Bio 3201 UNIT 1A pt3

Viral Reproduction

Viruses reproduce via TWO methods.

a. The Lytic Cycle

b. The Lysogenic or Provirus Cycle

A. The Lytic Cycle (Ex: T4 Bacteriophage)

 A short cycle – about 30 minutes

 Produces up to 200 viruses

 Virus injects its DNA/RNA into a host cell.

 Genetic material of virus takes over machinery of cell and produces new viral cells.

 HOST CELL IS DESTROYED.

The Five (5) stages of the Lytic Cycle

1. Attachment – The virus attaches itself to the host’s cell wall.

2. Entry – The virus injects its DNA/RNA into the host cell.

3. Replication – The host’s cellular metabolism reproduces the viral DNA or RNA.

4. Assembly – New viral particles are made by the host cell’s machinery (protein synthesis).

5. Lysis or Release – The host cell’s plasma membrane and cell wall break open releasing

new viruses. The host cell dies.

B. The Provirus or Lysogenic Cycle

1. Viral DNA/RNA enters cell and becomes part of cell’s DNA. (Called a provirus when this

happens)

2. Viral DNA is reproduced with cell DNA.

3. Viral DNA can exist in this form for years without harm to host cell.

4. When active, the provirus separates from host cell DNA and undergoes Lytic cycle.

5. Host cell is NOT usually destroyed.

Advantages and Disadvantages of Reproductive Strategies

There are three potential advantages of sexual reproduction:

 Sexual reproduction offers a population a way to adapt to a changing environment. At

least some offspring, for example, may have a greater ability to resist parasites or toxins

in the environment or to take advantage of new food sources.

 Competition among siblings may be reduced if they are genetically diverse.

 Pairing of homologous chromosomes and crossing over offer opportunities to replace or

repair damaged chromosomes.

Asexual reproduction also offers certain advantages:

 Asexual reproduction often proceeds more quickly than sexual reproduction, and it does

not require the presence of a second parent organism.

 Asexual reproduction usually requires less energy than sexual reproduction.

 Many forms of asexual reproduction, such as vegetative reproduction and budding, help

to maximize the chances that individual offspring will survive. In these forms of asexual

reproduction, the daughter organism does not fully separate from the parent until it is

capable of independent survival.

For Discussion: Do organisms that alternate generations or reproductive phases have the best

of both worlds?

Activity 12.2 -Page 478 Comparing Reproductive Strategies. (Google slides)

Reproduction in the Flowering Plants

Flowering Plants

 These plants are known as the angiosperms (Covered seed plants).

 These plants undergo sexual reproduction.

 These plants have evolved to be away from water, thus no dependence on water for

reproduction.

 These flowers have evolved specialized structures for reproduction.

Reproductive Structures of a Flowering Plant

Perfect Flower: A flower possessing BOTH male and female reproductive structures. This type

of flower is capable of undergoing self – pollination (fertilization).

Imperfect Flower: A flower possessing EITHER male or female reproductive structures only.

In general, flowers have four organs: sepals, petals, stamens, and one or more pistils.

Sepals protect the flower bud and can look like small leaves or even the flower’s petals.

Petals are usually colourful structures that attract pollinating insects and provide them with a

surface on which to land.

Pistil: The female reproductive structure of a flower. Made up of the Stigma, Style and Ovary.

1. Stigma: Sticky portion on top of the style in a flower.

Function: Trap pollen from the air for reproduction.

2. Style : Long slender tube leading from the stigma to the ovary in a flower.

Function: Passage way for pollen leading to the ovary.

3. Ovary: Reproductive structure of a flower that contains eggs. Eggs are created

by meiosis in this structure. Eggs contain the haploid (n) chromosome number.

Function: Site of fertilization within a flower.

Stamen: The male reproductive structure of a flower. Made up of the Anther and Filament.

1. Anther : Reproductive structure of a flower that contains pollen. Pollen is

created by meiosis.

Function: Create pollen for fertilization.

Pollen: Small round structure located within the anther of a flower that

contains the male sex cells (sperm). Pollen contains haploid (n) the

chromosome number as the parent cell.

2. Filament : Long slender stalk that holds up the anther.

Pollination occurs when the pollen (male gametophyte) lands on the female reproductive

structure of a plant of the same species.

Creation of Egg and Pollen within Flowering Plants

NOTE: Both Eggs and pollen are created through MEIOSIS within the flowering plants.

Egg formation within a flower

Within the ovule of a flower, a diploid (2n) cell undergoes meiosis to create 4 haploid (n) cells.

Three of these cells die. The remaining haploid cell undergoes mitosis three times to create a

total of 8 haploid structures within the ovary. One of these will be the egg.

Pollen Formation within a Flower

A diploid (2n) cell within the pollen sac of the anther undergoes meiosis to produce 4 haploid

(n) cells called microspores.

Each microspore undergoes mitosis to create two haploid nuclei known as the tube nucleus

and the generative nucleus.

The outer wall of the microspore hardens and is now called a pollen grain.

Sexual Reproduction/Fertilization in the Flowering Plants

The following events happen during reproduction in a flowering plant.

 Pollen grains reach the stigma and stick to the surface of the style.

 The tube nucleus begins to “dig” a pollen tube down through the style on its way to the

ovary.

 At the same time the generative nucleus divides by mitosis to produce two haploid

sperm nuclei (sperm).

 The sperm nuclei make their way down through the pollen tube and into the ovule.

 One of the sperm fertilizes the egg and produces a diploid (2n) zygote. The other sperm

joins with the two polar nuclei to form a triploid (3n) structure known as endosperm.

The endosperm provides food for the developing seed.

 After fertilization, the ovule develops into the seed, and the ovary develops into the

fruit.