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

Sister chromatidshomologouschromosomes

double strandedhomologous chromosomes

eye color(brown?)

eye color(blue?)

• 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

karyotype

Human female karyotype

46 chromosomes23 pairs

XX

46 chromosomes23 pairs

XY

Human male karyotype

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?• What if a complex multicellular organism (like

us) wants to reproduce?– joining of egg + sperm

• Do we make egg & sperm by mitosis?

46 46+ 92

egg sperm zygote

What if we did, then….

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

gametes

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

2346

egg

sperm

46

meiosis 46

fertilization

23

23

zygote

Stages of Meiosis

The Phases of Meiosis

Interphase •the cell replicates its chromosomes.

•After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

Interphase

The Phases of Meiosis

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

Prophase I

•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.

Prophase I

The Phases of Meiosis

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

Metaphase I

The Phases of Meiosis

Prophase I + Metaphase 1•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

The Phases of Meiosis

Anaphase I•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*

Anaphase I

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

Telophase I

Meiosis 1 overview• 1st division of meiosis

4 chromosomes diploid 2n 2 chromosomes

haploid 1n

Copy DNA during interphase

Line Up in tetrads

Divide 1

prophase 1

metaphase 1

telophase 1

The Phases of Meiosis

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

Prophase II

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.

Metaphase II

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.

Anaphase II

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.

Telophase II

• 2nd division of meiosis– looks like mitosis

Meiosis 2 overview

2 chromosomes haploid 1n

gametes

Line Up 2

telophase 1

metaphase 2

telophase 2

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 & 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

• 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