B3d

Question 1

What is the difference between simple organisms which are unicellular and more complex organisms which are multicellular?

Answer 1

Unicellular - single cells/ simple/ e.g. bacteria

Multicellular - multiple cells/ specialised organ system

Question 2

What are the advantages of being multicellular?

Answer 2

Being multicellular means you can be bigger, meaning you can travel further, get your nutrients in a variety of ways and fewer things can eat you.
Being multicellular allows for cell differentiation. Instead of being just one cell that has to do everything, you can have different types of cells that do different jobs. Your cells can be specially adapted for their particular jobs.
Multicellular organisms can be more complex - they ca have specialised organs, different shapes and behaviour, and so can be adapted specifically to their particular environment.

Question 3

Why does becoming multicellular require the development of specialised organ systems?

Answer 3

It needs…

• a system to communicate between different cells.
• a system to supply cells with the nutrients they need.
• a system that controls the exchange of substances with the environment.

Question 4

What do most body cells contain?

Answer 4

Chromosomes in matching pairs.

Question 5

What are new cells for growth produced by?

Answer 5

Mitosis.

Question 6

What is mitosis needed for?

Answer 6

Replacement of worn out cells, repair to damaged tissue and asexual reproduction.

Question 7

In mammals, what are body cells?

Answer 7

Diploid, two copies of each chromosome.

Question 8

Prior to mitosis, how does DNA replication take place?

Answer 8

The DNA double helix first ‘unzips’ to form two single strands. New nucleotides then join on using complementary base pairing, making an exact copy of the DNA on the other strand. The result is two double-stranded molecules of DNA that are identical to the original molecule of DNA.

Question 9

Why must DNA replication take place before cells divide?

Answer 9

So that each new cell still has the full amount of DNA. Also to give each chromosome two arms again.

Question 10

What happens in mitosis?

Answer 10

The DNA coils into double-armed chromosomes. These arms are exact copies of each other. The chromosomes line up at the centre of the cell, and then divide as cell fibres pull them apart. The two arms of each chromosome go to opposite ends of one cell. Membranes form around each of these two different sets of chromosomes. The cytoplasm divides, and you get two new cells containing exactly the same genetic material.

Question 11

What happens in sexual reproduction?

Answer 11

Gametes join in fertilisation.

Question 12

What do gametes have?

Answer 12

Half the number of chromosomes of body cells.

Question 13

How do you produce a unique individual through sexual reproduction?

Answer 13

Half the genes come from each parent.

Question 14

Why are sperm cells produced in large numbers?

Answer 14

To increase the chance of fertilisation.

Question 15

What are gametes produced by?

Answer 15

Meiosis.

Question 16

What are gametes?

Answer 16

Haploid - they only have one copy of each chromosome.

Question 17

Why does fertilisation result in genetic variation?

Answer 17

At fertilisation male and female gametes combine to form a diploid zygote. The characteristics of the zygote are controlled by the combination of genes on its chromosomes. Since the zygote will have inherited chromosomes from two parents, it will show features of both parents, but won’t be exactly like either of them.

Question 18

How is the structure of a sperm cell adapted to its function?

Answer 18

Sperm are small and have long tails so they can swim to the egg. Sperm have lots of mitochondria to provide the energy needed to swim the distance. Sperm have an acrosome at the front of the head, which ca release the enzymes they need to digest their way through the membrane of the egg cell.

Question 19

Why, in meiosis, is the chromosome number halved and each cell genetically different?

Answer 19

In the first division the pairs split up - the chromosomes in each pair move to opposite poles of the cell. In each of the two new cells, there are no pairs at all - just one of each of the 23 different types. Each new cell ends up with a mixture of the mum and dads chromosomes, but only half the usual number of chromosomes.
The second division of meiosis - each chromosome splits in half and one arm ends up in each new cell.
You end up with four new cells - two after the first division and then each of those splits again.