DIVISION OF

GENETIC MATERIAL

Mitosis and Meiosis

Lecture Overview

Orientation to cells

What is the genetic material?

Chromosome structure

Orientation to the life cycle of a cell

Cell cycle

How do cells reproduce?

Mitosis and regulation of cell cycle

Meiosis and gamete formation

CELLS

Fig. 2-1

CHROMOSOMES

A chromosome…

…is a molecule of DNA

…can exist as chromatin (diffuse) or as condensed

chromosomes

…is a unit of inheritance

Exist in homologous pairs in diploid organisms

Diploid: 2 complete sets of chromosomes (2n)

Homologs: Pairs of like chromosomes

Exceptions to the Rules

Not all organisms (or cells) are diploid

Haploidy

Polyploidy

Alternation of generations

Not all chromosome pairs are homologous

Sex-determining chromosomes (XX vs. XY)

Chromosome Comparisons Between Species

Common Name Scientific Name Diploid Number

Fruit fly Drosophila melanogaster 8

Garden pea Pisum sativum 14

Cat Felis catus 38

Human Homo sapiens 46

Yeast Saccharomyces cerevisiae 32

Chicken Gallus gallus 78

Geometrid moth Nemoria sp. 224

Basic Chromosome Structure

Centromere

Constricted region of DNA

Point of adherence of sister chromatids

Formation point of kinetochore

Attachment point for spindle fibers

Locus (Loci)

Basic Chromosome Structure

Classified by location of centromere

Fig. 2-3

Basic Chromosome Structure

Classified by location of centromere

Fig. 2-3

Karyotype (Karyogram)

Fig. 2-4

Clicker question: What is the

functional significance of the

centromere?

• A) It’s where transcription begins

• B) It’s where spindle fibers attach during mitosis

• C) It’s where chromosomes begin condensing

• D) It’s where chromosomes attach to the

plasma membrane

CELL CYCLE

Fig. 2-5

Interphase

Mitosis

Karyokinesis

Cytokinesis

Mitosis

Result of mitosis:

Production of 2 genetically identical daughter cells

Reasons for mitosis:

Growth

Tissue repair

Replacement of old cells

Asexual reproduction of protists, fungi, plants,

some animals

Mitosis

• Parent cells must provide their daughter cells with:

– Hereditary instructions encoded in DNA

– Enough metabolic machinery to start up their own operations.

Mitosis

• http://www.johnkyrk.com/mitosis.html

• Cell division occurs by precise steps which

distribute one set of chromosomes to each

of two daughter cells

Clicker question: At which

phase of the cell cycle are

chromosomes NOT paired as

sister chromatids? • A) G1

• B) G2

• C) Prophase of mitosis

• D) Metaphase of mitosis

Clicker question: At which

phase of the cell cycle are sister

chromatids formed?

• A) G1

• B) S

• C) Prophase of mitosis

• D) Anaphase of mitosis

Stages of Mitosis

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Fig. 2-7

Prophase

Centriole migration (in animals)

Spindle fiber formation

Nuclear envelope breakdown

Chromatin condenses to chromosomes

Fig. 2-7

Interphase Prophase

Fig. 2-7

Prometaphase

Attachment of spindle fibers

to kinetochores; one fiber

to each sister chromatid

Chromosome migration

toward equatorial plane

Fig. 2-7

Figure 2-8

Prometaphase

Fig. 2-7

Metaphase

Chromosomes are

aligned at equatorial

plane (a.k.a. the

metaphase plate)

Fig. 2-7

Metaphase

Fig. 2-7

Anaphase

Disjunction: Sister chromatids separate at centromere and are pulled to opposite ends of the cell

After separation, sister chromatids are now considered separate chromosomes

Fig. 2-7

Anaphase

Fig. 2-7

Telophase

“Reverse prophase”

Two new nuclei form

Cytokinesis follows or is simultaneous

Animals: Furrowing

Plants: Cell plate formation

Fig. 2-7

Telophase (two different preparations)

Fig. 2-7

Clicker question: In which phase

of the cell cycle do

chromosomes become visible?

• A) G1

• B) S

• C) Prophase of mitosis

• D) Anaphase of mitosis

Clicker question: In which phase

of mitosis do sister chromatids

separate from each other?

• A) G1

• B) S

• C) Prophase of mitosis

• D) Anaphase of mitosis

Clicker question: In which phase

of mitosis chromosomes begin

to move toward the equatorial

plate? • A) Prophase

• B) Prometaphase

• C) Metaphase

• D) Anaphase

• E) Telophase

Clicker question: What process

occurs either simultaneously

with or following telophase?

• A) DNA synthesis

• B) Breakdown of nuclear envelope

• C) Karyokinesis

• D) Cytokinesis

Cell Cycle Regulation

Why is the regulation of cell replication so

vitally important?

Cell Cycle Regulation

Length of the cell cycle

Fig. 2-6

Fig. 2-5

Cell Cycle Regulation: 3 Checkpoints

Cyclin-cdk complexes

Cyclin-dependent kinases

Encoded by “cell division cycle” (cdc) genes

Cyclins

Proteins that bind and activate cdk proteins, which

then activate other target proteins

Made and degraded in cyclic fashion

Cyclin-cdk complexes help cells transition through

checkpoints

Mitosis Review

http://www.sumanasinc.com/webcontent/animations/content/mitosis.html

Clicker question: What is the

outcome of mitosis?

• A) Two daughter cells genetically identical to

original (parent) cell

• B) One daughter cell genetically identical to

parent cell

• C) Four daughter cells genetically similar to –

but not identical to – the parent cell

• D) Infinite numbers of daughter cells all

genetically identical to parent cell

Clicker question: What prevents

mitosis from occurring in

defective cells?

