Biology Topic 3

Question 2

Describe structure and function of the nucleus

Answer 2

Large organelle surrounded by nuclear envelope( double membrane), which contains many pores. The nucleus contains chromatin and the nucleolus.

Function: controls cell activities by controlling transcription of DNA. Pores allow substances such as RNA to move between the nucleus and cytoplasm. The nucleolus makes ribosomes.

Question 3

Describe structure and function of lysosomes

Answer 3

Round organelle surrounded by a membrane, with no clear internal structure.

Function: contains digestive enzymes, which can be used to digest invading cells or break down worn out components of the cell.

Question 4

Describe structure and function of ribosomes

Answer 4

Very small organelle that either floats free in cytoplasm or is attached to rough ER. Made up proteins and RNA and isn’t surrounded by a membrane.

Function: Site where proteins are made

Question 5

Describe structure and function of rough ER and smooth ER

Answer 5

Rough ER: system of membranes enclosing a fluid filled space. The surface is covered in ribosomes.
Function: folds and processes proteins that have been made at the ribosomes.

Smooth ER: similar to rough but doesn’t contain ribosomes on surface.
Function: synthesises and processes lipids.

Question 6

Describe structure and function of the golgi apparatus

Answer 6

Group of fluid-filled, membrane bound, flattened sacs.

Function: processes and packages new lipids and proteins. Also makes lysosomes

Question 7

Describe structure and function of mitochondria

Answer 7

Double membrane. Inner membrane folded to form cristae. Inside is the matrix, which contains enzymes involved in respiration.

Function: Site of aerobic respiration, where ATP is produced. Found in large numbers in very active cells that need lots of energy.

Question 8

Describe structure and function of centrioles.

Answer 8

Small hollow cylinders made of microtubules ( tiny protein cylinders). Found in animal cells, but only some plant cells.

Function: involved with separation of chromosomes during cell division.

Question 9

Describe protein production in the cell in 6 steps.

Answer 9

1. Proteins made at ribosomes.
2. Ribosomes on rough ER make proteins that are excreted from cell. Free floating ribosomes make proteins that remain in cytoplasm.
3. New proteins folded and processed in rough ER.
4. Proteins then transported from the rough ER to the golgi apparatus in vesicles.
5. Undergo further processing at the golgi apparatus.
6. The proteins enter vesicles again and are transported either to where they are needed in the cell or excreted from the cell.

Question 10

Which structures are sometimes present in prokaryotes that are never present in eukaryotes?

Answer 10

Circular DNA: floats in DNA as prokaryotes don’t have nucleus.

Flagellum: for movement

Plasmids: small DNA loops containing genes for things like antibiotic resistance that can be transferred between prokaryotes.

Pili: hair like structures that help prokaryotes to stick and can be used for transfer of genetic material.

Slime capsule: defends against attacks from immune system.

Mesomes: inward folds in plasma membrane that increase surface area for respiration.

Also contain smaller ribosomes than eukaryotes

Question 11

Differences between light and electron microscopes?

Answer 11

Light: uses light
Cheaper
Lower magnification and resolution

Electron: uses electrons
Expensive
Higher magnification and resolution

Question 12

Differences between transmission electron microscopes (TEMs) and Scanning electron microscopes ( SEMs)?

Answer 12

TEMs: Use electromagnets to focus beam of electrons through specimen.
Denser areas absorb more electrons so appear darker.
Higher resolution
Can only be used on thin specimens.

SEMs: scan beam of electrons across specimen.
Images show surface of specimen and can be 3D
Lower resolution
Can be used on thick specimens

Question 13

How do you use eyepiece graticule and stage micrometer?

Answer 13

1. Line up eyepiece graticule and stage micrometer.
2. Note value of each division on stage micrometer e.g. 0.1 mm per division.
3. Count how many divisions on eyepiece graticule align with a know number of divisions on the stage micrometer.
4. Calculate value of one division on the eyepiece graticule with formula:

1 division of eyepiece graticule = length on stage micrometer ÷ number of graticule divisions

E.g. if 0.1 mm on stage micrometer lines up with 4.5 eyepiece divisions.
0.1÷4.5= 0.022mm per eyep division

So if specimen equal 20 eyepiece divisions long
20 × 0.022= 0.44 mm long

Question 14

Cell organisation?

