Year 10 Cells
Name: __________________________________ Form:_______________
Multiplying and dividing
How can characteristics be common to members of the same species and also appear in successive generations?
Cells have the ability to replicate the DNA they contain. To do this the bases of the double helix separate from each other,
in much the same way as a zip opens, leaving the bases exposed. Within each nucleus there are bases floating freely. These
floating bases pair with the exposed bases and also attach themselves to
the sugar and phosphate molecules of the adjacent base, causing two
identical double helices to form.
The amount of DNA has doubled inside the nucleus and the cell is now
able to divide. Cell division can occur in one of two ways: by mitosis, or
by meiosis.
Mitosis is the process that most cells use to divide. It is the process
whereby a single cell divides to form two identical daughter cells. These
daughter cells carry the same diploid number of chromosomes and
exactly the same genetic information as the parent cell. It is important for
unicellular cells like bacteria and protozoans to reproduce asexually.
Mitosis is necessary for growth and repair tissue cells.
The DNA within the nucleus has already duplicated itself when the cell
starts to divide. The chromosomes shorten and
thicken, becoming visible under a microscope.
Each chromosome can now be seen to consist
of two strands attached at only one point, the
centromere. Each chromosome has made a
copy of itself and now consists of two
identical strands or chromatids.
The process is precise. The ‘double-stranded’
chromosomes line up down the middle of the
cell and split into their two chromatids. The
chromatids move to opposite ends of the cell and when one of each chromatid reaches its
destination, the cell divides into two identical
cells, each with the same number of
chromosomes as its parent.
Meiosis is a special type of cell division
used only for the production of gametes—
sperm or egg cells. When gametes unite at the
point of fertilisation their nuclei combine. If
two human gametes each had 46 chromosomes
and were to combine at fertilisation they
would produce a single fertilised cell (or
zygote) with 92 chromosomes. This cell would
no longer be the cell of a human. Hence, it is
important for the gametes to be produced with
only a haploid number of chromosomes so that
the zygote that is produced has the correct
diploid number of chromosomes.
A = Duplication of Chromosomes – single to double strands
B = Pairing up of Homologous chromosomes + Cross over (exchange of genetic materials)
+ Separation of Homologous chromosomes
C = Splitting up of chromosomes – a total of 4 daughter cells with only half the chromosome number as their parent.
Meiosis is similar to mitosis except that there are two divisions with four haploid cells being produced. The first division
involves the separation of the pairs of chromosomes and the second division the separation of the chromatids.
Year 10 Genetics Homework - Mitosis and Meiosis
Name: ___________________________
Using the information provided to answer the following questions:
1.
What actually makes the cell division possible in living things?
(Hint: the structures and chemicals found inside the nucleus)
2.
Why does a DNA molecule have a double helix structure?
3.
What is Mitosis?
4.
What is the meaning of “Diploid number” of chromosomes? What is the number in an normal human cell?
5.
Name the structure that holds the duplicated chromosomes together.
Draw a simple diagram to show the replication of a
chromosome from one strand to double strands and
the splitting up of chromosomes. Label the diagram.
6.
Give 2 uses of Mitosis.
7.
What is the purpose of the cell division by Meiosis?
8.
What are gametes? Are they the same in both male and female animals?
9.
What is the meaning and importance of “fertilisation”? (Don’t answer like: make baby)
10. What is a zygote?
11. How many chromosomes are found in:
a) a sperm =
b) an unfertilised egg =
c) a fertilised egg =
d) a red blood cell with no nucleus =
12. Give 3 differences between mitosis and meiosis – using a table
Mitosis
Where it occurs
No of daughter cells formed
(2 or 4)
The number of chromosomes in daughter
cells (diploid = 46 or haploid = 23)
Genetic variation of daughter cells
(yes or no)
Pairing up and separation of homologous
chromosomes (yes or no)
13. What is the advantage of meiosis over mitosis as the method to reproduce?
Meiosis