Chapter 12Cell Continuity

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Cell Continuity

All cells develop from pre-existing cells

3 steps to form a new cell

1. Produce materials it will need

2. Grows larger

3. Reproduces to form a new cell

Cell continuity is the ability of cells to divide and survive from one generation to the next. 2

Chromosomes

• Structures in Nucleus, made of DNA & Protein

• Not dividing = Chromatin (long thin threads)

• When dividing = Chromatin forms a numbers of clearly distinguishable Chromosomes

• Each species has a definite no. of Chromosomes, Humans = 46 chromosomes

• Each Chromosomes has 1000s of genes

• What’s in a Nucleus3

TERMS

• Chromatin - When the cell is not dividing, chromatin exists as a mass of very long, thin fibers that are not visible with a light microscope.

• Sister chromatids - Replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis.

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Learning Check

1. What does cell continuity mean?

2. What 3 stages are involved in cell continuity?

3. What is a chromosome?

4. When chromosomes are not dividing what are they known as?

5. How many chromosomes are found in a typical human cell?

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Haploid• A Haploid cell has one set of

chromosomes, i.e. has only one set of each type of chromosomes in the nucleus.

• Haploid is symbolised by letter ‘n’ and number of chromosomes in the cell is given as e.g. n=23

• In humans, eggs and sperm are haploid n=23

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Diploid• A Diploid has two sets of chromosomes,

i.e. It has two of each type of chromosome in the nucleus

• Chromosomes are in pairs in diploid cell, called homologous pairs.

• Diploid is symbolised as ‘2n’ and total no. of chromosomes in cell is given as e.g. 2n=46

• In humans body / somatic cells are diploid 2n=46 9

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In diploid cells, 1 chromosome from the homologous pair comes from the mother and the other from the father.

Learning Check1. What is meant by haploid?2. What is meant by diploid?3. How many chromosomes does a haploid

human cell have?4. What is the correct way to write this?5. How many chromosomes does a human

diploid cell have?6. What is the correct way of writing this?7. If a cell has a diploid number 2n=6 what is

its haploid number?11

Cell Cycle

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Cell Cycle• The cell cycle describes the cell’s

activities in the state of non-division (interphase) and division (mitosis).

• At Interphase when the cell is not dividing, there is only one cell, but after Mitosis there are two identical cells called daughter cells .

• Mitosis is a form of nuclear division in which one nucleus divides to form two nuclei, each containing identical sets of chromosomes. 13

InterphaseLongest phase in cell cycle as before mitosis can take place, the cell needs to store enough energy to drive the chemical processes during the cell division.

1.chromosomes become visible in the nucleus short, thick and duplicated(DNA )

2.Produces new organelles e.g. mitochondria, centrioles

3.Produces chemicals needed for growth.

4.Chromosomes under microscope 14

INTERPHASE

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Learning Check

1. How many stages are there in the cell cycle?

2. What part of the cell cycle does the cell spend most of its time in?

3. What happens in this stage?4. What is the stage when the cell is

actually dividing called?5. What kind of cells are produced by

mitosis?16

Stage 1 - Prophase

Each chromosome has replicated during interphase and is therefore composed of two sister chromatids containing identical genetic information together at region called the centromere

1.The nuclear membrane and nucleolus are no longer visible.

2.The Centrioles / spindle apparatus has migrate to opposite poles of the cell.

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PROPHASE

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Stage 2 - Metaphase

• Chromosomes migrate towards the cell centre.

• The spindle fibres attach themselves to the centromeres of the chromosomes, 1 from each side of the cell.

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METAPHASE

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Stage 3 - Anaphase

• The spindle fibres shorten and

• the centromere splits,

• separated sister chromatids are pulled to opposite ends of cell.

• Hence, identical set of genes pulled to each end of the cell.

• Stages of Mitosis

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ANAPHASE

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Stage 4 - Telophase

• The chromosomes reach the poles of their respective spindles.

• Nuclear envelope forms around each of the 2 sets of chromosomes

• Chromosomes elongate within each nucleus

• Cleavage furrow formation in animal cells,

• cell plate formation in plant cells.

• Mitosis is complete

• Once complete, original cell divides to form 2 cells 23

TELOPHASE

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Mitosis Sequence of Stages 1. Interphase2. Prophase 3. Metaphase 4. Anaphase 5. Telophase

I Party Monday And Tuesday

Mitosis Song

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• Mitosis Rap song

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Learning check

1. How many stages are there in mitosis?

2. What happens in each stage 1?

3. What happens in each stage 2?

4. What happens in each stage 3?

5. What happens in each stage 4?

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Parts of the chromosome involved in mitosis

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Functions of Mitosis

In Unicellular Organisms• mitosis allows the organisms to multiply

e.g. AmoebaIn Multicellular Organisms• mitosis is responsible for growth,

renewal and repair e.g. production of new cells.

