase 1 • Chromosomes become visible.

• the two chromosomes combine to form tetrads --synapsis

• Tetrads contain one pairs of chromosomes (4 chromatids)

• chromosomes cross over at points called chiasmata.

• Crossing over allows the chromosomes to exchange genetic material, allowing for more different combinations of genetic material.

Prophase 1: Synapsis and Crossing Over

Crossing Over

During crossing over, homologous chromosomes line up and switchtheir genes. When they separate, they end up with new gene

combinations.

Metaphase 1 • In metaphase I the tetrads are again arranged across the center by the movements of the kinetochores (1 per chromosome rather than 1 per chromatid).

• Chromosomes are aligned with the two centromeres opposite each other, but this time sister chromatids will not be pulled apart as in mitosis.

Anaphase 1• The two homologous

chromosmes of each tetrad are pulled apart and separated from each other into separate poles –disjunction.

• Since the chromosomes from each parent can go into either pole this is another means to increase genetic diversity.

• Chromosome # is beginning to be cut in half

Telophase 1• In this phase, like in

mitosis the chromosomes are moved into opposite poles and the nuclear envelope reforms and the spindle is broken down

• Chromosome # is haploid but each chromosome is duplicated.

Kinetochore and Spindle Microtubules

Meiosis 2

• The two cells produced by meiosis 1 now enter a second meiotic division.

• Unlike meiosis 1, neither cell undergoes interphase or DNA replication before undergoing meiosis 2.

• After meiosis 2, the two haploid (N) cells produced during meiosis 1 form four haploid (N) daughter cells.

• Meiosis 2 is virtually identical to mitosis

Prophase 2 • In meiosis the cell goes directly from telophase I to prophase II without the interphase.

• The nuclear envelope is again dissolved and the spindle network is set up again.

• Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes. 

Metaphase 2 • Again (just like mitosis) spindle fibers attach to kinetochores and move chromosomes to center of cell

• Now the chromatids of each chromosome will be pulled apart during anaphase 2. 

Anaphase 2 • The kinetochores and spindle fibers pull the chromosomes apart towards the poles,

• The sister chromatids are split up and pulled toward opposite ends of the cell.

Telophase 2• Chromatids concentrate

in the poles, the nuclear envelope is reformed and the spindle again is dissolved.

• The cells divide for the last time, leaving a total of four haploid cells, which have half the chromosomes of the diploid mother cell.

• Unlike the daughter cells from mitosis, the daughter cells produced here cannot immediately cycle back to interphase

Summary of Meiosis

Gametogenesis: the name of the game

• Gametogenesis: The production of four haploid gametes from one diploid body cell in the gonads.

• Spermatogenesis: The production of 4 haploid sperm from one diploid body cell in the testes.

• Oogenesis: The production of 1 haploid egg (and three haploid polar bodies) from one diploid body cell in the ovaries.

Spermatogenesis• Spermatagonium- Diploid body cell in testes

(primordial germ cell). Becomes primary spermatocyte during prophase 1.

• Primary spermatocyte- Diploid cell formed during prophase 1

• Secondary spermatocyte- Haploid daughter cells produced after telophase 1.

• Spertatid-Daughter cells produced after telophase 2. Sperm without tails yet.

• Spermatozoa- Differentiated spermatids, have flagella for swimming.

Oogenesis• Oogonium: Diploid primordial germ cell in

ovary. Becomes primary oocyte during prophase 1

• Primary oocyte: diploid cell produced during prophase 1

• Secondary oocyte: haploid daughter cell produced after telophase 1

• Polar bodies: non functional egg cells, degenerate to provide nourishment to egg cell.

• Ootid: Immature egg• Ovum: Mature egg