BIOSC 041

v Today’s lecture

§  Review questions

§  Chapters 12-13, Mitosis & Meiosis

§  Quiz, beginning of class Monday (covers material thru today)

v Today’s lab

§  Check out plates to finish Scientific Method lab from last week

§  Lab manual exercises 3-4 (Microscopes)

Review: Cells

Which of the following organelles belong to the endomembrane system?

a.  Rough ER, smooth ER, mitochondria

b.  Rough ER, smooth ER, Golgi, lysosomes

c.  Nuclueus, DNA, RNA

d.  Ribosomes, mitochondria, Golgi

Review: Cells

Which of the following organelles belong to the endomembrane system?

a.  Rough ER, smooth ER, mitochondria

b.   Rough ER, smooth ER, Golgi, lysosomes

c.  Nuclueus, DNA, RNA

d.  Ribosomes, mitochondria, Golgi

Review: Cells

Mitochondria meet the cell’s energy needs by producing

a.  Glucose

b.  Oxygen

c.  ADP

d.  ATP

Review: Cells

Mitochondria meet the cell’s energy needs by producing

a.  Glucose

b.  Oxygen

c.  ADP

d.   ATP

Review: Cells

Which of the following is true of cilia vs. flagella?

a.  Cilia are typically smaller and more numerous than flagella

b.  Flagella occur only in pairs

c.  Cilia occur only in pairs

d.  Sperm use cilia for locomotion

Review: Cells

Which of the following is true of cilia vs. flagella?

a.   Cilia are typically smaller and more numerous than flagella

b.  Flagella occur only in pairs

c.  Cilia occur only in pairs

d.  Sperm use cilia for locomotion

Review: Cells

Which of the following is true of prokaryotes vs. Eukaryotes?

a.  Prokaryotes may be multi-celled

b.  Eukaryote cells are typically larger and more complex than prokaryotic cells

c.  Prokaryotic cells contain a nucleus, while Eukaryotic cells do not

d.  Bacteria are Eukaryotic

Review: Cells

Which of the following is true of prokaryotes vs. Eukaryotes?

a.  Prokaryotes may be multi-celled

b.   Eukaryote cells are typically larger and more complex than prokaryotic cells

c.  Prokaryotic cells contain a nucleus, while Eukaryotic cells do not

d.  Bacteria are Eukaryotic

BIOSC 041 The Cell Cycle & Meiosis

Reference: Chapters 12 & 13, a little bit of Ch. 46

Outline

v  The Eukaryotic Cell Cycle

§  Phases, or Periods, of the Cell Cycle

§  Mitosis

§  Control of the Cell Cycle

v  Sexual Reproduction and the Process of Meiosis

§  Sexual vs. Asexual Reproduction

§  Chromosome Numbers

§  Meiosis and the production of gametes

§  Gametogenesis: Spermatogenesis and Oogenesis

The Cell Cycle

v  You began life as a single cell, but there are now more cells in your body than stars in the Milky Way

§  The body is a community of about 100 trillion cells

§  Of these, only about 10 trillion are human, give or take a million or two

§  The rest are all the microorganisms with which we coexist (primarily bacteria, but other small organisms too)

§  You are a walking ecosystem!

The Cell Cycle

v  Just in the past second, millions of your cells have divided in two

v  The process of cell division shown below is very similar to what is occurring inside your tissues at this very moment

The Eukaryotic Cell Cycle

v  The Cell Cycle: the “life cycle” of a cell v  Genes control every step of the cell cycle v  The cell cycle consists of 3 Major Periods, or Phases

1.   Interphase

§  The period during which the cell carries out its function in the body

2.  Cell Division

§  Mitosis: the replication of nuclear DNA and division of the nucleus

§  Cytokinesis: the division of the cytoplasm into two daughter cells

§  Following cell division, the cell returns to Interphase

3.  Death

§  All cells eventually die; in fact, the inability to die is a hallmark of cancer

