iScience Education

AS Biology For WJEC

Version 1.0. July 2012

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Contents

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By the end of this section you will be able to:

Know the stages of mitosis and meiosis Identify the stages of mitosis from drawing and microscope images. Draw cells in the different stages of mitosis. Now the structure of a chromosome Describe how meiosis creates genetic variation.

1.0 Introduction

There are two different types of cell division – mitosis and meiosis. In this chapter we will look in detail at mitosis and learn about the structure of chromosomes and the features of a cell that is undergoing mitosis. We will also cover some aspects of meiosis, but not in as much detail as mitosis.

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2.0 Chromosomes and the cell cycle Before we look at mitosis it is important to understand the structure of a chromosome and the cell cycle. Much of what we will cover later on mitosis will involve following what is happening to the chromosomes in the cell.

2.1 The structure of a chromosome The nucleus of a human somatic cell (also known as a body cell, for example, a muscle or skin cell) contains 46 chromosomes. The 46 chromosomes arrange themselves in pairs which means humans have 23 pairs of chromosomes. Each pair of chromosomes is called an homologous pair – homologous means identical. The 46 chromosomes of the human is the full set of chromosomes as is called the diploid number of chromosomes. 2n is the abbreviation for the diploid number of chromosomes.

Why are our chromosomes in homologous pairs?

We are all a product of sexual reproduction. We, therefore, obtain chromosomes from both our mother and father. Chromosomes that we inherit from are mother a called maternal chromosomes and those from our father are called paternal chromosomes. During fertilisation maternal and paternal chromosomes come together to form the homologous pairs.

The structure of a chromosome is very important to remember. You may be asked to draw one or label one in an exam. You will also need to recognise chromosomes in diagrams and microscope images of cells undergoing mitosis – this is vital in determining what stage of mitosis the cell is in. On the next page I will take you through the structure of a chromosome, in doing so I will need to introduce you to a small amount of information that will be covered in greater depth later when we look at the cell cycle.

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The cell cycle has a number of stages to it, one of these stages in called interphase. When a cell is about to undergo mitosis the DNA replicates itself during interphase so that the DNA content of the cell doubles. We, therefore, need to look at the structure of a replicated and non-replicated chromosome.

Non-replicated paternal and maternal chromosome not in an homologous pair

A non-replicated chromosome can be represented in a number of ways - I have chosen to depict it as a slightly curve elongated structure. I have coloured the maternal and paternal chromosomes differently as is the normal convention. Please note that I have drawn the chromosomes the same size to indicate that they are homologous. When the homologous chromosomes come to lie close to each other they are in homologous pairs.

The structure of a non-replicated chromosome

Maternal chromosome Paternal chromosome

Non-replicated chromosome in an homologous pair as they have come to lie close together.

Homologous pair of chromosomes

Centromere

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Structure of replicated chromosome

Centromere

Sister chromatids

Replicated maternal and paternal chromosomes not in homologous pairs

Homologous Pair of chromosomes

Replicated maternal and paternal chromosomes in homologous pairs as they have come to lie close together.

The replicated chromosome structure is the structure you will have most to do with when studying mitosis.

The replicated chromosome is made up of two sister chromatids joined at the centromere. This structure od often known as an “X” shape structure.

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2.2 The Cell Cycle The cell cycle is the sequence of events that prepare the cell for cell division and the act of the cell dividing which is called mitosis. The cell cycle can be represented as a pie chart (figure 1) or as an X Y graph (figure 2)

G1

S

G2

Mitosis

Interphase

Figure 1. The cell cycle as represented as a pie chart.

24 Hours

The cell cycle lasts approximately 24 hours but this can vary depending on the cell type. It consists of two made stages – interphase and mitosis. Interphase is the stage of the cell cycle in which the cell is preparing to undergo cell division or mitosis. Interphase is the longest stage of the cell cycle lasting approximately 23 hours. Interphase can be divided into 3 part designated G1, S and G2. You are not required to know about these stages just be aware of their existence.

Mitosis last for approximately 1 hour and be divided into 4 stages. The stages are called Prophase, metaphase, anaphase and telophase.

Please take note of the arrow in figure 1. It shows the direction of the cell cycle.

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Nucleus

Cytoplasm

Nuclear Envelope

Cell Membrane

Nucleolus

Figure 2. The cell cycle to show the changes in DNA during interphase and mitosis.

Qua

ntit

y of

DN

A (a

rbit

rary

uni

ts)

Time (Hours) 0 24 23

Figure 2 shows that interphase is the longest stage of the cell cycle (23 hours) and during this stage the DNA as doubled as shown by the quantity raising from 1 to 2. During mitosis (which lasts 1hour) the quantity of DNA is halved so returns back to 1 arbitrary unit.

