Year 10 Genetics HW 2016
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 by
Mitosis.
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 chromatids (double-stranded)
attached at the middle centromere.
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. NB: Red blood cell consists of no nucleus, no DNA, therefore it can’t divide.
Meiosis is a special type of cell division
used only for the production of gametes (sexcells)—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 (half the
Diploid
number = 23) so that the zygote that is
Haploid
46
produced has the correct diploid number of
23
chromosomes.
Haploid
A = Duplication of Chromosomes – single (one chromatid) to double strands (2 chromatids)
23
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 a 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? What do you call the gametes in male and the gametes in females?
9.
What is the meaning of “fertilisation”? (Don’t answer like: make baby)
10. Why is it important to reduce the chromosomes to half (from diploid 46 to haploid 23) in the formation of sperms and
eggs?
11. How many chromosomes are found in:
a) a sperm =
b) an unfertilised egg =
c) a fertilised egg (zygote) =
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