Background information for Sexual Reproduction Mr. Shrout

Mitosis Mitosis produces genetically identical cells and is involved in growth and asexual reproduction. This form of cell division can quickly produce new cells (organisms), but does not produce any variations. Variations are produced by the mechanisms involved in Sexual Reproduction

Chromosome Numbers All organisms have a specific number of chromosomes Homologous pairs: In organisms that reproduce sexually, those chromosomes occur in pairs, one member of each pair originating with each parent Examples: Humans have 46 chromosomes, 23 homologous pairs Fruit flies have 8 chromosomes, 4 homologous pairs

Homologous chromosomes Each pair of homologous chromosomes has genes for the same traits. Homologous Chromosomes eye color (brown?) eye color (blue?) Sister chromatids homologous chromosomes double stranded homologous chromosomes

karyotype A karyotype is a photographic inventory of an individual’s chromosomes Human female karyotype- 46 total, 23 pairs Notice the chromosomes in this picture aren’t duplicated

Human female karyotype 46 chromosomes 23 pairs XX

Human male karyotype 46 chromosomes 23 pairs XY

Cell Types Organisms have 2 main cell types: Somatic cells: cells found in normal body tissues that aren’t associated with reproduction These cells have the full number of chromosomes Germ Cells: cells that produce cells that function in sexual reproduction These cells produce gametes: sperm (or pollen) and egg Gametes don’t have the full number of chromosomes of an organism Why?

How about the rest of us? + 46 46 92 What if a complex multicellular organism (like us) wants to reproduce? joining of egg + sperm Do we make egg & sperm by mitosis? What if we did, then…. 46 + 46 92 egg sperm zygote Doesn’t work!

Some cells have different numbers of chromosomes Normal body cells are Diploid- they have the full amount of chromosomes abbreviated 2n But all gametes (sperm and egg cells) are Haploid- they have half the normal amount of genetic information abbreviated 1n One copy of each chromosome This is so that when the gametes combine the offspring has the correct amount of genetic information

How do we make sperm & eggs? Must reduce 46 chromosomes  23 Sperm and eggs must be haploid: having half the number of chromosomes of the organism 23 46 zygote 23 46 egg 23 meiosis 46 23 fertilization sperm gametes

Stages of Meiosis

Interphase the cell replicates its chromosomes. The Phases of Meiosis Interphase Interphase the cell replicates its chromosomes. After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

Prophase I The chromosomes coil up and a spindle forms. The Phases of Meiosis Prophase I Prophase I The chromosomes coil up and a spindle forms. As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

Prophase I The Phases of Meiosis The chromatids in a tetrad pair tightly In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

The Phases of Meiosis Metaphase I Metaphase I During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber. The spindle fibers pull the tetrads (homologous pairs) into the middle

Prophase I + Metaphase 1 The Phases of Meiosis Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. Genetic recombination results from crossing over during prophase I and metaphase 1 of meiosis This increases variation further

Anaphase I The Phases of Meiosis begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell. This critical step ensures that each new cell will receive only one chromosome from each homologous pair. *Random assortment*

The Phases of Meiosis Telophase I Nucleus reforms, chromosomes may or may not unwind The spindle is broken down and the cytoplasm divides to yield two new cells Cells are now HAPLOID- half the amount of genetic information

Meiosis 1 overview 1st division of meiosis 4 chromosomes diploid 2n Divide 1 Line Up in tetrads Copy DNA during interphase prophase 1 metaphase 1 4 chromosomes diploid 2n telophase 1 2 chromosomes haploid 1n

The Phases of Meiosis Meiosis II Prophase II Meiosis II The second division in meiosis is simply a mitotic division of the products of meiosis I. During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes. Nucleus dissolves

The Phases of Meiosis Metaphase II The chromosomes, still made up of sister chromatids, are pulled to the center of each cell and line up randomly at the equator.

The Phases of Meiosis Anaphase II the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles of each cell.

The Phases of Meiosis Telophase II nuclei reform, the spindles break down, and the cytoplasm divides. At the end of meiosis II, four haploid cells have been formed from one diploid cell. These haploid cells will become gametes, transmitting the genes they contain to offspring.

Meiosis 2 overview 2nd division of meiosis gametes looks like mitosis telophase 2 telophase 1 Line Up 2 2nd division of meiosis looks like mitosis metaphase 2 2 chromosomes haploid 1n gametes

The Phases of Meiosis At the end of meiosis II, four haploid cells have been formed from one diploid cell. These haploid cells will become gametes, transmitting the genes they contain to offspring.

Gamete Formation Formation of Gametes (egg and sperm- sexual reproductive cells) Meiosis in males occurs in the testes and produces 4 sperm cells. The process is called spermatogenesis. Meiosis in females occurs in the ovaries and produces 1 egg and 3 polar bodies, which disintegrate. The process is called oogenesis. Variation: Meiosis allows for crossover and genetic recombination (gene mixing), which creates the differences between parents and offspring.

Meiosis to make gametes Meiosis & mitosis Meiosis to make gametes sperm & egg Mitosis to make copies of cells growth repair development

Meiosis 1 (diploid -> haploid)

Meiosis 2 (haploid 2 cells -> haploid 4 cells)

Comparison of Mitosis & Meiosis

Key Features of Meiosis Genetic Recombination- a set of processes that produce genetic variation during sexual reproduction (producing variation is the whole point of sexual reproduction!!) Random Assortment- during anaphase 1, homologous chromosomes move randomly to opposite ends not all of the chromosomes from one parent go in one direction

Key Features cont.. Crossing Over- During prophase 1 and metaphase 1, sections from homologous chromosomes can be exchanged genes from one parent can be exchanged with genes from the other parent Fertilization- combination of gametes from the two parents leading to the formation of a zygote (fertilized egg) This combines genetic information from two individuals

Crossing Over

Random Assortment

Fertilization