Bio Unit 5

5.1: The Structure of Chromosomes


·      You have heard the term DNA but what is it and why is it important?

·      Think back to our 4 major macromolecules: carbohydrates, proteins, lipids, and nucleic acids

·      Nucleic acids are made up of nucleotides

·      DNA is also made up of nucleotides, therefore, it is a type of nucleic acid

·      DNA stands for deoxyribonucleic acid

·      DNA is made of a double strand of nucleotides in the shape of a double helix

·      genes.gifDNA is important because it codes for proteins and genetic information

·      DNA is located in the nucleus

·      DNA is made up of thousands of genes

·      Genes – a segment of DNA that codes for a protein or an RNA molecule

·      Remember, each gene is like one car of a DNA freight train



·      A gene can also be defined as a region of DNA that controls a hereditary characteristic (trait)

·      This characteristic matches a sequence used in the production of a specific protein or RNA

·      A gene carries biological information in a form that must be copied and transmitted from each cell to all its offspring



·      Before we can talk about how cells divide we need to understand what chromosomes are

·      We call cells that are about to divide the parent cell

·      The parent cell then divides in to daughter cells

·      Think of your parents having you as their child!

·      Before the parent cells can divide, the DNA must arrange itself properly

·      DNA is a long double helix strand

·      When cells get ready to divide, DNA wraps tightly around itself until it creates chromosomes

·      Chromosomes are made up of two chromatids

·      Chromatids – exact copies of DNA that make up chromosomes

·      The two chromatids are joined together at the centromere to make a chromosome

·      Centromere – the point where the identical chromatids are connected to make a chromosome

·      This is a picture of a karyotype.  It shows all the chromosomes that you have

·      There is a pair of chromosomes at every number.  This is because you have homologous chromosomes

·      Homologous chromosomes – are a pair of chromosomes having the same gene sequences each derived from one parent

·      The reason you have homologous chromosomes is because you get one chromatid from each parent

     Remember a chromatid is only one side of the chromosome 


Text Box: To make this

Text Box: Look like this



Text Box: The chromatids have to replicate themselves




Answer the following questions.

 What is DNA made up of? 

    Where is DNA located?

     what is a gene?

          What is a chromosome? 

     Draw a picture of a chromosome.  Label the sister chromatid and centromere.

     What are homologous chromosomes?

    Where do our chromatids come from?



·       Yesterday we talked about DNA, chromosomes, and genes

·       We said that DNA is made up of nucleotides and contains thousands of genes

·       Genes are segments of DNA that code for different traits

·       And chromosomes are tightly coiled pieces of DNA

·       Chromosomes are found in the nucleus of all cells

·       They are made up of two chromatids that are exact copies of each other

·       During replication, homologous chromosomes are formed

·       Earth is large and contains a lot of different things like a cell

·       Each country is unique in its own way like a chromosome

·       Each state has its own rules like a chromosome segment

·       Each city has their own way of interpreting the states’ rules like a gene

·       The people of the city make up the rules like the nucleotide base pairs



·       Your parents give you one chromatid of each chromosome when you are conceived (created)


Chromosome Number

·       Every organism has a certain number of chromosomes

·       Humans have 46 chromosomes or 23 pairs of homologous chromosomes

·       This is shown in a karyotype

·       If you look closely at a human karyotype you can see the last two chromosomes are not the same

·       This is because this chromosome codes for sex cells

·       Females have two “X” chromosomes while males have one “X” and one “Y”

·       This karyotype is that of a male because there is an “X” and a “Y” chromosome

·       Gametes are organisms’ reproductive cells

o   For males it’s sperm; females eggs

·       Gametes have only one set of chromosomes, so they are called haploid

·       Haploid – cells that contain only one set of chromosomes

o   These do NOT have a homologous chromosome

·       Somatic cells are all the other cells in your body.  So any cell that is not a gamete,
reproductive cell

·       Somatic cells all have two sets of chromosomes, so they are called diploid

·       Diploid – cells that contain two sets of chromosomes

o   These cells DO have a homologous pair

·       Two chromatids (one from each parent)

