Asexual Reproduction

Mitosis

The Cell Cycle

With an introduction to:

&

Asexual ReproductionWhat is Asexual Reproduction?

•All living things need to reproduce themselves to continue their species; they do this sexually, asexually, or use a combination of both

•Asexual reproduction is the opposite of sexual reproduction in that it does not involve the fusion of both male and female gametes (sex cells)

•Asexual reproduction means (1) that an individual, or part of an individual can produce an offspring that has identical genetic makeup to itself, or (2) grow or regenerate new tissue that also is genetically identical

• An identical offspring is known as a clone.

•Uses a process called Mitosis to produce new cells

Asexual Reproduction

Advantages Disadvantages

•Organism doesn’t need to invest time and energy in a partner

•Organism doesn’t have to travel to propagate its species

•Less chance of harmful genetic mutation

•Less chance of anything going wrong as it is a simpler process

•Maintains desirable traits

•Organisms can reproduce faster

In comparison to sexual reproduction….

•No opportunity for genetic variation by mixing of genes

•Little genetic change within a species gene pool means little chance of species improvement and evolution

•Lack of genetic change is particularly harmful to species when environmental conditions change and the organism can’t adapt

Asexual Reproduction

Types of asexual reproduction are:

•Budding

•Fission

•Spore formation

•Fragmentation

•Vegetative reproduction

•Parthenogenesis

Asexual ReproductionBudding

•This occurs when a new organism is formed from an outgrowth from the original parent

•When the outgrowth is large enough it frees itself from the parent

•Examples include; yeast cells and some types of worms

Yeast cell in the process of budding

Budding scar

Asexual ReproductionBinary Fission•Occurs in bacteria and some

protista

•The cell duplicates its DNA and then extends its cell membrane from the centre

•The cell elongates from the centre in each direction until it is long enough to cleave

•This is a very fast method of reproduction with the population of a colony sometimes doubling every 15-20 minutes.

Asexual ReproductionBinary FissionImagine this one bacteria doubling

every 15 minutes

How long will it take to fill this rectangle?

Asexual ReproductionSpore Formation

•Spores are single cells produced by an organism and released into the environment to propagate

•Organisms that produce spores include prokaryotes, protists, fungi and plants such as ferns and mosses

A cloud of spores being released

Asexual ReproductionFragmentation

•Takes place in plants and animals

•Occurs when the organism breaks into 2 or more pieces

•A whole new organism is produced from each piece

•Examples include; sponges and some types of worms

Asexual ReproductionVegetative Reproduction•Happens only in plants

•Occurs when a new plant is produced from part of another plant

•Examples include:

•grafts – used commercially to produce desirable traits in flowers and fruit trees

•bulbs – such a daffodils, garlic

•tubers – such as potatoes; new plants can grow from the tuber

•runners – such as strawberries; new roots grow from a runner to make a new plant

Asexual ReproductionParthenogenisi

s•When an individual is produced from an unfertilized egg, therefore known as the virgin birth

•Mostly occurs in invertebrates

•Examples includes bees; the queen lays eggs that will produce females if fertilized and males, or drones, if not fertilized

Asexual Reproduction

•Before we can understand how organisms actually reproduce themselves asexually at a cellular level, we need a little background knowledge:

•(1) We need to understand the regular cycles that cells make within their life cycle – This is known as the cell cycle

•(2) We need to have a look at the actual process of nuclear division – This is called mitosis

Some background information…………..

1. The Cell Cycle•Most cells replicate themselves over and over. Each complete replication is known as the cell cycle

•The length of time the cell cycle takes depends on the type of cell, but it can vary from hours to days

•The best way to study the cell cycle is to think of it as a 24hr clock

StartFinish

•There are 2 main stages……

1. The Cell CycleInterphaseMitosis

•The cell cycle can be divided into 2 main parts Interphase and Mitosis

•Interphase can be divided into 3 main sections; G1, S phase and G2

G1

S Phase

G2

• Let’s have a closer look at what happens during the Interphase part of the cell cycle……..

