How it develops and progresses.

Describe the phases of meiosis and the mechanisms of recombination of genetic material during meiosis

A few words before we start…..

• Haploid: half of a full

set (only 1 set of DNA)

• Diploid: full set (1 set from

each parent (2 sets)

Haploid – number of chromosomes in sex cells represented by n indicating one chromosome

Diploid - number of chromosomes in somatic cells represented by 2n indicating a set or double number of chromosomes

Cell Reproduction

is either sexual or asexual

Advantages?

Disadvantages?

Asexual Reproduction (one parent)

• Binary Fission-

• Vegetative Propagation

• Regeneration

Binary Fission - split in two (bacteria)

Budding- (Yeast)

Vegetative Propagation - (Plants)

Regeneration -

Asexual reproduction

• Advantages:

• Doesn ’t require a mate Takes less time
• Doesn ’t require a mate
• Takes less time

• Disadvantages:

• All offspring are the same (genetically)
• All offspring are the same (genetically)

Sexual Reproduction

• 2 “parents” or 2 sets of DNA
• Examples: humans, plants, dogs

Sexual reproduction

• Advantages

• DIVERSITY!!!!!!!!!! Offspring are genetically different from parents.
• DIVERSITY!!!!!!!!!! Offspring are genetically different from parents.
• Offspring are genetically different from parents.

• Disadvantages?

• Need a mate Takes longer
• Need a mate
• Takes longer

Meiosis

Sexual Reproduction (two parents)

Why Meiosis?

• Meiosis -

• The production of gametes
• The production of gametes

Meiosis is the production of sex cells (gametes). During meiosis, the number of chromosomes in the daughter cells are reduced from the number of chromosomes in the parent cell (this is extremely important).

Interphase

• Cell growth
• DNA replication  sister chromatids held together by centromere

Prophase I

• The chromosomes coil up and a spindle forms.
• Homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad .

Prophase I – crossing over

• Crossing over occurs between homologous chromosomes  new genetic combinations that are different from the parent.

1. CROSS - OVER

• During prophase of meiosis I, homologous pairs of chromosomes form bivalents (meaning they pair up closely)

• Chromatids twist around each other. The point where they join are called chiasmata
• Fragments of non-sister chromatids swap over and so genes are exchanged

CROSSINGOVER

• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.
• Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene.
• In crossing over, homologous portions of two nonsister chromatids trade places.
• Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome.

Crossing Over

Meiosis

Before Meiosis

Gametes produced

• Homologous chromosomes line up during meiosis
• Parts of maternal and paternal chromosomes migrate

Metaphase I

• The spindle fibers pull the tetrads into the middle , or equator of the cell.
• There is no rule to who is on left or right side  independent assortment .

Anaphase I

• Homologous chromosomes separate and move to opposite ends of the cell.

Telophase I

• The spindle is broken down, the chromosomes uncoil slightly, and the cytoplasm divides to yield two new cells.
• Each cell has half the DNA as the original cell because it has only one chromosome from each homologous pair.  Haploid

Remember mitosis?

• Meiosis II looks just like mitosis except there is no DNA replication during interphase

Prophase II

• A spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.

Metaphase II.

• Sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

Anaphase II

• The sister chromatids separate and move to opposite poles.

Telophase II

• Finally nuclei reform, the spindles breakdown, and the cytoplasm divides.
• Four haploid cells have been formed from one diploid cell

• All are genetically different from each other and from the parent cell.
• All are genetically different from each other and from the parent cell.

Why meiosis is important

• Forms gametes for sexual reproduction
• Crossing over and independent assortment  different genetic combinations.  diversity!!!

Fertilization

Mom

Dad

23

23

Formation of a new individual through the union of parent cells

The number of chromosomes are animated so you can ask students the answers before it comes up

46

New Individual

28

Sexual reproduction

Haploid gametes

(n=23)

Sperm Cell

Meiosis

Meiosis

Egg Cell

Fertilization

Diploid zygote

(2n=46)

Mitosis and Development

Multicellular

diploid adults

(2n=46)

Mitosis vs. Meiosis

Event

Mitosis

Produces 4 cells

Meiosis

Produces 2 cells

New cells identical

New cells different

New cells have the same number of chromosomes as parent

New cells have fewer chromosomes than parent

√

√

√

√

The answers are animated so you can have students answer the question before you reveal the answer.

√

√

Mitosis vs. Meiosis

Event

Mitosis

Beginning cells diploid (2n)

Meiosis

Beginning cells haploid (n)

Ending cells diploid (2n)

Ending cells haploid (n)

Produces egg and sperm

Produces body cells

√

√

√

The answers are animated so you can have students answer the question before you reveal the answer.

√

√

√

31