Cell division and Survival

Reproduction of cells is essential if an organism is going to grow, repair itself and replace cells that have died. As far as the species is concerned it is even more essential that the organism replaces itself with a younger replacement.

1. Cells of Producers (plants) and Consumers (animals) are similar in many respects. Label the organelles in the two types of cells below and answer the questions that follow.

What are the three organelles plant cells have, that animal cells do not?

i)______ii)______

iii)______

2. Fill in the gaps in the following sentences explaining where genes

are and cell division by Mitosis.

Asexual Reproduction –reproduction without sex

Because every cell in an organism contains a complete set of genetic instructions on how to make the entire organism you can grow one exactly the same from a single cell (this is called cloning). However it is easier to do it from a whole group of cells. Because you are reproducing a new organism without sex it is called asexual reproduction.

Plants and some animals (aphids) naturally reproduce asexually.

1. Label the following diagrams of plants that naturally use asexual reproduction to colonise an area of ground.

Clover Plants / Twitch grass / Potato plants

2. Man also uses the method of asexual reproduction to mass produce plants which grow up identical to their parents. They are the same plant simply growing on a different set of roots. Label the following diagrams and complete the sentences below showing how cuttings are made.

A branch of a plant that is required to be
a______
r______is chosen and a small length is cut off that contains a growing
m______, an
ax______bud and
a couple of le______. / The c______
has exactly the same
g______in every
cell’s n______as the parent plant it was taken from. It will grow up looking exactly the same because it is a
c______. / The cutting must make
ad______
r______on its stem if it is going to suck up water and minerals to survive. It is placed carefully in damp sand to allow for these roots to develop. / The c______
has a plastic bag over the whole pot to prevent the cutting losing too much water before it has grown it’s
a______
r______and is able to survive as a separate new
identical c______of the parent plant.

Advantages and Disadvantages of Asexual Reproduction

  1. Examine the drawing above and complete the following sentence.

Because all c______have exactly the same g______on their ch______

the major disadvantage of a______r is that any disease or climatic event that kills one individual will kill them all.

2. Examine the drawing above and complete the following sentence.

If the individuals do not have the same g______on their ch______some will survive to reproduce and the species will carry on in a modified way. This is the basis of Evolution – selection by survival of the fittest.

3. Natural and artificial asexual reproduction is not all bad. There are significant advantages as well.

4. Complete the following sentence to revise the other major advantages of

asexual reproduction.

If the growing season is shortened by a long winter, or if there are no pollinating

insects, if there is only a single individual left, it cant do sexual r______The

The individual can still do a______r______and the species

su______for another year and colonises the area of ground,

keeping out competing plants.

Sexual Reproduction using Meiosis

Sexual reproduction produces a new individual that is different from its parents. Every cell has two sets of chromosomes (Diploid 2n). One set came from the female parent and one set came from the male parent. The single sets have to be packaged in a single cell as a gamete (Haploid n ). Meiosis is the cell division that enables these packages (gametes) containing a single set of chromosomes to be made from cells that contain two complete sets of chromosomes.

Mendelian Genetics and Punnett Squares

All body cells are diploid (have two complete sets of chromosomes). They therefore contain two genes (two sets of instructions) for everything the organism looks like and does. All gametes are haploid because they only contain one set of chromosomes. The gametes therefore must contain only one complete set of genes each. When gametes from two different parents fuse by fertilisation the zygote again becomes diploid and has two sets of instructions (genes). A punnett square is a simple device to look at the likely outcomes from a mating of two different individuals. The genes are given single letters and there are always two letters (signifying diploid) given for each parent e.g. TT, Tt or tt.

1. Complete the table linking the scientific term with its correct definition (arrow) and write in the letter, letters or words from the diagram above that correspond with the definition

(one has been done for you).

Scientific Term / Definition / Letter, letters or words
Dominant gene / is two letter shorthand for the combination of Genes in every cell of the organism the zygote grows into. The dominant gene is always written first. / _ _
_ _
_ _
Recessive gene / is written with a capital letter and as the name suggests is the set of instructions the zygote cell uses to make something look the way it does / _
Phenotype / is when both the genes in every cells nucleus are the same. (Hint think homosexual = likes the same sex) / T T
t t
Genotype / is a fertilised egg (the first diploid cell made when two gametes fuse ). / _ _
_ _
_ _
Homozygous / is written with a small letter. This gene will never be used as a set of instructions to change its looks unless the zygote cell has no alternative. / _ _
Heterozygous / The cell produced by meiosis that is haploid (contains only one set of chromosomes) eg sperm, egg, ovule, pollen. / _
_
Gamete / is the Physical expression of the gene (what the zygote looks like when it has grown up). / ______
Zygote / is when both the genes in every cells nucleus are different. (Hint think heterosexual = likes the different sex) / _ _

Punnett Square practice

1. Complete the following sentences.

These two plants are both peas. They look Physically different because the P______of the one on the left is short and the p______of the one on the right is tall. The tall p______plant must have at least one d______gene in its g______. It would therefore be either T t or _ _. The tall plant therefore can be either Tt which is called h______or TT called h______d______. The short ______plant can not have a dominant gene in it so it must have a g______of _ _. This genotype is called ho______r______.

2. If these two plants are crossed the gametes they produce will be transferred as pollen and fertilise the ovules of the other plant. The zygote that develops from the fertilised ovule grows into a seed that would then grow into an offspring pea plant. Complete the Punnett squares below and predict both the fraction split for genotype and phenotype as was done on the

previous page.

a)

b)

Sex determination in Humans

1.Complete the Punnet Square below for sex determination in humans. and fill in the Offspring table showing fractions as usual.

We all know there is a girl out there just waiting for us (and a boy if you are a girl) and the Punnet Square predicts that half the babies born will be female XX and half XY.

The Punnet Square Offspring box therefore predicts the proportions of likely numbers of different phenotypes that will occur in a population. eg 50% male : 50% female.

2. Complete the following sentences.