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3.0 Interphase Interphase is the stage of the cell cycle that prepares the cell for undergoing mitosis. A number of events occur during interphase which you will be expected to sate in an exam. You need to be able to recognise a cell in interphase from diagrams as well as from microscope images and describe the cells appearance. You also need to sate several processes that occur during interphase. A cell in interphase looks like that depicted in Figure 3 and that under the microscope in figure 4.

Nucleus

Cytoplasm

Nuclear Envelope

Chromatin

Cell Membrane

Nucleolus

Figure 3. Diagram of a cell in interphase.

The features of a cell in interphase.

A cell in interphase has a clear nucleus with a nucleolus and chromatin threads. Under the microscope a cell in interphase has a grainy appearance in the nucleus which is the chromatin threads. There are no visible chromosomes at the stage.

Chromatin nucleolus

Processes that occur during interphase.

1. Organelles are produced. 2. Mitochondria and chloroplasts replicate. 3. Protein synthesis. 4. Centrioles replicate to form two pairs. 5. ATP produced by respiration. 5. The cell increases in size. 6. DNA replication r the DNA content doubles.

Centrioles

Figure 4. Microscope image of a cell in interphase.

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Figure 4. Microscope image of a cell in interphase. 4.0 Mitosis Now that the cell as completed interphase it can begin the process of dividing which is called mitosis. Mitosis forms two genetically identical daughter cells. The cells are genetically identical because the DNA within the nucleus of each daughter cell is identical due to the process of DNA replication that copies the DNA to produce two identical copies during the interphase stage of the cell cycle. Mitosis can be divided into 4 stages called prophase metaphase anaphase and telophase. After telophase the cell undergoes a cleavage process called cytokinesis where the formation of two daughter cells occurs. For each stage of mitosis you will need to recognise what the cell looks like from a diagram and microscope images as well as describe what events are occurring during each stage.

To accurately describe the events that occur during mitosis the cell is described as having two poles and an equator as shown in figure 5.

Pole Pole Equator

Figure 5. The poles and equator of a cell.

The equator is the middle of the cell and the poles are the two end of the cell.

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4.1 Prophase

Features of a cell in prophase

In addition to the chromosomes becoming visible the following features/events occur during prophase:

The pairs of centrioles migrate to opposite poles of the cell. Spindle fibres begin to form from the centrioles. The function of the

spindle fibres will be covered in metaphase and anaphase stages. An aster is formed which is a star like arrangement of spindle fibres

radiating from the centrioles. The nucleolus had disappeared. The nuclear envelope starts to breakdown (as indicated by the dashed line

in figure 6) and will be completely gone by the end of prophase. This is essential for metaphase where the chromosomes must be free in the cytoplasm of the cell.

A real cell undergoing mitosis has the appearance of that shown in figure 7. The chromatin is no longer spread thought the nucleus but has form distinct chromosomes

Figure 6. Diagram to show a cell in prophase of mitosis

Figure 7. Microscope image of a cell undergoing prophase of mitosis

Condensed chromatin to form visible chromosomes

Aster

Spindle

Prophase is the first stage of mitosis and is the first time that the chromosomes become visible. This occurs due to the chromatin threads shortening and condensing. The chromosomes have the characteristic “X” shape with two sister chromatids joined at the centromere. In figure 6 You will see 4 chromosomes. It is customary to use just a small number of chromosomes to aid clarity. There are two homologues pairs indicated by chromosomes of the same size, the different colours represent the maternal and paternal chromosomes.

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4.2 Metaphase

Figure 8. Diagram to show a cell in metaphase of mitosis

Figure 9. Microscope image of a cell undergoing metaphase of mitosis

Features/ events of a cell in metaphase

Metaphase is the second stage of mitosis and the chromosomes are now free in the cytoplasm. This permits the chromosomes to align along the equator of the cell in a single row with the chromosomes one on top of the other.

The spindle fibres have now completely formed and span the cell from pole to pole.

The chromosomes are attached to the spindle fibres by the centromere.

The appearance of a cell in metaphase can be seen in figure 8 and under the microscope in figure 9.

The cell in now reading to undergo anaphase.

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4.3 Anaphase

Figure 10. Diagram to show a cell in anaphase of mitosis

Figure 11. Microscope image of a cell undergoing anaphase of mitosis

Features/ events of a cell in anaphase

Anaphase is a very dramatic and impressive stage of mitosis if viewed as a time lapsed video.

The sister chromatids are pulled to opposite poles of the cell by shortening of the spindle fibres. The centromere leads the way and the chromatids have the arrow head like appearance indicating the stong pullin force of the spindle fibres.

The centromeres divide.

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4.4 Telophase and cytokinesis

Features/ events of a cell in telophase

During telophase the chromatids reach the poles of the cell and the cell beings to undergo cytokinesis by forming a cleavage furrow, figure11.

Figure 11. Diagram to show a cell in telophase of mitosis Continues on page 14

Cleavage furrow both top and bottom of the cell

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Features/ events of a cell in telophase

Cytokinesis occurs to divide the one cell into two daughter cells.

