Cell Cycle And Mitosis

Question 1

What is the three stages of the cell cycle

Answer 1

Interphase, nuclear division and cytokinesis

Question 2

What is the interphase

Answer 2

The longest stage in the cell cycle. Interphase is when the organelles double, the cell grows and then the DNA replicates.
Also known as the synthesis phase.

Question 3

What is nuclear division?

Answer 3

Nuclear division can be either mitosis, creating two identical diploid cells or meiosis, creating four genetically different haploid cells.

Mitosis creates cells with identical DNA for growth and repair where as meiosis creates gametes.

Question 4

What is cytokinesis?

Answer 4

This is the final stage. The division of the cytoplasm to create the new cells.

Question 5

What are the four key facts of mitosis?

Answer 5

• involves one round of cell division
• genmetically identical cells are made
• diploid cells are made
• growth and repair where

Question 6

What is prophase

Answer 6

In this stage the chromosomes condense and become visible. In animal cells, the centrioles separate and move to opposite poles of the cell.

The centrioles are responsible for creating spindle fibres which are released from both poles to create a spindle apparatus. The nuclear envelope also breaks down.

Plants have a spindle apparatus, but lack the centrioles.

Question 7

What is the metaphase.

Answer 7

The chromosomes align along the equator of the cell between the two centrioles. The spindle fibres released from the poles now attatch to the centromere and chromatid.

Spindle fibres are a type of microtubule. The spindle fibres are collectively known as the spindle apparatus.

Question 8

What is the Anaphase?

Answer 8

The spindle fibres start to retract and pull the centromere and chromatids they are bound to towards the opposite poles. This causes the centromere to divide in two and the individual chromatids and pulled to each opposite pole. These separated chromatids and now referred to as chromosomes.

This stage requires energy in the form of ATP which is provided by respiration in the mitochondria.

Question 9

What is telophase?

Answer 9

The chromosomes are now at each pole of the cell and become longer and thinner again.

The DNA no longer needs to be condensed so the chromosomes are no longer visible as the chromatin spreads out.

The spindle fibres disintegrate and the nucleus starts to reform. The final stage in the cell cycle is when the cytoplasm splits in two to create the two new genetically identical cells.

Question 10

What is the mitotic index

Answer 10

The mitotic index can be calculated by counting down how many cells are visible in the field of view and the number of cells tha are in the stage of mitosis.

Mitotic index = the number of cells in mitosis ÷ total number of cells

Question 11

What is binary fission

Answer 11

The process of cell division (binary fission) in prokaryotic cells.

It is much simpler than mitosis in eukaryotic cells because Prokaryotic cells do not possess:
A nucleus
Chromosomes
Membrane-bound organelles
Spindle fibres

Question 12

What are the DNA molecules like in Prokaryotic cells?

Answer 12

They have a single, circular DNA molecule and plasmids.
Plasmids are smaller, circular DNA molecules that are also replicated and inherited

The number of copies of plasmids that each cell inherits can vary, although this number is regulated

Question 13

What is the process of binary fission?

Answer 13

The single, circular DNA molecule undergoes DNA replication

Any plasmids present undergo DNA replication

The parent cell divides into two cells, with the cytoplasm roughly halved between the two daughter cells

The two daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids

Question 14

How do viruses replicate

Answer 14

All viruses are parasitic, meaning they can only reproduce by infecting living cells (known as host cells)

Viruses replicate by injecting their nucleic acid into a host cell

Question 15

What is the process of virus replication.

Answer 15

First, a virus uses attachment proteins on its surface to bind to complementary receptor proteins on the surface of a host cell

The virus then injects its DNA or RNA into the host cell

The host cell then uses its nucleic acid and protein-building machinery (ribosomes) to produce new viral particles

Question 16

What are the two types of gene mutation?

Answer 16

• deletion
• substitution (nonsense, silence or missense)

Question 17

What are the two types of chromotome mutation

Answer 17

• polyploidy
• non disfunctional

Question 18

What is silent substitution?

Answer 18

Also the wrong nucleotide has been placed in the amino acid code.

The same amino acid is being coded for. Because the genetic cod is degenerate, there is no change in the polypeptide being produced.

