Chromosomes

http://ed.ted.com/lessons/the-twisting-tale-of-dna-judith-hauck

Chromosome Overview• A chromosome is a coiled molecule of

DNA that contains many genes• A gene is a segment of DNA that

determines an organism’s traits.

QUESTION!

Since chromosomes contain DNA, where do you think you would find chromosomes inside of the cell?

Chromosome OverviewStructure of a Chromosome:• chromatin- thin strands of DNA & protein

found in the nucleus– copied before cell division– chromatin is “thin and thread-like”

• chromatid- one of 2 exact copies of DNA that make up a chromosome

• chromosome- coiled up chromatin• centromere- where the two chromatids attach

(it holds them together)

 

Chromosomal Hierarchy

DNA Chromatin Chromatid Chromosome

Centromere

Chromosome

Draw and label in your notes:

Chromosome Overview

• chromosomes are found in your somatic cells

• somatic cells are “body cells” that make up your liver, heart, skin, hair, etc etc – ALL of your cells

EXCEPT for sperm or eggs

Chromosome Overview• humans have 23 pairs

of chromosomes in every somatic (body) cell– this is equal to 46

chromosomes total in every somatic cell

– you receive one of each pair from your mom and one of each pair from your dad

Chromosomes and Cell Cycle

Cell DivisionCell CycleMitosis

Cytokinesis

Cell Basics• There are trillions of cells in your body.• Cells are microscopic• Cells have DNA inside a structure called

the nucleus• The nucleus is “surrounded” by a structure

called the nuclear envelope• Cells are filled with a jelly like substance

called the cytoplasm

Nucleus

DNA

Cytoplasm

A Cell Nuclear Envelope

Cell Division• Cell division is the splitting of a single

cell into two daughter cells that are identical to each other – mitosis – process of nuclear division

(dividing the nucleus)– cytokinesis - process of division of the

cytoplasm

Cell DivisionCells divide because the organism needs to:

– Grow- our cells don’t get bigger in size, they get bigger in number

– Repair- needed because of worn out or injured cells (your skin cells are replaced every 28 days; your stomach every 7)

– Reproduce:• asexual – one parent; offspring

identical to parent – mitosis or binary fission

• sexual – combination of genetic material from two parents – meiosis (more on this later!)

Asexual

• asexual reproduction– binary fission

Asexual

• asexual reproduction– mitosis

Why are cells small?Cells are small to allow the surface

area to volume ratio to remain high. This allows the cell:

• To take in sufficient nutrients and oxygen to maintain life

• To remove waste• To move molecules across the cell

efficiently

When cells in an organism divide, growth and repair result.

High Surface Area to Volume Ratio?????

Surface area = 6 x height x width = 6

Volume = l x w x h = 1

Ratio = 6:1 = 6

Surface area = 6 x height x width = 150

Volume = l x w x h = 125

Ratio = 150:125 or 1.2

6 is higher than 1.2!

The Cell CycleCell Cycle- • occurs in somatic cells

– What are somatic cells?• a set of events that results in two new

daughter cells, which then start the process again.– Interphase

• G1 S G2– Prophase– Metaphase– Anaphase– Telophase– Cytokinesis

Growth and Preparation- 90% of the time spent here

Mitosis- division of the nucleus

Division of the cytoplasm

The Cell Cycle

Interphase– 90% of the time, the cell is in this phase– the cell grows – performs operations unique to the type of cell

(stomach cells make digestive enzymes, some white blood cells make antibodies, etc)

Three stages of interphase:- G1 (growth stage 1)- S (synthesis)- G2 (growth stage 2)

The Cell Cycle• GROWTH 1 STAGE – G1

– decides whether or not the cell will divide– makes its structural proteins and enzymes

to perform its functions• a pancreas cell will produce and secrete

insulin• salivary gland cells will produce and

secrete enzymes in the mouth to aid in digestion

The Cell CycleS Synthesis (DNA Replication)–each of the

chromosomes is copied

The Cell CycleGROWTH 2 PHASE – G2

– DNA replication is checked by DNA repair enzymes– cell prepares for mitosis– proteins organize themselves to form a series of

fibers called the spindle

DNA correctly replicated?All proteins built for cell division?

