Chapter 5.1

Question 1

Chromosome Structure

Answer 1

• Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells)
• The main proteins present are the large positively charged globular proteins called histones, their role is to organise and condense the DNA tightly so that it fits into the nucleus
• The other proteins are enzymes used in copying and repairing the DNA —The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of

Question 2

S-Phase (interphase)

Answer 2

• (S phase) the DNA replicates to create two identical strands of DNA called chromatids, joined together by a narrow region called the centromere
• The two chromatids that make up the double structure of a chromosome are known as ‘sister chromatids’
• It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as one chromatid goes into one daughter cell and one goes into the other daughter cell during mitosis, ensuring the daughter cells are genetically identical
• Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes

Question 3

What is “important or essential” to make sure that cell division takes place during interphase

Answer 3

• It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as one chromatid goes into one daughter cell and one goes into the other daughter cell during mitosis, ensuring the daughter cells are genetically identical
• Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes -called telomeres

Question 4

telomeres

Answer 4

The ends of the chromatids in chromosomes are ‘sealed’ with protective structures

Question 5

The Importance of Mitosis:

Answer 5

The process of mitosis is of great biological significance and is fundamental to many biological processes:

• Growth of multicellular organisms
• replacement of cells and repair of tissues
• Asexual reproduction

Question 6

The Importance of Mitosis: Growth of multicellular organisms

Answer 6

• The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell
• This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms
• Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Question 7

The Importance of Mitosis: Replacement of cells and repair of tissues

Answer 7

• Damaged tissues can be repaired by mitosis followed by cell division
• As cells are constantly dying they need to be continually replaced by genetically identical cells
• In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut
• Some animals can regenerate body parts

Question 8

The Importance of Mitosis: Asexual reproduction

Answer 8

-Asexual reproduction is the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent -For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring -For multicellular organisms (as seen with many plant species) new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways. Some examples of these are budding in Hydra and yeast and runners from strawberries

Question 9

Mitosis

Answer 9

is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus

Question 10

The Cell Cycle

Answer 10

is the regulated sequence of events that occurs between one cell division and the next The cell cycle has three phases:

• interphase
• nuclear division (mitosis)
• cell division (cytokinesis) =Mitosis is part of a precisely controlled process known as the cell cycle

Question 11

The length of the cell cycle is very variable

Answer 11

depending on environmental conditions, the cell type and the organism

-For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)

Question 12

what causes the change in phases

Answer 12

The movement from one phase to another is triggered by chemical signals called cyclins

Question 13

during Interphase

Answer 13

the cell increases in:

• mass, size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)
• Interphase consists of three phases:

—G1 phase-Cells make the RNA, enzymes and other proteins required for growth

—S phase-It is at some point during the G1 phase a signal is received telling the cell to divide again -The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)– S stands for synthesis (of DNA) -The S phase is relatively short -

—G2 phase the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired -Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)

-Interphase = G1 + S + G2

Question 14

Nuclear division (mitosis)

Answer 14

• Follows interphase
• Referred to as the M phase

– M stands for mitosis

-Cell growth stops during the M phase

Question 15

Cytokinesis

Answer 15

• Follows M phase
• Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells
• In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

Question 16

telomers are made of

Answer 16

non-coding DNA (DNA that does not contain genes) that is made up of short base sequences that are repeated many times (multiple repeat sequences) In telomeres

-one strand is rich in the base guanine (G) and the other strand is rich in the complementary base cytosine (C)

Question 17

-The main function of telomeres

Answer 17

is to ensure that the very ends of the DNA molecules are included in DNA replication during mitosis (the copying enzyme responsible for DNA replication is unable to run right to the very end of the DNA molecule and stops a little short of the end)

• If this end part of the DNA molecule contained an important gene, that piece of genetic information would be lost during DNA replication
• In each subsequent cell division, a little more genetic information would be lost
• Telomeres therefore act as a ‘buffer’ region of non-essential DNA and ensure that no important coding sections near the ends of the DNA molecules are left out of the replication process -This ensures no genes are lost during cell division (the loss of vital genes can even result in cell death) and allows for continued replication of a cell