A new nuclear envelope forms around the chromatids figure 12. It is worth noting at this point the chromatids are now called chromosomes. Note that the chromosomes have the single structure of a non-replicated chromosome described to you on page 4.

The two new daughter cells will know enter interphase again (figure 13) and so the cell cycle repeats.

Note form Figure 12 that the DNA content of the two daughter cells has reduced to the normal diploid number i.e. the chromosome number has halved (see figure 2).

Figure 14 shows the appearance of two daughter cells as viewed under the microscope.

Figure 12. Diagram to show a cell in telophase of mitosis with the cell nuclei reforming around the chromosomes.

Figure 13. Diagram to the two new genetically identical daughter cells formed by cytokinesis.

Figure 14. Two new daughter cells in interphase.

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4.5 The functions or significance of mitosis. Mitosis is the type of cell division that is responsible for growth and repair. Mitosis increases the number of cells in an organism and so caused an organism to grow. Mitosis allows the replacement of cells that have been damage and causes the healing of wounds. Please remember that mitosis doesn’t just occur in animals it occurs in plants and prokaryotes (bacteria) as well. In plants and prokaryotes mitosis is the means by which they can reproduce – this is called asexual reproduction and, as previously, stated forms genetically identical cells and so is cloning.

5.0 Meiosis For your WJEC BY1 exam you only need to know some basic principles of meiosis. Primary you need to know what the function of meiosis is and that it’s the cell division that creates genetic variation. With this in mind you will then need to make comparisons between meiosis and mitosis.

5.1 Meiosis and it’s function Meiosis is the cell division that produces gametes (sperm and ovum), so is involved with sexual reproduction. The production of gametes by meiosis produces 4 daughter cells that are genetically different to the mother cell and have half the number of chromosomes. Gametes with half the number of chromosomes are called haploid and have the symbol “n”. in human gametes the haploid number would be 23 chromosomes as 23 is half of 46 which is the diploid number.

Meiosis is also called reduction division as it reduces the number of chromosomes by half. To reduce the chromosome number meiosis must has two division steps called meiosis I and meiosis II. Meiosis I and II have the same stages as mitosis i.e. prophase, metaphase, anaphase, telophase and cytokinesis. To distinguish between meiosis I and II the stages are designated I or II., e.g. prophase I and prophase II. At the end of meiosis I there are two daughter cells, these cells the divide once more to produce 4 daughter cells that are haploid and show genetic variation (figure 15).

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5.2 Meiosis creates genetic variation

Mother cell which is diploid (2n)

Two daughter cells showing genetic variation after meiosis I.

4 daughter cells (gametes) showing genetic variation after meiosis II. The gametes are haploid (n).

Meiosis I Meiosis II

Figure 15. Diagram to show the sequence of events during meiosis. The different coloured circles represents genetic variation in those cells.

Meiosis generates genetic variation in a number of ways, these are listed below:

1. Crossing over during prophase I.

2. Independent assortment of homologous chromosomes during metaphase I.

3. Mutation

4. Random fusion of haploid gametes.

Each of these genetic variation processes will be discussed on the following pages.

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Bivalent

Synapsis

Chiasmata

Crossing over of chromatids

Genetic material has been exchanged between maternal and paternal chromatids.

Crossing over during Prophase I

Refer to figure 16 while reading this passage.

During prophase one homologous chromosomes come to lie close together by a process of synapsis (step 1) to form a structure called a bivalent. The chromatids from maternal and paternal chromosomes cross over (step 2). The point at which the chromatids join is called a chiasmata. Genetic maternal is then exchange between the maternal and paternal chromosomes. (step 3). This has created genetic variation as now there is some maternal chromosome on the paternal chromosome and vice versa.

Figure 16.. Crossing over during prophase I.

STEP 1

STEP 2 STEP 3

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Independent assortment during metaphase I

Refer to figure 17 while reading this passage.

During metaphase I of meiosis chromosomes align along the equator in bivalents (that homologous pairs). The way in which the bivalents arrange themselves is a random process so bivalents could arrange like that is arrangement 1 or equally as arrangement 2. What is happening here is that the two smaller chromosomes have arrange the other way round along the equator so that the blue coloured chromosome is facing the right pole and that red chromosome is facing the left pole.

Arrangement 1 Arrangement 2

Figure 17. Independent assortment during metaphase I.

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Mutation and random fusion of gametes

During meiosis mutations can occur in the DNA so creating genetic variation. During fertilisation when haploid gametes fuse they do so randomly this adds to creating genetic variation

Important point

Frequently in exam questions you are asked the importance of gametes being haploid. The importance is that is preservers the diploid number of chromosomes. Remember that haploid means half so when two haploid gametes fuse during fertilisation this restores the diploid number. For example the human diploid number is 46 and the haploid is 23. So 23 + 23 gives you 46 chromosomes.