Cell Reproduction
Mitosis and Meiosis
Cells

Your body is made up
of between 10 and 100
trillion cells.
(100,000,000,000,000)

One-thousandth of
them die every day
Cells

Where do all these cells come from?
Cellular Reproduction

New cells are made
from copies of old
cells

This process began
occurring before you
were born and will
continue until after
you die
Types of Cellular Reproduction

MITOSIS – results in
two identical cells;
used for the majority
of cells in the body

MEIOSIS – results in
four cells that are
different from the
“parents”; used for
sex cells only
Important Definitions





Chromosome: a single DNA double helix
together with proteins that help to organize
the DNA
Chromatid: one of the two identical strands
of DNA and protein that forms a replicated
chromosome
Gene: a unit of heredity; specifies the
amino acid sequence of proteins and hence
particular traits
Allele: one of several alternative forms of a
particular gene
Locus: the physical location of a gene on a
chromosome
Important Definitions (cont.)
Autosome: chromosome not related to sex
 Diploid: referring to a cell with pairs of homologous
chromosomes
 Haploid: referring to a cell that has only one member of
each pair of homologous chromosomes
 Homologous Pair: two homologues that are similar in
appearance and genetic information that pair during
meiosis
 Heterozygous: carrying two different alleles of a given
gene
 Homozygous: carrying two copies of the same allele of
a given gene

Importance of Chromosomes

All chromosomes come in
pairs

Humans have 23 pairs, so a
normal cell has 46
chromosomes

All cells need a full set to
function properly (except sex
cells)
Chromosome Number

Before mitosis begins, cells
have two of each chromosome
(2N)

After chromosomes duplicate,
cells have four of each
chromosome (4N)

After mitosis, cells have two of
each chromosome (2N)
Mitosis






Mitosis is the division of chromosomes in a
cell
It can take a few minutes or a few days
Usually followed by cytokinesis (cell division)
Daughter cells produced after mitosis have
the same number of chromosomes as the
original cell
Purpose of mitosis is replication of somatic
cells and asexual reproduction
Somatic cells include all cells in the body
except the egg or sperm
Mitosis
Chromosome
Genes
Total bases
Sequenced bases[54]
1
3,148
247,200,000
224,999,719
2
902
242,750,000
237,712,649
3
1,436
199,450,000
194,704,827
4
453
191,260,000
187,297,063
5
609
180,840,000
177,702,766
6
1,585
170,900,000
167,273,992
7
1,824
158,820,000
154,952,424
8
781
146,270,000
142,612,826
9
1,229
140,440,000
120,312,298
10
1,312
135,370,000
131,624,737
11
405
134,450,000
131,130,853
12
1,330
132,290,000
130,303,534
13
623
114,130,000
95,559,980
14
886
106,360,000
88,290,585
15
676
100,340,000
81,341,915
16
898
88,820,000
78,884,754
17
1,367
78,650,000
77,800,220
18
365
76,120,000
74,656,155
19
1,553
63,810,000
55,785,651
20
816
62,440,000
59,505,254
21
446
46,940,000
34,171,998
22
595
49,530,000
34,893,953
X (sex chromosome)
1,093
154,910,000
151,058,754
Y (sex chromosome)
125
57,740,000
22,429,293
Before Mitosis Begins

Chromosomes are
duplicated within the cells
(exact copies)

Held together at center

Two strands are called
“sister chromatids”

Crash Course Biology - Mitosis
Stages Of Mitosis
Interphase
 Prophase
 Metaphase
 Anaphase
 Telophase

Mitosis

Step 1: INTERPHASE
– During Interphase the cell
carries out normal activities
such as metabolism and
protein synthesis
– The DNA in each
chromosome is replicated
Mitosis

Step 2: PROPHASE
– Sister chromatids thicken
– Nuclear membrane breaks
down
– Two centrioles move apart
– Fibers (strands of protein)
form between centrioles
Mitosis