Answer 14

1.Organelles

2. Cells
3. Tissues- group of similar cells working together to carry out a specific function.
4. Organ: group of different Tissues working together to carry out particular function.
5. Organ system: organs work together to form Organ systems, e.g., digestive system.

Question 15

What happens at each stage of interphase?

Answer 15

Gap phase 1 (G1): cell grows and new organelles and proteins are made.

S phase: synthesis- cell replicates its DNA, ready to divide by mitosis.

Gap phase 2 (G2): cell keeps growing and proteins needed for cell division are made.

Question 16

What happens at each stage of mitosis?

Answer 16

1. Prophase: chromosomes condense. Centrioles move to opposite sides of cell, forming network of protein fibres called the spindle. Nuclear envelope breaks down and chromosomes lie free in the cytoplasm.
2. Metaphase: chromosomes line up along middle of cell and become attached to spindle by their centromere.
3. Anaphase: centromeres divide separating each pair of sister chromatids. The spindle contracts pulling chromatids to opposite poles of the spindle.
4. Telophase: chromatids decondense and are now called chromosomes again. Nuclear envelope forms around each group of chromosomes so there are now two nuclei. Spindle fibres disappear. The cytoplasm divides to form two identical daughter cells (cytokinesis)

Question 17

How to observe mitosis in a root tip?

Answer 17

1. Cut 1cm from tip of growing root.
2. Prepare boiling tube containing 1M HCl and incubate in water bath at 60 degrees Celsius.
3. Transfer root tip into boiling tube and incubate for 5 minutes.
4. Rinse root tip with cold water using pipette.
5. Place root tip on microscope slide and cut 2mm from very top. Get rid of rest.
6. Use mounted needle to macerate.
7. Stain with toluidine blue.
   8 place cover slip over and press firmly.
8. Observe under microscope and calculate mitotic index.

Question 18

Mitotic index formula

Answer 18

Mitotic index = number of cells with visible chromosomes÷ total number of cells observed

Question 19

How are sperm cells specialised for their function

Answer 19

1.Flagellum- allows sperm to swim.
2.Lots of mitochondria for respiration to produce ATP needed for sperm to swim.
3.Acrosome contains digestive enzymes that break down the zona pellucida allowing the sperm to penetrate the egg.
4. Haploid nucleus

Question 20

How are egg cells specialised for their function?

Answer 20

1. Cytoplasm contains lots of organelles and nutrients to support cell division after fertilisation.
2. Zona pellucida helps prevent more than one sperm from entering (polyspermy)
3. Haploid nucleus
4. Cortical granules released from vesicles, which cause the zona pellucida to harden after one sperm has penetrated the egg, to avoid polyspermy.

Question 21

Describe fertilisation in 5 steps.

Answer 21

1. Sperm swim towards egg cell in the oviduct.
2. When one sperm has contact with the zona pellucida, the acrosome reaction occurs, releasing enzymes that digest the zona pellucida.
3. Sperm head fuses with egg cell membrane, triggering the cortical reaction, in which the egg cell releases the contents of vesicles called cortical granules, causing the zona pellucida to harden.
4. Egg cell now impenetrable to other sperm.
5. Sperm nucleus fuses with egg nucleus this is fertilisation.

Question 22

Outline meiosis in 6 steps

Answer 22

1. DNA replicates so there are two identical copies of each chromosome called chromatids.
2. DNA condenses to form double-armed chromosomes made from two sister chromatids.
3. Chromosomes arrange themselves into homologous pairs.
4. First division homologous pairs separated.
5. Second division pairs of sister chromatids separated.
6. 4 new daughter cells that are genetically different are produced. These are gametes

Question 23

Define chromosome , chromatid , sister chromatids, homologous pairs

Answer 23

Chromosomes: tightly packaged bundle of DNA

Chromatid: one half of a copied Chromosome. Cell makes copy of each chromosome when about to divide. Each chromosome looks like an x now. Each arm of x = chromatid.

Sister chromatids: two identical arms of copied chromosome joined at the centromere.

Homologous pair: Two chromosomes in a pair – normally one inherited from the mother and one from the father. 23 pairs total.

Question 24

Describe in terms of crossing over how meiosis produces genetically different cells.

