4. What is the common
structure between the 4
cells ?
5. ALL cells have:
(1)Plasma membrane.
(2)Genetic material (CHROMOSOMES) that provides instructions
for making substances that the cell needs.
(3) Cytoplasm
(4)break down molecules to generate energy to be able to perform
vital functions.
6. Which structure in my body gives me my appearance and makes me different from someone
else’s appearance?
The nucleus of every cell contains threads called chromosomes. Each chromosome is made up of one very
long molecule called DNA, this DNA carries codes (genes) that instructs the cell about which kind of protein it
should make.
7. When were the chromosomes discovered??
Chromosomes were discovered in the nineteenth century. After inventing microscopes, scientists were able to
see very small things, such as the structure of cells.
Are the chromosomes visible all the time??
NO, When you look at cells using a microscope, you almost certainly did not see any chromosomes. This is
because chromosomes only become visible with a light microscope when a cell is dividing. Chromosomes
also need to
be coloured using special stains, in order to be able to see them.
11. V.V.V. Important Key definition:
Chromosome:
• Long thread like structure, found in the nuclei of cells which are made of DNA and contain genes.
• Thread like structure of DNA (one DNA molecule) , carrying genetic information in the form of genes.
DNA:
• The chemical from which genes and chromosomes are made; its structure constitutes a code that
determines the protein made in a cell.
• Molecule of nucleotides that carry a code that instructs the cell about which kind of protein it should
make.
Gene:
• A length of DNA on a chromosome that codes for a protein made by the cell.
• Part of a DNA molecule coding for one protein
• Length of DNA on a chromosome which contain a code that determines which protein a cell makes;this
in turn determines some of the characteristics of the cell and the organism.
12. Cell division
You began your life as single cell (zygote) composed of 46 chromosomes (diploid) that is formed after fertilization of
egg and sperm (haploid). The nuclei of each of those gametes (reproductive cells) contains single complete set of 23
chromosomes.
As you grow up from being a zygote till the end of your life your cells undergo cell division, each single cell divides
into 2 new cells, and each time the 2 new cells are provided with a complete perfect copy of the whole
chromosomal set of chromosomes.
13.
14. The process of copying the DNA is called replication.
Importance of replication:
It ensures that an exact copy of DNA is passed from cell to cell
during growth and from generation to generation. It occurs before
mitosis ( somatic cell division) and meiosis (reproductive cell
division).
1. Growth, increasing the number of genetically identical cells.
2. Tissue repair.
3. Cell replacement.
4. Asexual reproduction.
15. How many chromosomes are found in the human being??
46= (44+XX) or (44+XY)
The chromosomes in a cell of a man or woman are arranged in
descending order of their size.
16. Identify the gender; example males may be potentially
disguised as females in female champions.
Screen for genetically inherited sex linked diseases like red
green color blindness.
Sex determination
18. This photograph shows a plant cell that is just beginning to
divide into two cells. The cells have been stained with a
special dye that colors the chromosomes dark red.
You cannot really see any of the other structures in the cell.
19. Have u ever asked your self what is
the rule of creation of the fly or
fruit fly??
This photograph shows a chromosome from a fruit fly.
Fruit flies have especially thick chromosomes, called giant
chromosomes, which are much easier to see than most
chromosomes.
You can see that this chromosome is a long, striped thread.
20. Do all creatures have the same number of chromosomes??
No, Different species of organism have different numbers of chromosomes in the nuclei of their cells.
Human cells have 46 chromosomes.(23 pairs of chromosomes) we get 23 chromosome from each parent)
Each chromosome carries approximately 600-2000 genes, each gene reflects only 1 criteria.
Fruit fly cells have 8 chromosomes.
Mango trees have 40 chromosomes.
21. Do all cells contain nucleus with
chromosomes ??
Why ???
22. Cells that lack DNA :
• mature red blood cells and
• cornified cells (the outer tough layer) which are
found in hair, skin, and our nails.
The absence of a nucleus is an adaptation of the red blood cell for its role. It allows the red blood cell to
contain more hemoglobin and, therefore, carry more oxygen molecules. It also allows the cell to have its
distinctive bi-concave shape which aids diffusion.
Rbcs are unable to divide and they live only 120 days.