• A) cdk-cyclins

• B) S phase

• C) spindle fibers

• D) checkpoints

Clicker question: What can

happen when a cell is arrested

at a checkpoint?

• A) the cell dies

• B) the cell fixes the problem

• C) the cell starts the cell cycle over

• D) A and B only

• E) A, B and C

Meiosis

Produces genetically unique haploid (n) gametes

Reduces the genetic material by half

Two divisional phases

Allows genetic recombination

Gametes combine for sexual reproduction

Increases genetic diversity within a population

Meiosis: Summary of Key Events

Meiosis I

Synapsis: pairing of homologous

chromosomes

Crossing over: exchange of chromosomal

material between homologs

Reductional division: number of

chromosomes is cut in half

2n n

Meiosis

Meiosis II

Equational division

Number of chromosomes remains the same

n n

Clicker question: What is the

outcome of meiosis?

• A) Two daughter cells genetically identical to

original (parent) cell

• B) One daughter cell genetically identical to

parent cell

• C) Four daughter cells containing half the

genetic material of the parent cell

• D) Infinite numbers of daughter cells all

containing half the genetic material of the

parent cell

Stages of Meiosis

Prophase I

Prometaphase I

Metaphase I

Anaphase I

Telophase I / Prophase II

Prometaphase II

Metaphase II

Anaphase II

Telophase II Fig. 2-7

Stages of Prophase I

1: Leptonema (leptotene stage)

Chromatin begins to condense into chromomeres

Fig. 2-9

Stages of Prophase I

2: Zygonema (zygotene stage)

Homologs pair roughly

Form bivalents (# = n)

Fig. 2-9

Stages of Prophase I

3: Pachynema (pachytene stage)

Formation of synaptonemal

complex

Synapsis

Chromatids become apparent

Tetrads

Crossing over between non-sister

chromatids

Fig. 2-9

Stages of Prophase I

4: Diplonema (diplotene stage)

Sister chromatids begin to separate from each other

Formation of chiasmata

Attached points reveal where crossover occurred

Fig. 2-9

Stages of Prophase I

5: Diakenesis

Nuclear envelope breakdown

Spindle fibers attach to centromeres

Maximal chromosome contraction

Chiasmata move to ends of

chromosomes (terminalization)

Fig. 2-9

Metaphase I

Homologs aligned at

metaphase plate

Homologs held together

by chiasmata between

non-sister chromatids

Fig. 2-10

Anaphase I

Segregation (disjunction)

of tetrads

Dyads are formed

Point of reductional

division (2n n)

Fig. 2-10

Telophase I

Daughter cells are n

Not all species undergo

this stage – some skip

from Anaphase I to

Prophase II

Fig. 2-10

Prophase II Metaphase II

Akin to mitosis

All chromosomes align at

metaphase plate

What’s the difference

between metaphase and

metaphase II?

Fig. 2-10

Anaphase II

Dyads segregated

Monads = individual

chromosomes

Equational division

(n n)

Fig. 2-10

Telophase II & Cytokinesis

Gamete formation (n)

Compared to parent cell,

1/2 the number of

chromosomes but still 1 of

each type

Contain combinations of

maternal & paternal DNA

How many potential

gametes from one parent

cell? Are they all

genetically distinct, or are

any the same as another?

Fig. 2-10

Clicker question: During

crossing over, genetic material

is exchanged between…

• A) sister chromatids

• B) non-sister chromatids of non-homologous

chromosomes

• C) non-sister chromatids of homologous

chromosomes

Clicker question: During

metaphase I, what line up next

to each other at the metaphase

plate? • A) sister chromatids

• B) homologous pairs of chromosomes

• C) non-homologous pairs of chromosomes

Clicker question: During

anaphase II, what is it that

separate from each other?

• A) sister chromatids

• B) homologous chromosomes (with sister

chromatids attached)

• C) non-homologous chromosomes (with sister

chromatids attached)

• D) homologous chromosomes (w/ only one

copy of the chromosome

• E) non-homologous chromosomes (w/ only one

copy of the chromosome)

Mitosis / Meiosis Comparison

Fig. 2-11

Mitosis / Meiosis Comparison

GAMETE FORMATION

Spermatogenesis

Oogenesis

Spermatogenesis

Occurs in testes

Continuous process

beginning at puberty,

continuing throughout life

Fig. 2-11

Oogenesis

Occurs in ovaries

Not continuous or

equivalent in humans

Primary oocyte formation:

Meiosis initiated as embryo

Arrested in prophase I

Resumes at sexual maturity

Fig. 2-11

Occurs in ovaries

Not continuous or

equivalent in humans

Primary oocyte

Secondary oocyte formation:

Meiosis arrested at metaphase

II until fertilization

Upon fertilization, meiosis

resumes to completion, then

egg and sperm nuclei combine

Oogenesis

Fig. 2-11

Occurs in ovaries

Not continuous or

equivalent in humans

Primary oocyte

Secondary oocyte

First & second polar bodies

Equal division of genetic

material but not cytoplasm

Not used for reproduction

Oogenesis

Fig. 2-11

Clicker question: What is the

genetic content of secondary

spermatocytes?

• A) diploid

• B) haploid

• C) triploid

Clicker question: What is the

genetic content of a primary

oocyte?

• A) diploid

• B) haploid

• C) triploid

Meiosis Review

meiosis

Meiotic Division & Genetic Variation

Potential combination of chromosomes in a

gamete = 2n

n = haploid number of chromosomes

If n = 2 then 4 possibilities

If n = 23 then 8.4x106 possible combinations

Combine both parental gametes = 7x1013 possibilities

Does not include variation due to crossing over!!!