Reproductive Advantages

Acceleration in 7th Grade Life Science

Reproduction:One of the reasons cells divide is for reproduction (unicellular organisms).In fact, some cells can divide every 20 minutes (bacteria).

Asexual Reproduction:• From one parent.• “Like begets like.”• More common in single-celled organisms.

• 6 Types

Six Types of Asexual Reproduction:

1.Binary Fission: simplest form; bacteria (prokaryotes) make a copy of their DNA, then the bacteria splits into 2 new cells.

Six Types of Asexual Reproduction:

2.Budding: yeast (fungi); “pinching” off a small part of the cell with a copy of the DNA. Some animals (cnidarians) do this often.

Six Types of Asexual Reproduction:

3.Spores: fungi; like a “mini” cell or seed.; a cell wall forms around a copy of the chromosomes (DNA).

• Often the spores are formed asexually after a fertilization (sexual) event.

Six Types of Asexual Reproduction:

4.Vegetative propagation: plants strawberries; plants send out runners that take root (for example: Kudzu).

Six Types of Asexual Reproduction:

5.Fragmentation (Regeneration): starfish; growing a new individual from a fragment. Regeneration is re-growing a body part that was lost or damaged.

Six Types of Asexual Reproduction:

6.Mitosis: in somatic cells. Somatic Cells are all of the cells of the body except the reproductive cells.

Why Asexual Reproduction?:• Asexual reproduction allows for

the rapid growth of a population in a relatively stable environment, where the offspring should be equally able to survive as the parent due to their identical genetic inheritance.

Why Sexual Reproduction?:• Sexual reproduction allows for a

greater genetic diversity within the population due to:1.The shuffling of genetic

material during gamete formation (meiosis), and

2.The combining of two individual’s genetics at fertilization.

Sexual Reproduction:• From 2 parents.• Gametes: sex cells

• Sperm: male sex cell.• Egg (ovum): female sex cell.

Sexual Reproduction:• In sexual reproduction gametes

combine to form one cell which must have the same amount of genetic material as other body cells. Therefore the gametes can only contribute half of the amount of genetic material to the new zygote , so the zygote won’t have too much genetic material.

Sexual Reproduction:• Meiosis is the process by which

gametes are formed with only half of the amount of genetic material of a regular body cell. It is similar to meiosis, but involves two divisions instead of one.

• Spermatogenesis: the process of forming sperm cells by meiosis.

• Oogenesis: the process of forming an ovum (egg) by meiosis.

Sexual Reproduction:• Eventually the 4 cells will

develop into mature gametes, either 4 sperm cells or 1 egg and 3 polar bodies, each containing 23 individual chromatids (haploid,n).

4 Sperm Cells

1 Egg (Ovum) 3 Polar Bodies

Sexual Reproduction:• Genetic Recombinations: a major

source of genetic variation among organisms caused by reassortment or crossing over during meiosis. When tetrads are formed for meiosis, the pairing of homologous chromosomes is called synapsis. Occasionally the homologous chromatids twist around each other, forming X-shaped regions called chiasmata. Here the homologous (but non-sister) chromatids break and exchange genetic material. This is known as crossing over.

Sexual Reproduction:• Crossing over:

Figure 13.11

Prophase Iof meiosis

Nonsisterchromatids

Tetrad

Chiasma,site ofcrossingover

Metaphase I

Metaphase II

Daughtercells

Recombinantchromosomes

Sexual Reproduction:• In humans, the sperm and the egg will

each contribute 23 chromosomes to the offspring. They have two choices for each chromosome to pass on, therefore, not including crossing over, there are 223 (8 000 000) possible sperm cells and 223 (8 000 000) possible egg cells that can be produced. When these possibilities are combined at fertilization there are now:

223 (8 000 000) x 223 (8 000 000) = 64 000 000 000 000 possible

zygotes. That is a lot of genetic variation and that doesn’t even account for crossing over.