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Learning Check

1. What is the function of mitosis in Multicellular organisms?

2. What is the function of mitosis in unicellular organisms?

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Everyone together

• The Mitosis Song 'I Will Divide'

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Cancer

• Rate of cell division (mitosis) is carefully controlled.

• Sometimes a cell or group of cells lose the ability to control the rate of cell division.

• They form a mass of cells called a tumour which can be benign or malignant.

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Benign tumours

• Benign means kind, they are not life threatening and do not invade other tissues. E.g. warts, skin tags.

• Cells stop dividing after some time• Cancer is a group of disorders in which

certain cells lose their ability to control both the rate of mitosis and the number of times mitosis takes place.

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Malignant tumours

• Uncontrolled multiplication of abnormal cells

• Malignant tumours (cancers) invade other cells and move around the body

• Movement of these cells called Metastasis.

• Cancer cells divide indefinitely.

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Causes of Cancer• Caused when normal genes are altered to

form cancer-causing genes called oncogenes

• Brought about by cancer causing agents called carcinogens e.g. cigarette smoke, asbestos fibres, ultraviolet radiation and some viruses

• Most cancers can be cured with Radiation (burn out cancer), Chemotherapy (Chemicals slow down mitosis) and surgery.

• Cancer

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Learning Check

1. How do tumours form?

2. What is the difference between benign and malignant tumours?

3. When cancer cells spread it is called m......................?

4. Can you name 3 possible causes of cancer?

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Meiosis• Is a form of nuclear division in which the

daughter nuclei contain half the chromosome number of the parent nucleus.

• Human cells have 46 chromosomes

• Meiosis occurs in the ovaries and testes to produce gametes called eggs and sperm so there are 23 chromosomes in each egg and sperm

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THE STAGES OF MEIOSIS• Involves 1 replication and 2 cell divisions,

producing four haploid (n) cells:Meiosis 1

Prophase 1: - nuclear membrane dissolves– centrioles move to poles – spindle fibres forming – DNA has been replicated and forms chromosomes

made of 2 identical chromatids – Homologous chromosomes line up– Where chromatids overlap it allows for CROSSING

OVER of genetic information between chromosomes

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• Metaphase 1: - homologous chromosomes (4 chromatids) line up on the equator

• Anaphase 1: - homologous chromosomes separate and each chromosome (2 chromatids) move towards opposite poles (SEGREGATION)

• Telophase 1: - cytoplasm divides to form 2 new cells with 46 chromosomes each– the nuclei may not form – NOTE: the cells are not identical

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Meiosis 2

• Interphase 2: no doubling of DNA, it is just the time between meiosis I/II

• Prophase 2: - spindle fibres forming• Metaphase 2: - chromosomes (2 chromatids)

line up on the equator• Anaphase 2: - chromatids separate, each one

moves to opposite pole• Telophase 2: - nuclear membrane forming

– 4 daughter cells are produced, haploid, (n) – 4 sperm but 1 egg (3 polar bodies)

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Functions of Meiosis

In Multicellular Organisms:

1.Allows sexual reproduction without increasing the number of chromosomes in the offspring

2.Allows new combinations of genes

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Compare Mitosis & MeiosisMitosis Meiosis

No. Of Chromosomes in resulting cell

Same as parent cell i.e. kept constant - Diploid

Half the number of the parent cell - HAPLOID

Genes present on the Chromosomes in resulting cell

Identical to the parent cell Different to the parent cell

Result 2 identical daughter cells 4 haploid cells

Role Asexual Rep in unicellular organisms,Growth/replacement in multicellular organisms

Produce haploid gametes for reproduction,genetic variation

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Learning Check

1. What is meiosis?

2. How does meiosis differ to mitosis?

3. What is the function of meiosis?

4. What parts of a human would you expect meiosis to occur in?

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Syllabus Can You?.................. (OL)

• Explain the terms: cell continuity & chromosomes.• Define the terms: haploid & diploid number.• Describe the cell activities in he state of non-division:

Interphase and Division (mitosis).• Define the term: mitosis.• Explain the process in simple terms with some

diagrams.• Define cancer and state causes.• State the primary function of mitosis for single-celled

vs multi-cell. Organisms.• Define the term: meiosis.• State the functions of meiosis.

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Syllabus Can You?............... (HL)

• Explain the terms: cell continuity & chromosomes.• Define the terms: haploid & diploid number.• Describe the cell activities in the state of non-division:

Interphase and Division (mitosis).• Define the term: mitosis.• Define cancer and state causes.• State the primary function of mitosis for single-celled vs multi-

cell. Organisms.• Define the term: meiosis.• State the functions of meiosis.• Explain the process in more detailed terms with diagrams (be

able to name each stage and chromosome parts)

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http://biology.about.com/library/blmitosisanim.htm

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• Mitosis in a Winter Wonderland

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