·       They replicate to make sister chromatids or homologous chromosomes

·       The cell will split and the new cells will have a copy of each chromatid



·       Humans have 23 pairs of homologous chromosomes

·       That is a total of 46 chromosomes in each somatic cell

·       There are only 23 chromosomes in each gamete


Haploid & Diploid

·       Every organism has its own number of chromosomes

o   Some organisms have the same number, but that doesn’t make
them the same organism

o   For example: potatoes, plums, and chimpanzees all have 48 chromosomes in each cell

o   Their diploid number is 48 and their haploid number is 24

·       The easiest way to remember this is with a simple formula

o   Haploid = n

o   Diploid = 2n

·       The number of chromosomes in diploid cells is always twice that of haploid cells



Haploid (n) Chromosomes

Diploid (2n) Chromosomes











Chromosomes – Key Points

·       Before a cell can divide the DNA needs to be copied exactly

·       The DNA tightly wraps itself up into chromatids

·       The chromatids replicate to make a chromosome

·       Somatic cells have a diploid number of chromosomes

·       Gametes have a haploid number of chromosomes

Answer the following questions.

In your own words how can a cell, chromosome, chromosome segment, gene, and nucleotide base pairs be compared to Earth?

What is the difference between gametes and somatic cells?

What is the difference between haploid and diploid?

Fill in the following chart.



Haploid (n) Chromosomes

Diploid (2n) Chromosomes







Rattlesnake Fern












 5.2 Cell Cycle COMPLETE

Cell Cycle

-Just as we humans have a life cycle, cells have a life cycle as well.

-What does the word cycle mean?

- Something that repeats

-The cell cycle is repeating cycle of growth and division for a cell.


Eukaryotic Cell Cycle

- There are 5 different parts of the cell cycle.

G1- First Growth

S- Synthesis

G2- Second growth



- Quick way to remember the order: Get Some Good Money, Cool!



- The first 3 stages (first growth, synthesis, second growth) are known as interphase.

- The cell spends over 90% of the time in interphase




Phase Name

What Happens

What we see






G 1

- A cell grows rapidly and carries out it’s normal functions

- G1 occupies most of a cell’s life

- Cells that are not going to divide remain in G1

- In this stage, the D.N.A. is still a long double helix.







- In the S stage, we are making another copy of DNA.

- Now each chromosome has 2 chromatids attached at the centromere







-The cell grows in order get ready for cell division, which is called mitosis.

-To get ready for mitosis, we make preparations in the nucleus and the chromosomes.

-Microtubules are formed. Microtubules provide structure and support in the cell.







-This is when the nucleus is divided into two nuclei.

-One parent cell makes 2 of the same daughter cells.

-Each daughter cell has the same number of chromosomes as the parent.







- Cytokinesis is the process when we divide the cytoplasm and the cell membrane, so we end up with 2 identical cells.





-To make sure that everything is going correctly in the cell cycle, there are checkpoints.

-They are like traffic lights:

Green=good=go                        Red = bad= stop

-This is when the cell makes sure the conditions are good enough to move on.

-There are 3 checkpoints:

G1 Checkpoint

G2 Checkpoint

Mitosis Checkpoint

-If everything is good, you move forward, if there is a problem, you stop and fix it.


G1 Checkpoint

-At the G1 checkpoint, you decide if the cell is healthy enough to proceed.

-Green light: cell is large and healthy it will go to S:Synthesis.

- Red light: cell is not large and healthy, it stays in G1 and does not make copies of the DNA


G2 Checkpoint

-In the G2 Checkpoint, you check to make sure the DNA was copied correctly in S:synthesis.

-Green light: Move onto mitosis

-Red light: There is a problem and DNA repair enzymes fix it.


Mitosis Checkpoint

-After mitosis, we check to make sure the daughter cells were made properly.

-Green light: Daughter cells undergo cytokinesis and then begin their own growth

-Red light: Do not enter cytokinesis


Why are checkpoints important?

-If one of the checkpoints do not work properly, the cell is unhealthy and will continue to grow and divide, even though it’s not supposed to.

-This will lead to lots and lots of cells dividing.

-This leads to cancer

-Cancer is the uncontrolled growth of cells.