1. The Cell Cycle

G1

S Phase

G2

Mitosis

•In G1 – or Gap 1 – the cell grows in size. Chromosome number is haploid, written as 1N

•In S Phases – or synthesis phase – the DNA in the nucleus replicates or duplicates itself and becomes diploid, written as 2N

•In G2 – or Gap 2 – the cell gets ready for cell division

1. The Cell CycleInterphaseMitosis

•The Mitosis section of the cell cycle can also be divided up into a few sections, this time into 4 sections ……………

1. The Cell Cycle

Prophase

Metaphase

Anaphase

Telophase Mitosis: Nuclear division

•A mnemonic is useful in remembering the order of the stages of mitosis. If you can’t think of your own try: (He) probably met Ana (at the) telethon!

Cytokinesis: Division of the cytoplasm

2. Mitosis

•Mitosis is the division of the genetic material in the nucleus of the cell

•Each parent cell gives rise, or produces, two identical daughter cells

•Mitosis has 4 major stages: prophase, metaphase, anaphase and telophase

•When the nucleus in a cell is not undergoing mitosis it is said to be in interphase

•Mitosis occurs in all the somatic cells (or body cells) and not in the gametes (or sex cells) in humans

2. Mitosis

cell membrane

nuclear membrane

nucleolus

A simplified cell

nucleus

•Mitosis occurs in the nucleus of cells

centrioles

2. MitosisInterphase

Cell membrane

Nuclear membrane

DNA

•The cell has replicated its DNA in S Phase and is now in G2 Phase preparing for Mitosis

•The DNA condenses by folding on itself even more than usual

•You can start to see the chromosomes, they look like spaghetti on a plate!

2. MitosisEarly prophase

•The 6 chromosomes in this nucleus have been colour coded in their homologous pairs so we can follow them in this display; 2xblue, 2xgreen and 2xred

•The coloured pairs are also called homologous chromosomes; one shade is from mum and one shade is from dad

one blue chromosomefrom mum

one blue chromosome from dad

2. MitosisProphase

•The chromosomes have condensed as much as they can; they now look like the letter ‘H’.

•The two vertical arms of the chromosome are called sister chromatids and they are connected by a centromere

•The centrioles duplicate themselves

centromere

sister chromatids from

mum

from dadHomologous chromosomes

2. MitosisProphase

•The chromosomes start to line up on the metaphases plate that runs down the centre of the cell

•The centrioles start to move in opposite directions

•The nuclear membrane begins to break down

metaphase plate

2. Mitosis

Late Prophase

•Centrioles continue to move to opposite ends of the cell

•Nuclear membrane breaks down even more

2. Mitosis

Late Prophase

•Centrioles arrive at opposite ends of the cell

•Nuclear membrane has completely broken down

2. Mitosis

Metaphase

•Chromosomes are lined up on the metaphase plate

•Centrioles build spindle fibres

•Centromeres join to spindle fibres

Spindle fibres

2. Mitosis

Anaphase

•Chromosomes are pulled apart and sister chromatids move to opposite ends of the cell along the spindle fibres

2. Mitosis

Anaphase

•Sister chromatids continue to pull part

•Cell membrane starts to pinch or cleave

cleavage

2. Mitosis

Telophase

•Cleavage continues and a cell plate is formed in plants cells only(this becomes the cell wall)

•New nuclear membranes form around the new nuclei

•Two daughter cells are now evident

cell plate forms in plant cells

2. Mitosis

Telophase

•Chromosomes start to de-condense

•Nuclear membranes properly formed

2. Mitosis

Interphase

•The nucleolus returns

•Chromosomes are no longer visible

•Two new daughter cells identical to their parent

2. Mitosis

•So nuclear division, or Mitosis starts from an interphase cell coming out of G2 and finishes as an interphase cell ready for G1.

•Let’s have a look again at the process of Mitosis, this time we won’t stop…………..(click as fast as you like to work the animation!)………..

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

The end