Download Meiosis - Lemon Bay High School

Meiosis
How Cells Divide for Sexual Reproduction
Learning Goals
• Describe the process of meiosis, including independent
assortment and crossing over. Explain how reduction
division results in the formation of haploid gametes or
spores.
• Over the next few days we will contrast the number of
chromosomes in body cells and in gametes, summarize
the events of meiosis, contrast mitosis and meiosis, and
describe how alleles from different genes can be inherited
together.
What is Meiosis?
• Meiosis is a type of cell division, known as reduction
division, that reduces the number of chromosomes
by half, creating four haploid cells.
• Each cell is genetically distinct from the parent cell
that gave rise to them.
• This process occurs in all sexually reproducing singlecelled and multicellular eukaryotes, including
animals, plants, and fungi.
Meiosis Vocabulary
• Somatic cells – cells that make up all
the internal organs, skin, bones,
blood and connective tissue, etc. in
an organism.
• Germ cell - any biological cell that
gives rise to the gametes of an
organism that reproduces sexually.
• Gametes – sex cells (sperm & egg)
• Zygote – a fertilized egg – develops
into an embryo
Meiosis Vocabulary
• Chromosome – thread-like
structure of DNA and proteins that
store genetic information
• Homologous – set of chromosomes
(one maternal chromosome and
one paternal chromosome) that
pair up with each other inside a
cell during meiosis
• Humans have 23 pairs of homologous
chromosomes (46 chromosomes all
together)
Meiosis Vocabulary
• Diploid – cell that contains both
sets of homologous chromosomes.
• Diploid number = 2N
• Haploid - cell that contains only
one set of chromosomes, therefore
only has one set of genes.
• Haploid number = N
• N = number of chromosomes in one set
• Human somatic cells – diploid (2x23 = 46 chromosomes)
• Human sex cells – haploid (23 chromosomes)
Diploid v. Haploid
Meiosis Vocabulary
• Tetrad – a set of four chromatids.
This occurs after each
chromosome duplicates itself and
pairs with its corresponding
homologous chromosome.
• Crossing over – process in which
chromosomes exchange portions
of their chromatids during meiosis.
Duplication of
each
homologous
chromosome
Tetrad – pair of
duplicated
homologous
chromosomes
Phases Meiosis
• First comes Interphase (A)
• Meiosis involves two distinct divisions:
• Meiosis I (B)& Meiosis II (C)
• At the end of Meiosis II, the original
diploid cell has divided into four
haploid cells (gametes)
Phases Meiosis
INTERPHASE
(Cell Growth & Preparation)
MEIOSIS I
(First Division)
MEIOSIS II
(Second Division)
Interphase
• Prior to Meiosis, Interphase occurs
and DNA is duplicated.
Prophase I
MEIOSIS I
• Just like MITOSIS, Meiosis I
goes through the four
PMAT phases, except they
all have a “I” after them.
• The BIG difference is that
the chromosomes have
crossed-over and created
new allele combinations.
• Homologous chromosomes pair up,
forming tetrads
• Crossing-over occurs, and new
combinations of alleles are formed
Metaphase I
• Spindles form and attach to each
tetrad.
• Paired homologous chromosomes
line up across the middle of the cell
Anaphase I
• Homologous chromosomes move
apart
Telophase I & Cytokinesis
• The nuclear envelope forms around
each cluster of chromosomes.
• Cytokinesis forms two new cells
Between Meiosis I & Meiosis II
• Meiosis I results in 2 new cells that are
genetically different from each other
AND from the original cell.
• After Meiosis I, the cell DOES NOT go
into interphase again, therefore, DNA
does not replicate again.
MEIOSIS II
• Just like Mitosis and Meiosis
I, Meiosis II goes through
the four PMAT phases,
except they all have a “II”
after them.
• Meiosis II results in FOUR
haploid daughter cells,
these are the gametes
(eggs or sperm).
Prophase II
• Chromosomes condense as
nuclear envelope breaks down
Metaphase II
• Spindles form and attach to each
chromatid
• Chromosomes line up across the
middle of the cell
Anaphase II
• Chromosomes are pulled apart
Telophase II & Cytokinesis
• Nuclear envelope forms around
chromatids as they relax
• Cytokinesis occurs, forming FOUR
haploid daughter cells
Phases Meiosis
INTERPHASE
(Cell Growth & Preparation)
MEIOSIS I
(First Division)
MEIOSIS II
(Second Division)
Mitosis vs. Meiosis
Mitosis
• One cell divides into two cells
• Diploid cell  Diploid cells
• DNA in daughter cells are
identical to parent cell
• Each daughter cell has the
same amount of chromosomes
as the parent cell
Meiosis
• One cell divides into four cells
• Diploid cell  Haploid cells
• DNA in daughter cells are not
identical to parent cell, alleles
have been “shuffled” around
• Each daughter cell has half
the amount of chromosomes
VIDEO TIME
Pull out a blank piece of paper and draw the
following:
Interphase
G1 - Growth
Interphase
S phase – DNA
Replication
2
2
1
1
2
1
2
1
Interphase
S phase – DNA
Replication
Prophase I
(part one)
2
1
2
1