-Someone has cancer if they have a tumor somewhere in their body.

-What is a tumor?

-It is a lump created by the uncontrollable growth of cells because the checkpoints do not work.

-The drugs and radiation we use try to kill these cells that do not have working checkpoint


5.4 Mitosis COMPLETE


We learned about chromosomes.What’s a chromatid? Centromere? How many do we have?

We learned about the cell cycle. What’s G1, S, G2, M, C? What is the point?


Cell Growth:

In the first part of the cell’s life, G1, S, G2, the cell just grows.

We call these 3 stages, interphase.

Today, we are going to focus on cell division and the process of making daughter cells.

This is mitosis and cytokinesis


Cell Division:

- In mitosis, we divide the nucleus of the cell, so each daughter cell can get the genetic material

- In cytokinesis, we divide the rest of the stuff in the cell (cytoplasm and other organelles), so each daughter cell is complete.



There are 4 stages of mitosis

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

All of this focuses on the nucleus of the cell

Every body cell (somatic cell) undergoes mitosis, to make 2 identical diploid daughter cells.

They are diploid (2n)



Setting the stage

- In G1 (first growth) the cell grows and develops

- In S (Synthesis) we made a copy of the chromatids, so we have chromosomes

- In G2 (second growth) the cell continues to grow and creates microtubules (used for structure and support.

After this happens, and we clear all the check points, mitosis can begin.


I.    Mitosis is followed by CYTOKINESIS


Phase Name

What Happens

What we see







-DNA coils up and becomes visible as chromosomes

-Nuclear envelope dissolves

-spindle fibers form. These are microtubules that help move the chromosomes

- Prophase = Preview






Text Box: MITOSISMetaphase

-chromosomes move to middle of the cell

o   Line up at the equator

-spindle fibers link chromatids to opposite poles / ends of the cell

- Metaphase = Middle







-centromere divides

-sister chromatids separate and move along spindles to opposite poles

- Anaphase= Apart








-nuclear envelope reforms around chromosomes

-chromosomes begin to unwind

-spindle fibers break down

- The daughter cells are still connected

- Telophase = Together


Mitosis Checkpoint

- At the end of mitosis, we need to make sure everything is done correctly:

Green light: Daughter cells are made correctly and have the correctly genetic information

Red light: There was an error! Do not go to cytokinesis!!

Cytokinesis (in animals)

-Cell membrane grows to enclose each cell

- Cytoplasm divides in half.

- The cell looks like in pinches in half in animal cells

- Forms 2 new identical daughter cells

-2 cells are identical to parent cell


Cytokinesis (In plants)

- What do plants have, that animals don’t?

- In cytokinesis in plants, we need to divide the 2 daughter cells by growing a cell plate, which then turns into the cell wall.


5.5: Meiosis



·       There are two types of cells in our bodies

·       Gametes – haploid cells that have one set of chromosomes

o   Examples: sperm and egg

·       Somatic cells – diploid cells have two sets of chromosomes because they have a homologous pair

o   Examples: blood cells, liver cells, hair cells

·       Mitosis – process of cell division which results in the production of two identical daughter cells from a single parent cell

·       Mitosis is how somatic cells divide and create other somatic cells

o   Start with a parent cell and separate it in to two identical daughter cells

o   Each daughter cell has the exact same DNA as the parent cell

·       But in order to make gametes, you need to cut your number of chromosomes in half

·       Meiosis is the form of cell division that halves the number of chromosomes in reproductive cells, called gametes

·       Homologous chromosomes are the same size and code for the same genes, but one comes from the mother and one from the father

·       In diploid cells, or somatic cells, you have both homologous chromosomes

·       In haploid cells, or gametes, you only have one of the homologous chromosomes

·       Meiosis is the process by which the homologous chromosomes are split up into gametes


Meiosis Cell Cycle

·       Meiosis takes place in the same part of the cell cycle as mitosis and cytokinesis

·       That means that G1, S, and G2 have already occurred

o   Remember those three stages make up interphase

·       So the DNA has already been replicated and so there are now pairs of homologous chromosomes that we’re moving