Question 19

What is missense substitution?

Answer 19

This type of mutation results in a different amino acid being coded for. This could affect the structure and function of the polypeptide chain.

Question 20

What is nonsense substitution?

Answer 20

This results in a stop codon being coded for. This will prevent the rest of the polypeptide chain being produced.

Question 21

What is polyploidy chromosome mutation?

Answer 21

When there is a chnage in the whole set of cchromosome. Caused by there being 3 sets of chromosomes rather than 2.

Question 22

What is nondisfunctional chromosome mutation.

Answer 22

When indivitual homologous pairs of chromosomes fail to separate during mitosis. This usually results in the gamete having one more or one less chromosome

Question 23

What is a mutagenic agent.

Answer 23

The mutation rate can be increased by mutagenic agents. Eg uv rays

These disrupt funcyion of DNA and interfere with transcription. Thry can help with diversity however some mutations can cause danger.

Question 24

What is a cancer?

Answer 24

Cancers arise due to uncontrolled mitosis

Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells)

Cancers start when changes occur in the genes that control cell division. A change in any gene is known as a mutation. If the mutated gene is one that causes cancer it is referred to as an oncogene

Question 25

Stages of development of cancer

Answer 25

Malignant tumours 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 Metastasis is very dangerous as it can be very difficult to detect, locate and remove secondary cancers

Question 26

What is a gene

Answer 26

Small section of DNA that codes for one protein

Question 27

What is locus

Answer 27

The position of a gene on a chromosome or DNA molecule

Question 28

What is an allele

Answer 28

one of the many different forms of gene

Question 29

What is a homologous pair of

Answer 29

pair of chromosomes (one maternal and one paternal) that have the same gene loci.

Question 30

What is meiosis

Answer 30

Meiosis produces four genetically different daughter cells. Meiosis involves two nuclear divisions and creates four haploid daughter cells from a single diploid parent cell.

Question 31

What is a haploid and a diploid

Answer 31

Haploid - only one set of chromosomes. Diploid - two sets of chromosomes

Question 32

What is independant segregation

Answer 32

The homologous pairs of chromosomes line up opposite each other at the equator of the cell. It is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie. These pairs are separated so one of each homologous pair ends up in the daughter cell. This creates a large number of possible combinations in the daughter cells produced.

Question 33

What are the two mechanisms that introduce variation

Answer 33

- independent segregation of homologous chromosomes - crossing over between homologous chromosomes

Question 34

What is crossing over

Answer 34

When homologous pairs line up opposite each other at the equator in meiosis 1, parts of the chromatids can become twisted around each other. This puts tension on the chromatids, causing pairs of the chromatid to break. The broken parts of the chromatid recombine with the another chromatid. This results in new combinations of alleles.

Question 35

First four stages of meiosis

Answer 35

1. Interphase - DNA and organelles are replicated. 2. Sister chromatids are attached by a centromere. 3. Chromosomes lines up in homologous pairs along the equator (of the cell)- Independent segregation. 4. Crossing over can occur. Alleles are exchanged between maternal and paternal chromosomes.

Question 36

Last four stages of meiosis

Answer 36

5. Spindle fibres attach to the centromeres, connecting each chromosome to one centriole. 6. Spindle fibres contract, pulling one chromosome from each homologous pair to either side (of the cell), forming 2 new cells. 7. Spindle fibres connect each centromere to both centrioles. When the fibres contract, sister chromatids are separated. 8. 4 genetically identical gametes are created, each with 23 chromosomes.

Question 37

What is chaisma

Answer 37

Chaisma forms a bivalent (connected to the homologous chromosomes). This starts during metaphase 1. This only occurs in the first round of mitosis.

Question 38

Meiosis compared to mitosis:

Answer 38

Meiosis Two nuclear divisions Haploid cell (one set of chromosomes) Introduces genetic variation Mitosis One nuclear division Diploid cells Creates genetically identical cells.

Question 39

Genetic variation in gametes

Answer 39

There are 2n possible chromosome combination in gametes It is random which egg and sperm will fuse in fertilisation. Therefore, variation is increased. There is actually (2n ) 2 combinations of chromosomes because of random fertilisation This is before crossing over is considered.