23. Scientists number the chromosomes in a cell according to how long the chromosomes are. (descending order of length).
In a human cell, the longest chromosome is chromosome 1, the next longest is chromosome 2, and so on.
We have two of each kind of chromosome.
This photograph shows all the chromosomes in a human cell. It has been made by cutting and pasting pictures of the
individual chromosomes.
They have then been lined up in size order.
They do not really line themselves up like this! Each chromosome looks like a cross shape, because – just before a cell divides
– each chromosome makes a copy of itself. The two copies stay joined together part way along their length.
24.
25.
26. Reading slide:
If you find in your book,
Chromosome 1 this means it is chromosome 1 maternal or
paternal
Chromosome 1s is both the maternal and the paternal copies
27. Why we cannot usually see the chromosomes in a
cell, even using a microscope?
1.You are examining cell that is not dividing now
2.Unstained cell
3.unnucleated cell
28. Activity
Chromosome in a nucleus model
Making models of chromosomes in a cell
1. Work in a group of two or three for this activity.
You are going to make a set of chromosomes in a cell of a chosen species of animal.
2. First, use the internet to find out how many chromosomes your species has in its cells.
3. You can use whatever materials you like to make the models of chromosomes.
For example, you could use string or wool. You will also need a large sheet of paper.
Next, draw an outline of a cell on a large sheet of paper. Don’t make it a perfect circle – draw it freehand. Draw
a large nucleus inside the cell.
4. Now make your chromosomes. Remember that there are two chromosomes of each type.
So, if your animal has 32 chromosomes, you need to make two sets of 16.
Stick the chromosomes inside the nucleus of the cell.
Write a heading on the sheet of paper, to say which species of animal your cell represents
29. Genes
Each chromosome is made up of hundreds of different genes. The genes are arranged in a
particular sequence along the chromosome.
Each gene helps to control a particular characteristic in the organism.
30. Scientists are still finding out which genes are found on each type of chromosome in humans.
On chromosome 1, for example, we know that there are about 2000 different genes.
Chromosome 15 is a much shorter chromosome, and it has about 600 different genes.
On this diagram of a short chromosome, the stripes represent different genes.
31. Do scientists know what does each gene do??
Discovering exactly what each of these genes do is not easy, but scientists are learning more all the time.
For example, we know that two genes on chromosome 15 help to determine eye colour.
Careful reading without studying
two genes on chromosome 15 help to determine eye colour; OCA2 and HERC2
OCA2 is believed to be the dominating factor for eye color determination. And HERC2 is believed to be a regulatory gene (DNA repair
regulation, pigmentation and neurological function).
32. Everyone has genes for eye colour in the same place (locus) on their chromosome 15s.
But there are different versions of these genes,
Each gene is formed of small subunits called nucleotides, the sequence of nucleotides in the same gene differ from one
person to another,
so one person could have a chromosome 15 with eye colour genes that give them blue eyes,
and another could have a chromosome 15 with eye colour genes that give them brown eyes.
That is why the two sisters in this photograph have different eye colours.
33. Chromosome 15 eye
color gene:
The sequence of blue
color
Chromosome 15 eye
color gene:
The sequence of brown
color
34. there is a range of different ways genes contribute to
characteristics and some genes do not have a direct impact on
an organisms characteristics while others are vital to an
organism’s survival.
Do all genes contribute to the organisms characteristics ???
35. DNA
Chromosomes are made of a chemical substance called DNA.
Each chromosome is one enormously long molecule of DNA. This means that genes are also made of DNA.
A DNA molecule has a shape like a twisted ladder. This shape is called a double helix.
One gene could be a length of DNA with about 2500 of these twists.
We cannot see these twists when we use microscopes to look at chromosomes, because DNA molecules
are much too small to see.
37. DNA was first discovered in the 1950s. Since then, scientists have found out a great deal about how
the DNA in genes helps to determine the characteristics of humans and other organisms.
The DNA in a cell determines what the cell does. It contains a complete set of instructions to make a
functioning cell, and a whole organism. If you continue to study science to IGCSE or O level, you will
find out much more
about this.
Questions
3 Explain the difference between a chromosome and a gene.
4 All fruit flies have a gene for wing shape, in the same place on chromosome 2, But some fruit flies
have normal wings, and some have very tiny wings – so small that they cannot fly. What can explain
this?