·       There are actually two separate divisions of the cell in meiosis

·       They are called

o   Meiosis I

o   Meiosis II

·       The good news is Meiosis II is just like mitosis


Meiosis I

·       Meiosis I has 4 stages:

1.     Prophase I                         Purple

2.     Metaphase I                    Monkeys

3.     Anaphase I                         Always

4.     Telophase I and cytokinesis          Talk

·       This is going to look similar to Mitosis, but there are differences that we need to look for


Meiosis I

Stage I: Prophase I

·       The DNA finishes coiling up to form chromosomes

o   The chromosomes are made of two identical chromatids

o   The chromosomes are now visible

·       The nuclear envelope breaks down

·       Homologous chromosomes pair up

·       Spindle fibers form and connect to centromeres

Stage 2: Metaphase I

·       The pairs of homologous chromosomes are moved to the equator of the cell

o   It is the spindle fibers that move them to the equator or center of the cell

·       Unlike Mitosis, the homologous pairs stay together

Stage 3: Anaphase I

·       Homologous chromosomes are pulled to opposite poles of the cell by the spindle fibers

·       The chromosomes themselves do not separate in to their chromatids

o   This is the major difference between meiosis and mitosis

Stage 4: Telophase I

·       A nuclear envelope forms around the chromosomes (still 2 chromatids)

·       Cytokinesis occurs, creating two new cells that have half the number of chromosomes as the parent cell

o   The cells are now haploid

o   Each cell has one of the homologous chromosomes from the parent cell that we started with

Meiosis I Recap

·       So started with the same types of cells as in mitosis

o   Have chromosomes made of two chromatids that are exact copies of DNA made in S phase

·       In mitosis, the chromatids that make up the chromosome would be split apart

·       In meiosis, we split apart homologous chromosomes

o   Now we have two cells that have half the amount of chromosomes as our parent cell


Meiosis II

·       Meiosis II has 4 stages:

1.     Prophase II                    Purple

2.     Metaphase II                    Monkeys

3.     Anaphase II                    Always

4.     Telophase II and cytokinesis          Talk

·       This is going to look similar to Mitosis, but there are differences that we need to look for


Meiosis II

Stage 5: Prophase II

·       Meiosis II is very similar to mitosis

·       A new spindle forms and the nuclear envelope dissolves

·       The pairs of homologous chromosomes are split up

o   There is one of the two homologous chromosomes in each cell

o   One big and one little in each

Stage 6: Metaphase II

·       The chromosomes line up along the equator of the cell

o   Where is the equator again?

·       The two chromatids of the chromosome have opposite spindles attach to it

o   Just like in mitosis

o   The cell is getting ready to split the chromatids apart

Stage 7: Anaphase II

·       The spindles pull the two chromatids apart at their centromeres

·       The two chromatids, which used to make up the same chromosomes, move towards opposite ends of the cell

o   This happens because the spindle fibers are shortening

Stage 8: Telophase II

·       A nuclear envelope forms around each set of chromosomes

·       The spindle breaks down

·       The two cells under go cytokinesis

·       FOUR haploid cells are created

Meiosis Wrap UP

Started With

Ended With

One cell

4 cells



Had Homologous chromosomes

Don’t have homologous chromosomes

Same cells that we start with fro mitosis

Have half the number of chromosomes as daughter cells made in mitosis

·       So this is how the sperm and the eggs that are necessary to make offspring (i.e. babies) are made

·       The gametes must have half the number of chromosomes of the diploid cell

o   This way when the two come together, they make a normal cell

5.6 Mitosis vs. Meiosis





Start with

1 parent cell (diploid)

1 parent cell (diploid)

End with

2 daughter cells that are DIPLOID (have homologous chromosome pairs)

4 daughter cells that are

HAPLOID (do not have homologous chromosome pairs)

Number of chromosomes at the end (for humans)

46 total

(23 pairs)

23 total

(no pairs!)

Identical to the parent?


No! only have half the number of chromosomes

Type of cells

Somatic cells (body cells)

Gametes (sex cells)

Like sperm and egg

How many cycles of division?

1 cycle- only separate the 2 chromatids.

2 cycles- first separate the homologous chromosomes. Then separate the chromatids.