Question 40

When do mutations in chromosomes arise

Answer 40

Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis. Non disjunction - This is when the chromosomes or chromatids do not split equally during anaphase

Question 41

How does polyploidy occur

Answer 41

Each homologous pair is doubled due to DNA replication in interphase. All of the chromosomes fail to separate equally during metaphase in meiosis 1. In meiosis 2, chromatids separate equally. Instead of creating haploid cells, its created a diploid cell as the chromosomes failed to separate in meiosis one. If a diploid gamete fertilises with a normal haploid gamete (one copy of each homologous pair). There will be three copies of every chromosome.

Question 42

How does nondisfunctional/aneuploidy occur

Answer 42

Each homologous pair is doubled due to DNA replication in interphase. One chromosome does not separate equally. Normal division in meiosis 2,m the chromatids seperate equally. Two cells will be a haploid with an additional chromosome Two haploid cells will be missing one chromosome

Question 43

What causes natural selection

Answer 43

1. Variation within a population - caused by a mutation 2. Selection pressure causes some of the population to die 3. Those with advantageous allele survive and reproduce 4. An advantageous allele is passed on to the offspring 5. Allele frequency therefore increases in the next generation.

Question 44

What is the evidence for natural selection

Answer 44

1. Organisms produce many more offspring than survive to be mature individuals 2. The individuals in a species are not all identical • random assortment of paternal & maternal chromosomes in meiosis • crossing over of segments of paternal & maternal chromosomes • random fusion of male & female gametes in sexual reproduction mutations 3. Natural selection results in offspring with favourable characteristics - favourable characteristics expressed in their phenotypes of some offspring.

Question 45

What is stabilising selection

Answer 45

• Stabilising Selection acts against both extremes in a range of phenotypes. As a result the variation about the mode is reduced. • After selection, the mode is in the same position: This is the most advantageous phenotype. Stabilising selection has reduced the variation about this modal value.

Question 46

What is directional selection

Answer 46

One of the extremes has the selective advantage. Occurs when there is a change in the environment. The modal trait changes eg antibiotic resistance in bacteria.

Question 47

What is a super bug

Answer 47

One bacterium contains a plasmid to be transferred A a connection is formed and the plasmid is copied. Both bacteria now contain plasmid. The recipient may even integrate the plasmid into its chromosome.

Question 48

Factors that cause varation - genemutatuion , independant segregation and crossing over

Answer 48

Gene Mutation - An error occurs during replication of the DNA in a gene. A change in the order of base alters the amino acid sequence in the protein coded for by the gene. Independent segregation - During meiosis, maternal and paternal chromosomes are reshuffled. The chromosomes and therefore alleles of genes can combine in new ways. Crossing over - During meiosis 1, sections of chromatids are interchanged. Blocks of genes are moved and linked alleles may separate and rejoin in new combinations.

Question 49

Factors that cause variation - chromosome mutation, random fertilisation and environmental factors

Answer 49

Chromosome mutation - During cell division, sections of chromosomes are displaced, e.g., during anaphase. This can result in genes being deleted, duplicated or inversion of a sequence. Random fertilisation - Each parent is genetically different and can produce huge numbers of gametes. Which gametes fuse at fertilisation is a matter of chance. Environmental factors - The expression of genes may be affected by diet, disease or temperature during development. Mutagens may cause gene mutations in somatic cells.

Question 50

Describe binary fission in bacteria

Answer 50

Replication of (circular) DNA; 2. Replication of plasmids; 3. Division of cytoplasm (to produce daughter cells

Question 51

Describe the role of the spindle fibres and the behaviour of the chromosomes during each of these phases

Answer 51

prophase, chromosomes condense; 3. (In) prophase OR metaphase, centromeres attach to spindle fibres; 4. (In) metaphase, chromosomes/pairs of chromatids at equator/centre of spindle/cell; 5. (In) anaphase, centromeres divide; 6. (In) anaphase, chromatids (from each pair) pulled to (opposite) poles/ends (of cell); 7. (In) prophase/metaphase/anaphase, spindle fibres shorten;

Question 52

Describe the appearance and behaviour of chromosomes during mitosis.