Question 18

To avoid the risk of losing genes most cells have an enzyme called

Answer 18

telomerase that adds additional bases at each end (the telomeres)

-Some cells (generally specialised cells) do not have telomerase to ‘top up’ their telomeres and therefore after a certain number of cell divisions the cell dies, this has been connected with the ageing process

Question 19

Stem Cells

Answer 19

is a cell that can divide (by mitosis) an unlimited number of times

• Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)
• This ability of stem cells to differentiate into more specialised cell types is known as potency

Question 20

potency

Answer 20

ability of stem cells to differentiate into more specialised cell types

Question 21

There are three types of potency

Answer 21

• Totipotency
• Pluripotency
• Multipotency

Question 22

There are three types of potency: Multipotency

Answer 22

-Multipotency

– multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent Multipotent adult stem cell

Question 23

There are three types of potency: Totipotency

Answer 23

-Totipotency

– totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta). The zygote formed when a sperm cell fertilises an egg cell is totipotent, as are the embryonic cells up to the 16-cell stage of human embryo development

Question 24

There are three types of potency: Pluripotency

Answer 24

-Pluripotency

– pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)

Question 25

Multipotent adult stem cells

Answer 25

• Having differentiated and specialised to fulfil particular roles, most adult cells gradually lose the ability to divide until, eventually, they are no longer able to divide
• Although these adult stem cells can divide (by mitosis) an unlimited number of times, they are only able to produce a limited range of cell types

– they are multipotent

Question 26

stem cell therapy

Answer 26

which is the introduction of adult stem cells into damaged tissue to treat diseases (eg. leukemia) and injuries (eg. skin burns)

Question 27

stem cells can be found throughout the body

Answer 27

in the bone marrow, skin, gut, heart and brain

Question 28

How a Tumour Forms

Answer 28

• cancers arise due to uncontrolled mitosis
• Cancerous cells divide repeatedly and uncontrollably, forming a tumour
• Cancers start when changes occur in the genes that control cell division. If there is a change in any gene this is known as a If the mutated gene is one that causes cancer it is referred to as an oncogen. Mutations are common events and don’t lead to cancer most of the time
• Most mutations either result in early cell death or result in the cell being destroyed by the body’s immune system
• As most cells can be easily replaced, these events usually have no harmful effect on the body
• The mutations that result in the generation of cancerous cells do not result in early cell death or in the cell being destroyed by the body’s immune system
• This means that the harmful mutation occurring in the original cell can be passed on to all that cell’s descendants
• A typical tumour contains around a thousand million cancerous cells by the time it is detected

Question 29

Carcinogens

Answer 29

are any agents that may cause cancer (eg. UV light, tar in tobacco smoke and X-rays). If the agent causes cancer it is described as carcinogenic

Question 30

Some tumours (such as warts) do not spread from their original site

Answer 30

these are known as benign tumours and do not cause cancer

Question 31

Some tumours spread through the body, invading and destroying other tissues

Answer 31

these are known as malignant tumours and cause cancer

Question 32

Malignant tumours

Answer 32

interfere with the normal functioning of the organ / tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels

• Malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body
• The spreading of cancers in this way is known as metastasis

Question 33

-Metastasis

Answer 33

• The spreading of cancers in this way
• is very dangerous as it can be very difficult to detect, locate and remove secondary cancers

Question 34

Stages in the development of cancer

Answer 34

1. Oncogenes arise due to carcinogens
2. cancerous cell does not respond to signals from other cells so continues to divide
3. mitosis
4. cncerous cells not removed by immune system
5. rapid mitosis
6. tumour gets bigger
7. tumor supplied with blood and lymph vessels. If it is a malignant tumour, tumour cells spread in blood and lymph to other parts of the body
8. Metastasis

Question 35

Oncogenes

Answer 35

Mutated genes that cause cancer

Question 36

Metastasis

Answer 36

tumour cells invade other tissues. secondary cancers form throughout the body