The Eukaryotic Cell Cycle: Interphase

v  Most of the cell’s “life” cycle is spent in Interphase

§  Interphase consists of G1àSàG2

§  G1: Normal cell function

§  Duration varies from seconds to years, depending on cell type

§  Mature cells arrest at G1, enter “G0”, and never divide again

§  S: Synthesis phase; DNA & Centrosomes (centrioles) replicate

§  Centrosomes are organelles that control the movement of the cytoskeleton

§  G2: Preparation for Mitosis

§  Synthesis of necessary enzymes and molecular structures to control mitosis

Interphase

Mitosis &Cytokinesis

G1 G2

SInterphase

Mitosis &Cytokinesis

G1 G2

S

More about the S phase of Interphase

v  During the S phase, each chromosome replicates, but stays together as a single, replicated chromosome

v  Each half of the replicated chromosome is called a chromatid

v  After mitosis, each sister chromatid becomes a single, independent chromosome in the new (daughter) cell

Cell Division: Overview

v  Four Critical Events

1.  Interphase, G1: Cell receives a signal to divide

2.  Interphase, S: chromosomes replicate

3.  Mitosis: Nuclear Division

4.  Cytokinesis: separation of the cell into 2 daughter cells

Interphase

Mitosis &Cytokinesis

G1 G2

SInterphase

Mitosis &Cytokinesis

G1 G2

S

Mitosis has 4 stages, followed by Cytokinesis

v  Mitosis is a continuous process, divided into 4 stages 1.   Prophase

§  Chromosomes become visible; mitotic spindle forms; nuclear envelope breaks down

2.   Metaphase §  Chromosomes connect to mitotic spindle and align at center

of cell 3.   Anaphase

§  Chromatids separate at the centrosome and move to opposite poles of the cell

4.   Telophase §  Mitotic spindle disassembles; nuclear envelope reforms

v  Cytokinesis

§  Separate from mitosis; the two daughter cells physically separate & complete the process of cell division

v  Mnemonic: Please Make A Telephone Call

Early Prophase: DNA condenses into a mitotic chromosome

DNA is usually bound up with proteins called histones to form nucleosomes

Centromere

Replicated

Nucleosomes wrap around each other to

form coils

Coils pack into a mitotic chromosome

proteins

nucleosome

Mitosis (Part 1) INTERPHASE (G2) Start Mitosis: PROPHASE LATE PROPHASE

Centrosomes Chromatin (duplicated)

Early mitotic spindle

Centromere

Kinetochore

Nucleolus Nuclear envelope Chromosome, with two

sister chromatids

Mitotic spindle

Mitosis (Part 2)

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE

Spindle

Metaphase plate Nucleolus

Cleavage furrow

Nuclear envelope

Cytokinesis

v  Organelles are distributed between the two daughter cells

v  The cell membrane pinches in like a drawstring

v  Separates the two daughter cells

Figure 8.8a

Cleavage furrow

Cleavage furrow Contracting ring of microfilaments

Daughter cells (a) Animal cell cytokinesis

Following Cell Division: Return to Interphase

v  The two daughter cells go on to perform their daily function in the body, usually side by side in the tissue of which they are a part

v  Frequency of Cell Division

§  Frequency and total number vary with the type of cell and age of the organism

§  Examples

§  Cells in the bone marrow continually divide to produce about 2 million red blood cells every second

§  Nerve cells and many muscle cells stop dividing at a certain point (G0) and never divide again

Cells have a finite number of cell divisions

v  The number of cell divisions is under genetic control and is known as the Hayflick limit

§  Limit depends on cell type but ranges from 10-60 in an adult, can be much greater in an embryo or juvenile

§  The cell keeps an internal count of the number of cell divisions

§  Once the limit is reached the cell stops dividing

v  The timing of cell division and the total number of times a cell will divide result from regulation of the cell cycle

Regulation of the Cell Cycle

v  The cell cycle is regulated by a control system with a series of check points through which the cell must pass before proceeding to the next step

v  Similar to a clock (or washing machine dial)

§  Only goes in one direction – forward

§  Directs the sequential events of the cell cycle

v  At each checkpoint the cell cycle pauses until the cell receives a go-ahead signal

The Major Checkpoints of the Cell Cycle

v  The G1 Checkpoint: to divide or not?