Answer 52

During prophase) 1. Chromosomes coil / condense / shorten / thicken / become visible; 2. (Chromosomes) appear as (two sister) chromatids joined at the centromere; (During metaphase) 3. Chromosomes line up on the equator / centre of the cell; 4. (Chromosomes) attached to spindle fibres; 5. By their centromere; (During anaphase) 6. The centromere splits / divides; 7. (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate; (During telophase) 8. Chromatids / chromosomes uncoil / unwind / become longer / thinner.

Question 53

Compare what happens durinh cytokenisis in animal and plant cells

Answer 53

In animal cells - cell membrane is pulled inwards across centre of the cell. This separates the cytoplasm into two halvrs each containing a new nucleus Plant cells - vesicles fuse to extend cell membranes across cytpp;assm. New cell walls develop

Question 54

Why would we prepare a root tip squash with a stain

Answer 54

To distinguish chromosomes because would not have been visible without the stain

Question 55

Explain how genetic variation is increased though meiosis

Answer 55

- homologous pairs pair up -independant segregation -crossing over -new comninqation of alleles are created

Question 56

and explain the factors that influence the duration of the cell cycle in multicellular organisms.

Answer 56

cell type; [1 mark] • A rapidly-dividing cell (e.g. hair follicle, bone marrow) versus a slow-growing one (nerve cells, striated muscle cell); [1 mark] • The organism; [l mark] o fast-growing vs slow-growing • The conditions; [l mark] • e.g. a growth phase in a foetus vs an adult / a tropic

Question 57

researcher pressed down on the sample tissue by applying pressure to the coverslip to...

Answer 57

Create a single / thin layer of cells/tissue; [l mark] • So light could pass through the sample and give a clear image/ distinguish cells/nuclei/chromosomes;

Question 58

researcher pressed down on the sample tissue by applying pressure to the coverslip to...

Answer 58

Create a single / thin layer of cells/tissue; [l mark] • So light could pass through the sample and give a clear image/ distinguish cells/nuclei/chromosomes;

Question 59

Explain the term metatesis

Answer 59

spreading of malignant OR cancerous tumours; [1 mark] • By a piece of tumour breaking off and travelling in blood / lymph; [1 mark] • To form a secondary tumour/growth (in other organs/tissues); [1 mark] • Greatly reduced survival chances because of the difficulty of detecting and treating secondary tumours; [1 mark]

Question 60

Explain what metatesis is

Answer 60

spreading of malignant OR cancerous tumours; [1 mark] • By a piece of tumour breaking off and travelling in blood / lymph; [1 mark] • To form a secondary tumour/growth (in other organs/tissues); [1 mark] • Greatly reduced survival chances because of the difficulty of detecting and treating secondary tumours; [1 mark]

Question 61

Compare and contrast the processes of binary fission and eukaryotic cell division.

Answer 61

Both replicate DNA / entire genome; [1 mark] • Identical daughter cells produced; [1 mark] No chromosomes Plasmids

Question 62

What is the binomial system?

Answer 62

Same genus shows close relationship.

Question 63

Why do different species look similar

Answer 63

1. Live in a similar environment 2. Have similar selection pressures 3. Similar alleles will have the selective advantage 4. Produces similar/same proteins and therefore have similar characteristics

Question 64

What are classification systems an example of

Answer 64

The heirarchy

Question 65

What is a heirarchy

Answer 65

- Smaller groups arranged within larger groups • No overlap between groups.

Question 66

Why is the classification system needed

Answer 66

- Understand relationships between organisms and keep track of changes • The system used needs to be universal • System is based on putting organisms into groups

Question 67

What are the ways to classify

Answer 67

Originally, based on visible similarities- e.g. appearance, behaviour, fossils. More modern and accurate classification methods are: - DNA sequence - mRNA sequence - Amino acids sequence - Immunological - comparing similarity in self-antibody shape

Question 68

What does the phylogenic classification do

Answer 68

Phylogenic classification arranges species into groups according to their evolutionary origins and relationships Phylogeny tells us who's related to whom and how closely related they are. All organisms have evolved from shared common ancestors (relatives). This is shown on a phylogenetic tree.