Step 2: METAPHASE
– Centrioles to opposite
ends of cell
– Fibers stretch between
– Sister chromatids
attach to fibers
– Sister chromatids line
up at center of the cell
Mitosis

Step 3: ANAPHASE
– Sister chromatids pulled
apart from partner
– Fibers pull chromatids
towards centrioles at
opposite ends of the cell
Mitosis

STEP 4: TELOPHASE
– Each side now has complete
set of chromosomes
– Fibers disappear
– Nuclear membrane re-forms
– Cell membrane pinches
inwards to form 2 cells
Mitosis in Plant Cells

2 Differences
– No centrioles
– Cell wall must re-form
between new cells
TELOPHASE
INTERPHASE
ANAPHASE
PROPHASE
METAPHASE

Bazemann Biology – mitosis and meiosis
simulation
Meiosis
Meiosis

Process that creates gametes (eggs and sperm)

Cells divide twice
Chromosome Number

Cells start with two of
each chromosome (2N)

Chromosomes double
(4N)

Pairs divide (2N)

Sister chromatids divide
(N)
Importance of Meiosis

During fertilization,
sperm (N) and egg
(N) fuse to create one
cell (2N)

New cell has complete
set of chromosomes
(half from mother,
half from father)
Diploid


Di= two sets
Cell that contains both sets of homologus
chromosomes
– Cell contains
 2 complete sets of chromosome
 2 complete sets of genes

Number of chromosomes in diploid cell
represented by 2N
Haploid
Means “one set”
 Refers to cells that contain only one set of
chromosomes
 Gametes (sex cells)
 Represented by N

Meiosis
2 distinct stages
 Meiosis I

– A diploid cell enters here

Meiosis II
– At the end of this, the diploid cell that entered
meiosis has become 4 haploid cells
Meiosis I



Before meiosis 1, each chromosome replicates
Then they divide like in mitosis
What happened in mitosis?
– PMAT

Tetrad
– STRUCTURE MADE WHEN EACH CHROMOSOME
PAIRS UP WITH ITS HOMOLOGOUS CHROMOSOME
– 4 CHROMATIDS IN A TETRAD
Independent Assortment
This is the random alignment of the
replicated chromosomes
 Chromosomes don’t line up in a particular
way, this leads to variation.

Prophase 1
Each chromosome pairs with its
homologous chromosome making a tetrad
 As they pair up in tetrads, chromosomes
exchange portions of their chromatids in
the process …. CROSSING OVER

Metaphase1

Spindle fibers attach to chromosomes
Anaphase 1

The spindles pull homologous
chromosomes apart to opposite
poles/ends
Telophase 1

Nuclear membranes form and cell
separates into two new cells
Now what do we have?
2 new daughter cells
 Each daughter cell has a set of
chromosomes and alleles different from
each other and different from the
parent diploid cell

Meiosis II
Unlike Mitosis, Neither cell goes through a
round of chromosome replication
 Each cell’s chromosome has 2 chromatids

Prophase II

Meiosis resulted in 2 haploid (N) cells,
each with ½ the number of chromosomes
in the original cell
Metaphase 2


Chromosomes line up in middle
Anaphase 2
Sister chromatids separate and move to
opposite poles
Telophase 2
Meiosis II results in 4 haploid (N)
daughter cells
 4 daughter cells contain haploid number
of chromosomes, just 2 each

Gamete Formation

Male
– Haploid gametes produced by meiosis are called
sperm

Female
– Haploid gamete produced by meiosis is called an
egg
– Cell divisions at the end of meiosis one and two
are uneven so one cell gets most of the cytoplasm
(the EGG) and the other three are called polar
bodies (don’t participate in reproduction)
Mitosis vs. Meiosis

Mitosis
– Results in the production of two genetically
identical DIPLOID cells
– Daughter cells have sets of chromosomes identical
to each other and to parent cell
– MITOSIS allows body to grow and replace other
cells
– Asexual reproduction

Meiosis
– Results in four genetically different HAPLOID cells
– MEIOSIS is how sexually reproducing organisms
make gametes