Answer 24

1. Before 1st division of meiosis, homologous pairs of chromosomes pair up.
2. Two chromatids in each homologous pair twist around each other.
3. Twisted bits break off their original chromatid and rejoin onto the other chromatid.
4. Chromatids still have same genes but have different combination of alleles.
5. Means that each of four new cells contains chromatids with different alleles.

Question 25

Describe in terms of independent assortment of chromosomes how meiosis produces genetically different cells.

Answer 25

1. 4 daughter cells formed from meiosis have completely different combinations of chromosomes. 2. When gametes are produced different combinations of paternal and maternal chromosomes go into each cell. 3. This is called independent assortment of the chromosomes.

Question 26

Explain gene linkage and sex linkage

Answer 26

Gene linkage: the closer the loci are of 2 different genes on a chromosome, the closer linked they are as less likely to be separated by crossing over. Sex linked: allele for characteristic found on sex chromosome. Men only have on x chromosome so only have to inherit one allele to have disorder e.g. haemophilia

Question 27

Explain difference between totipotency and pluripotency.

Answer 27

Totipotency: the ability to produce all cell types of specialised cell in an organism. Can give rise to entire organism. Pluripotency: the ability of a stem cell to give rise to most cell types of specialised cell in an organism

Question 28

Describe how stem cells become specialised through differential gene expression.

Answer 28

1. Stem cells all contain the same genes but not all are expressed because not all of them are active. 2. Genes can be activated or deactivated. 3. mRNA is only transcribed from the active genes. 4. This mRNA is translated into proteins. 5. These proteins modify the cell determining its structure and processes. 6. These changes produced by proteins cause the cell to become specialised.

Question 29

Describe how the lac operon in E. Coli demonstrates gene expression.

Answer 29

E. Coli is a bacterium that respires glucose, but can use lactose if glucose is unavailable. Genes needed to produc3 enzymes needed to respire glucose are found on the lac operon. Lactose not present: regulatory gene produces the lactose repressor which binds to operator site, preventing transcription because RNA polymerase can't bind to promoter region. Lactose present: when lactose is present it binds to the repressed, changing the repressors shape so it can no longer bind to the operator site. RNA polymerase can now bind to promoter region and transcribe the DNA.

Question 30

Describe adult and embryonic stem cells as sources for human stem cells.

Answer 30

Adult: obtained from adult body tissues e.g. bone marrow. Causes discomfort to remove but safe. If from patient's own tissues decreases risk of rejection. Can only develop into limited range of cells. Embryonic: obtained from early embryos created by IVF. Stem cells removed at 4-5 days and rest of embryo destroyed. Can develop into all types of specialised cell.

Question 31

Ethical issues raised by use of embryonic stem cells in medicine?

Answer 31

Some belive that at fertilisation a unique individual is formed with the right to life so it is wrong to destroy the embryo. Some have less objection to stem cells taken from egg cells that have been artificially activated rather than fertilised by sperm, as doesn't have potential for life.

Question 32

Role of regulatory authorities in stem cell medicine?

Answer 32

1. Allowing or not allowing research proposals. 2. licensing and monitoring centres to ensure only fully trained staff carry out research. 3. Producing guidelines and codes of practise. 4. Monitoring developments in scientific research to ensure guidelines are up to date with latest scientific understanding. 5. Providing advice to governments and professionals.

Question 33

How does increased methylation affect the transcription of a gene?

Answer 33

Methyl group attaches at a CpG site (where cytosine and guanine are next to each other in the DNA) Increased methylation changes the DNA structure, so that proteins and enzymes needed for transcription can't bind to gene so it is repressed.

Question 34

Difference between continuous and discontinuous variation?

Answer 34

Continuous: when individuals in a population vary within a range without distinct categories e.g. mass and height. Discontinuous: when individuals fall into distinct categories E.g blood group

Question 35

How does modification of histones affect gene expression?

Answer 35

Histones are proteins that DNA wraps around to form chromatin, which makes up chromosomes. Epigenetic modification if histones include the addition or removal of acetyl groups. 1. When histones are acetylated, the chromatin is less condensed. So proteins needed for transcription can bind, so the gene is expressed. 2. When acetyl groups are removed from histones the chromatin becomes condensed and transcription proteins can't bind, repressing the gene.