Inheritance of TraitsSexual reproduction

In organisms that reproduce sexually, traits are inherited through gametes.

Gametes (sperm and eggs) are produced by meiosis.

Asexual reproduction

Genetically identical clones of the parent.

In some exceptions, genetic material can be exchanged between clones.

Example: antibiotic resistance in bacteria can be transferred via plasmid DNA.

In sexual reproduction, haploid sperm and egg unite to form a diploid zygote.

Bacteria reproduce asexually by binary fission.

Binary Fission and Budding

Asexual reproduction: binary fission.

A single parent cell dividing into two daughter cells.

Asexual reproduction: budding.

Part of the parent body is ‘pinched off’ to produce a small individual exactly like the parent.

Binary fission in the bacterium Staphylococcus

Budding in Hydra

New individual

Sexual Reproduction Asexual Reproduction

Produces genetic variation.

Usually takes longer than asexual reproduction.

Quick.

No variation, so are more vulnerable when environment changes.

Some organisms, e.g. aphids and water fleas (Daphnia), reproduce asexually most of the time, but also go through sexual reproduction as well.

Sexual vs Asexual Reproduction

Daphnia with eggs. These diploid eggs are produced mitotically and hatch into clones of the adult.

Introduction to MeiosisThe purpose of meiosis is to produce haploid sex cells (gametes).

They have one copy of each chromosome.

In humans the haploid number is 23.

Meiosis only occurs only in the ovaries and testes.

Sperm surround an egg prior to fertilization

Meiosis IOne chromosome from each homologous pair is donated to each intermediate cell.

In prophase 1, homologues pair up to form tetrads in a process called synapsis. The sister chromatids can exchange genetic information during crossing over.

Anaphase 1 separates homologous chromosomes.

Interphase DNA replication

2N

Synapsis and crossing over

2N

Prophase 1

Tetrads line up on the equator of the cell.

2N

Metaphase 1

Intermediate cell

1N

Telophase 1

Intermediate cell

Anaphase 1

Homologues separate

Meiosis II

The second division of meiosis is similar to mitosis.

There is no chromosome duplication in meiosis II.

Sister chomatids (now separate chromosomes) are pulled apart and are donated to each gamete cell.

The gametes are haploid (1N).

1N

Intermediate cell

Individual chromosomes separate

Anaphase 2

Prophase 2

1N

Telophase 2

Gamete(egg or sperm)

1N

Metaphase 2

1N

Cell Division: An Overview

Femaleembryo

2N

Maleembryo

2N

Mitosis

Somatic cell production

Embryo

2N

Egg

1N

Sperm1N

Fertilization

Zygote

2NMitosis

Somatic cell production

Adult

2N

Mitosis

Mitosis

Meiosis

Meiosis

Maleadult

2N

Femaleadult

2N

Spermatogenesis

Oogenesis

AneuploidyNondisjunction during meiosis can lead to an abnormal chromosome number.These chromosomal defectsare known as aneuploidy.

XY

XY XY

XY

XX

X X X X

X X

XOXO XXYXXY

Aneuploidy in Sex Chromosomes

Aneuploidy can result in an abnormal number of sex chromosomes.

Turner syndrome affects females.

X

Klinefelter syndrome affects males.

XXY

Turner Syndrome karyotype

Aneuploidy in Autosomes

Aneuploidy can also affect autosomes (non-sex chromosomes).

In trisomy, the nucleus of the cells have one extra chromosome (2N+1).

Three forms of trisomy survive to birth:

Down syndrome (trisosmy 21)

Edward syndrome (trisomy 18)

Patau syndrome (trisomy 13)

Down Syndrome karyotype