The Cell Cycle & Mitosis

Ch. 12

Main goals for this lecture

1. Distinguish between chromosomes, chromatids, and chromatin

2. Correctly model and identify in pictures each stage of the cell cycle

The Main Ideas

1. Cell division results in 2 genetically identical daughter cells

2. The mitotic phase alternates with interphase in the cell cycle

The unity of life

• All living things reproduce

• Reproduction occurs at the cellular level

The many functions of cell division

• REPRODUCTION: In unicellular organisms - leads to an increase in the size of the population

• In multicellular organisms - – GROWTH: a single fertilized egg (zygote) divides to

form a new organism– REPAIR: Repairs and renews old or damaged cells

Cell Division results in identical daughter cells

• The DNA is copied and divided between the 2 new cells

• Each new cell (daughter) contains an exact copy of the original DNA

• In human cells, there is about 6 feet of DNA to copy and neatly and accurately divide between the 2 new cells!

Quick Think

What is the purpose of mitosis?

DNA• A cell’s genetic information

is called its genome– In bacteria (prokaryotic cells)

it is usually 1 long circular DNA molecule

– In plants, animals, and fungi (eukaryotic cells) is it usually several linear molecules of DNA

Chromosomes• The individual molecules of DNA are

packaged into chromosomes• Each species has a characteristic number of

chromosomes– Humans - 46– Camel 70– Alligator - 32– Chicken - 78– King crab - 208– Corn - 20

Quick Think

Is the complexity of the organisms directly

proportional to the number of chromosomes? Explain

your answer.

Chromosomes• Chromosomes

are made of chromatin– DNA + proteins

• THE DNA PART: Each chromosome is a single long piece of DNA coding for 100s or 1000s of genes

Chromosomes

• THE PROTEIN PART: the proteins help keep the shape of the chromosome and help control the activity of the genes

Quick Think

• So…what’s a chromosome?

• What is it made of?

• What is it’s purpose?

• Where is it found?

• How many do we have?

Chromosomes• When the cell is dividing the DNA is tightly

packed into X-shaped chromosomes• When the cell is not dividing, the DNA is more

loose - just chromatin strands

Quick Think

• How does the appearance of chromatin and chromosomes vary?

• Draw them…?

Getting the DNA ready for cell division

• Before a cell divides, the DNA (chromatin) condenses and folds into a neat, tightly packed chromosome

• Each chromosome is made up of 2 sister chromatids - identical copies of that particular chromosome

Chromosomes

• The 2 sister chromatids are attached with sticky proteins

• The place where they are attached is called the centromere

Quick Think

Distinguish between chromatin,

chromosomes, and chromatids?

Chromosomes• During cell division, the 2 sister

chromatids are pulled apart to opposite ends of the cell and repackaged into new nuclei

• They are now considered individual chromosomes

Mitosis

• The formation of 2 daughter nuclei

• Followed by the division of the cytoplasm and organelles called cytokinesis

Notice! Not 2 new cells. 2 new nuclei.

Mitosis and Interphase

• Mitotic (M) phase - ~10% of a cell’s life

• Interphase - ~90% of a cell’s life

Interphase• During this time

the cell:– grows– produces proteins

& organelles – copies its

chromosomes– Prepares for cell

division

Interphase• 3 phases:

– G1 phase = first gap• Cell is making proteins and

organelles

– S phase = synthesis• Cell is making proteins and

organelles• DNA IS COPIED

– G2 phase = second gap• Cell is making proteins and

organelles

Mitosis• 5 subphases:

– Prophase– Prometaphase– Metaphase– Anaphase– Telophase

What happens in each phase

• At the end of interphase:– DNA has been copied– DNA not in chromosomes yet– Nucleus intact– The centrosome (an organelle

that helps organize the mitotic spindle fibers) replicates

What happens in each phase• Prophase:

– Chromosomes now tightly coiled– Sister chromatids joined together– Nucleoli disappear– Mitotic spindle begins to form

What happens in each phase• Prometaphase:

– Nuclear membrane breaks up– Microtubles from spindle start to connect with

chromosomes– Each sister chromatid has a kinetochore protein in

the centromere where the spindle fiber attaches

• What is the primary difference between prophase and prometaphase?

What happens in each phase

• Metaphase:– The spindle fibers push

the sister chromatids until they are all lined up at the metaphase plate (the middle of the cell)

What happens in each phase

• Anaphase:– The sister chromatids are pulled apart by

the spindle fibers

What happens in each phase• Telophase:

– New daughter nuclei begin to form around each set of chromosomes

– Chromosomes start to unwind

Quick Think

Explain what happens to the chromosomes

during mitosis

What happens in each phase

• Cytokinesis:– Division of the

cytoplasm– In animal cells, the

two new cells “pinch” apart

– In plant cells, a new cell wall forms between the 2 new daughter cells

What phase is this cell in?

Mitosis Modeling

• One chromosome before the DNA has been copied

Mitosis Modeling

• 2 chromosomes AFTER the DNA has been copied

Mitosis Modeling

• 1 sister chromatid

Mitosis Modeling

• Centromere

Mitosis Modeling

• Interphase – G1

Mitosis Modeling

• Interphase – S

Mitosis Modeling

• Interphase – G2

Mitosis Modeling

• Prophase

Mitosis Modeling

• Prometaphase

Mitosis Modeling

• Metaphse

Mitosis Modeling

• Anaphase

Mitosis Modeling

• Telophase

Mitosis Modeling

• Cytokinesis

Chromosome Challenge!

• With your chromosomes….show me:

• Metaphase

Chromosome Challenge!

• With your chromosomes….show me:

• Early Anaphase

Chromosome Challenge!

• With your chromosomes….show me:

• Late Anaphase

Chromosome Challenge!

• With your chromosomes….show me:

• End of Telophase

Chromosome Challenge!

• With your chromosomes….show me:

• A cell before it has copied the DNA

Chromosome Challenge!

• With your chromosomes…show me:

• All four stages with the people in your row…in order