INTERPHASE IN AN ANIMAL CELL

INTERPHASE IN A PLANT CELLNote that the DNA is in the form of chromatin – loose and in long strands.The nucleolus is usually visible during interphase, but not during mitosis

Mitosis• Follows interphase when the cell is ready

to divide• 4 main parts

– prophase, metaphase, anaphase, telophase– P-MAT

PROPHASE

METAPHASE

ANAPHASE

TELOPHASE

MITOSIS

• PROPHASE– condensing of 2 sister chromatids

• chromatin coils up• chromosomes become visible

– centrioles replicate and begin to move to opposite sides of the cell

– nuclear envelope (the outside of the nucleus) disappears

Notice that the chromatin begins to coil up and you see “space” in the nucleus between what will soon be evident as separate chromosomes

MITOSIS

• METAPHASE–spindle fibers move the chromosomes

to the middle– this organization helps to ensure that in

the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome

Chromosomes in metaphase have their centromeres lined up in the middle and their long arms are trailing from each side. Some students think it looks like Chinese characters, others have compared it to stitches on a zombie mouth.

MITOSIS

• ANAPHASE–each chromosome is attached to a

spindle which moves it toward one pole–chromatids move apart from one

another– results in equal separation and

distribution of chromosomes

In anaphase the centromeres are in rows at each end of the cell and the arms of the chromosomes are trailing away toward the middle of the cellSome students think this looks like a scary zombie mouth opening

MITOSIS

• TELOPHASE– newly separated chromatids arrive at opposite

ends of cell– nuclear envelope reappears around the daughter

nuclei– the chromosomes uncoil and are no longer

visible– cytokinesis may also begin during this stage– this phase is opposite of prophase in the events

that happen– end with two new nuclei (one for each new cell)

In telophase the 2 new cells are preparing to enter interphaseThe chromosomes uncoilIn this picture you can see the cell wall forming between the two “wads” of chromatinThis is called a cell plate until it reaches both sides of the old cell and divides it into 2 new cells

Tissue sample showing cells in multiple phases of mitosis

CYTOKINESIS• Process in which the

cytoplasm divides and two separate cells (daughter cells) form

• In animals, it begins with the formation of a cleavage furrow

• In plants, a cell plate forms, then a new cell wall forms

Animal cell cleavagePlant cell plate

Challenge!

• What differences do you see between animal cell division and plant cell division?

(Hint: does one type of cell have something the other doesn’t?)

1.

2.

3.

Challenge!• Shape

– animal cells are round– plant cells are square

• Structures– animal cells have

structures called centrioles

– plant cells do not

• Telophase– a cleavage furrow

divides animal cells– a cell plate divides plant

cells

Can you identify the phase each cell is in?

MITOSIS—Summary PROPHASE- chromosomes visible (P for

Phat), nuclear membrane disappearing chromosomes condense/fatten and become visible)

METAPHASE- sister chromatids lined up in the middle/equator (M for middle, chromosomes lined up in the middle of cell)

ANAPHASE- sister chromatids pulled apart (A for Apart or Away because the chromatids pull apart and move away from center)

TELOPHASE- chromosomes are at ends of cell, cells prepare to separate (T for Two new nuclear envelopes are forming)

Cleavage furrow

Cell plate

Not all cells reproduce…

• some leave the cell cycle here and do not undergo cell division– red blood cells – which “kick

out” their nucleus to make room for the hemoglobin and therefore can’t divide

–brain and spinal cord cells – rarely if ever divide; called G0 (pronounced G naught)

A note about the “not”

• G0

– pronounced G “not” – there are some cells, like nerves cells, that do

not divide– those cells stay in the G0 phase

– for example, nerve cells typically do not undergo mitosis meaning your body cannot make more nerve cells• this is why if nerve damage is usually permanent

Other cells can’t stop reproducing!

Abnormal Cell Cycle: CancerWhat causes cancer?• Cancer is caused by mutations (changes) in

the DNA, including the genes that regulate the cell cycle

• Basically:– uncontrolled cell growth

• cancer cells grow and divide as long as they receive nutrients

• cancer cells crowd normal cells causing tissues and organs to stop working

• Environmental factors can increase the risk of cancer.

• Substances that are known to cause cancer are called carcinogens(kar SIH nuh junz). – Tobacco, tobacco smoke, and alcohol, are examples of

carcinogens

• Some viruses are linked to cancer– HPV (human papilloma virus) can lead to cervical cancer as

well as other forms of cancer

• Mutagens cause mutations, which can lead to cancer. – Radiation (x-rays, UV light) is a mutagen. So are some

chemicals.