§  If the cell receives a signal to divide, it moves through the checkpoint and enters the S phase

§  If the cell doesn’t receive the signal it either stays in G1 or moves to G0, where it will be “mature” and never divide again

The Major Checkpoints of the Cell Cycle

v  The G2 Checkpoint: the gateway to mitosis

§  The cell only carries out mitosis if

1.  Chromosomes and centrosomes have been fully replicated

2.  All mitosis components have been synthesized and are good to go

The Major Checkpoints of the Cell Cycle v  The M Checkpoint: During Mitosis

§  Right before Anaphase the cell pauses until it receives a signal that Metaphase is complete:

§  All chromosomes are connected to the mitotic spindle and aligned in the middle of the cell

Chemical signals regulate the Checkpoints

v  The signals the cell receives are chemicals §  Usually small protein molecules §  The signals bind to specific protein receptors on the cell surface

v  Signals come from both inside and outside the cell §  Signals can come from neighboring cells or even from glands far away

in another part of the body §  The cell produces internal signals as a go-ahead when key events, such

as chromosome replication, have been completed

Image Credit: Nicolle Rager, National Science Foundation

Example: Protein Growth Factors

v  Small proteins secreted by cells stimulate other, neighboring cells to divide

§  Important in building and maintaining tissues and organs

v  Cells capable of division have special protein receptors on their surfaces that bind only to these growth factors

§  When mature, a cell loses the receptors and can no longer respond to the growth factor

Loss of Cell Cycle Controls in Cancer Cells v  Cancer cells get it all wrong

§  Do not respond normally to the body’s cell-cycle control signals that limit cell division

§  The affected cell has lost the ability to respond and keeps dividing even when sent a signal not to divide

Or

§  It will respond in an inappropriate manner

§  Some cancer cells “help” each other along by secreting growth factors at the wrong time (or place)

§  Affected cells will divide uncontrollably, forming a tumor

Cell Cycle Summary

v  The Cell Cycle

§  Periods of the cell cycle

§  Process of cell division

§ Phases of Mitosis

§ Cytokinesis

v  Regulation of the Cell Cycle

§  Checkpoints

§  Cancer – a genetic disorder of the cell cycle

Reproduction v  Asexual = vegetative

§  Mitotic division produces clones

§ The goal is genetic constancy

§ Any variation is due to mutation

v  Sexual

§  Involves meiosis

§  Each parent contributes one cell (gamete)

§  Results in genetic diversity

§ Offspring are different from parents and each other

Chromosome Numbers v  Somatic Cells and Germ Line Cells

§  Contain two sets of chromosomes, one from each parent = homologous pair (2 homologs)

§  Cells with two sets are diploid (2n) § Most common ploidy in animals and many plants § Humans: 23 homologous pairs = 46 chromosomes

v  Gametes §  Germ Cells derived from Germ Line Cells §  Haploid (1n)

§ Contain only one set of chromosomes § This one set is a mix of parental chromosomes

Two non-sister chromatids in a homologous pair

Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3)

2n = 6

Two sister chromatids of one replicated chromosome

Pair of homologous chromosomes (one from each parent)

Describing Chromosomes

S phase

Meiosis: production of gametes v  Interphase

§  Chromosomes replicate v  Meiotic Division I halves the chromosome number

§  Homologous chromosomes pair up and form bivalents (4 chromatids) §  Crossing Over (aka Recombination) occurs §  Homologous pairs then separate but each chromosome remains intact

v  Meiotic Division II §  Sister chromatids separate

Meiosis: Crossing Over yields genetic diversity

v Chiasmata form in Prophase I §  These are regions on the

chromosome where the homologs physically join to exchange genetic material

chiasmata

Meiosis I

Meiosis I

Meiosis I

Meiosis II

Meiosis II

The Purpose of Meiosis

v  Products:

§  4 haploid (1n) gametes

§  Genetically distinct from one another and from either parent

v  Recombinant chromatids contain DNA from each parent

v  Independent assortment

§  Which homolog winds up in which gamete is a matter of chance

§  Homologs assort in a random manner and independent of one another

Mitosis vs. Meiosis

Spermatogenesis

Spermatogenesis

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v  Fun facts:

§  A normal human male produces about 45-200 million sperm per day, or about 2000 per second

§  Only about 25% survive for more than a few hours

§  The only part of the sperm that makes its way into the egg is the haploid nucleus

§  The male contributes nothing else; no mitochondria or other organelles

For males, quantity overrules quality

Oogenesis (Human)

Oogenesis

v Fun facts

§  The number of oocytes declines from about 7 million during prenatal development to fewer than 400,000 at birth

§  Over her lifetime, a woman can expect to produce only about 450 ova

For females, it pays to be choosy!

Meiosis Summary

v  Chromosome Counting

§  Diploid and haploid chromosome numbers

§  Gametes

v  Meiosis

§  Homologous pairs of chromosomes

§  Crossing over and independent assortment

§  Phases of Meiosis I and Meiosis II

v  Meiosis vs. Mitosis

v  Production of Gametes

§  Spermatogenesis in males

§  Oogenesis in females

Review: Mitosis

Which of the following best describes mitosis:

a.  Chromosome duplicates once, divides twice, and produces

four haploid (1n) cells b.  Chromosome duplicates twice, divides twice, and

produces four diploid (2n) cells

c.  Chromosome duplicates once, divides once, and produces two haploid (1n) cells

d.  Chromosome duplicates once, divides once, and produces two diploid (2n) cells

Review: Mitosis

Which of the following best describes mitosis:

a.  Chromosome duplicates once, divides twice, and produces

four haploid (1n) cells b.  Chromosome duplicates twice, divides twice, and

produces four diploid (2n) cells

c.  Chromosome duplicates once, divides once, and produces two haploid (1n) cells

d.   Chromosome duplicates once, divides once, and produces two diploid (2n) cells

Review: Meiosis

Which of the following best describes meiosis:

a.  Chromosome duplicates once, divides twice, and produces

four haploid (1n) cells b.  Chromosome duplicates twice, divides twice, and

produces four diploid (2n) cells

c.  Chromosome duplicates once, divides once, and produces two haploid (1n) cells

d.  Chromosome duplicates once, divides once, and produces four diploid (2n) cells

Review: Meiosis

Which of the following best describes meiosis:

a.   Chromosome duplicates once, divides twice, and

produces four haploid (1n) cells b.  Chromosome duplicates twice, divides twice, and

produces four diploid (2n) cells

c.  Chromosome duplicates once, divides once, and produces two haploid (1n) cells

d.  Chromosome duplicates once, divides once, and produces four diploid (2n) cells

Review: Mitosis & Meiosis

Which of the following is true of mitosis and meiosis:

a.  Mitosis is used for asexual reproduction, while meiosis is

used for sexual reproduction

b.  Meiosis is used for asexual reproduction, while mitosis is used for sexual reproduction

c.  Meiosis is used to repair tissue

d.  Mitosis is a form of sexual reproduction

Review: Mitosis & Meiosis

Which of the following is true of mitosis and meiosis:

a.   Mitosis is used for asexual reproduction, while meiosis

is used for sexual reproduction

b.  Meiosis is used for asexual reproduction, while mitosis is used for sexual reproduction

c.  Meiosis is used to repair tissue

d.  Mitosis is a form of sexual reproduction