Topic 3

Question 1

Nucleus:

Answer 1

-surrounded by a double membrane called the envelope, which contains pores enabling molecules to enter and leave the nucleus.
-the nucleus contains DNA wrapped around histone proteins in a complex called chromatin and a nucleolus which is the site of ribosome production.

Question 2

Rough endoplasmic reticulum:

Answer 2

• a series of flattened sacs enclosed by a membrane with ribosomes on the surface. The RER folds and processes proteins made on the ribosomes; often located close to the nucleus.

Question 3

Smooth Endoplasmic reticulum:

Answer 3

a system of membrane bound sacs that produces and processes lipids and steroids.

Question 4

Golgi Apparatus:

Answer 4

A series of fluid-filled, flattened and curved sacs with vesicles surrounding the edges. The golgi apparatus modifies and packages proteins (after the RER) as well as lipids. (e.g. is glycosiliates proteins or concentrates proteins) It also produces lysosomes.

Question 5

Mitochondria:

Answer 5

usually oval shapes and bound by a double membrane called the envelope. The inner membrane is folded to form projections called cristae, with a fluid matrix on the inside containing the enzymes needed for cellular respiration.

Question 6

Centrioles:

Answer 6

Hollow cylinders containing a ring of microtubules arranges at right angles to eachother. Centrioles are involved in cell division.

Question 7

Ribosomes

Answer 7

• in eukaryotic cells there are 80s ribosomes (in prokaryotic cells they are 70s). They are composed of two subuntis. The site of protein synthesis.

Question 8

Lysosomes:

Answer 8

-vesicles containing digestive enzymes, bound by a single membrane

Question 9

Protein trafficking:

Answer 9

1. Proteins are produced on the ribosomes
2. Proteins which are produced on the ribosomes on the surface of the RER are folded and processed in the RER
3. The proteins are then transported from the RER ti the golgi apparatus in vesicles (involves the fusion of membranes)
4. They are then modified (e.g. glycosiliation-carbohydrates are added to form a glycoprotein) in the golgi appartus
5. The golgi appartus packages the proteins into vesicles to be transported around the the cells to where they are required. Some proteins such as extracellular enzymes leave the cell by exocytosis. (the vesicle membrane fuses with the cell surface membrane to release the contents)

free ribosomes if they are intracellular, rer if they are extracellular

Question 10

Structure and organelles of prokaryotic cells:

Answer 10

Cell wall- the cells rigid outer covering made of peptidoglycan. Provides structure and support of the cell
Slime Capsule- protective slimy layer which prevents dehydration of the cell and enables adhesion to surfaces
Plasmid- circular pieces of DNA
Flagellum- a tail-like structure which rotates to move the cell (can also be in eukaryotic e.g. sperm cell)
Pilli- hair like structures which attach to other bacterial cells (allowing the exchange of plasmids)
70s ribosomes- composed of two subunits and site of protein synthesis
Mesosomes- infoldings of the inner membrane- their function is debated- believed by some scientists that they contain enzymes required for respiration.

Question 11

Features of the ovum:

Answer 11

• Contains the zona pellucida- a protective coating which the sperm have tp penetrate in order for fertilisation to occur, it then hardens after they have entered.
• Contains a haploid nucleus so that a full set of chromosomes is restored at fertilisation.
• Follicle cells form a protective coating around the egg.

Question 12

Features of the spermatozoa:

Answer 12

• sperm cells contain many mitochondria to provide enrgy for the rotation of the flagellum which enables the cell to move
• acrosomes contain digestive enzymes which break down the zona pellucida and allow sperm to penetrate the egg.

Question 13

The process of Mammalian Fertilisation:

Answer 13

1. Acrosome reaction- Enzymes digest the follicle cells and zona pellucida. The membrane of the acrosome fuses with the sperm cell membrane- digestive enzyme, acrosin, released by exocytosis.
2. Sperm bind to specific receptors on egg cell surface membrane and membrane fuses
3. Cortical reaction- corticle granules and lysosomes move to egg cell surface membrane, fuse and release contents (granules) by exocytosis. This causes the egg surface membrane and zona pellucide to harden to prevent polyspermy. The new, hardened membrane is called the fertilisation membrane.
   - there is a constant movement of ions trhough channels (more positive inside the egg cell)
4. Egg and sperm haploid (gamete) nuclei fuse together forming a diploid zygote.
   - there are mitochondria within each gamete cell which release ATP energy for reactions within the cell.
   - there are lipid droplets which provide energy for growth and development.

Question 14

What is a locus?

Answer 14

the location of genes on a chromosome

Question 15

What is autosomal linkage?

Answer 15

• alleles on the same chromosome are said to be autosomally linmked and are inherited (as if they were the same gene)
• the closer the loci of the genes on the chromosome, the more closely lonked they are. This is because they are far less likely to be separated during recombination in meiosis.

Question 16

What is sex-linkage?

Answer 16

They occur on the X chromosome. Some genetic disorders are sex-linked and therefore are much more common in men because men only need one allele to be expressed (X,Y) vs women need the allele on both X chromosomes to be expressed (X,X)
e.g. Haemophilia (absence of clotting factor so blood doesn’t clot properly) or red-green colourblindness

Question 17

The three stages of the cell-cycle:

Answer 17

• mitosis- prophase, metaphase, anaphas and telophase
• cytokinesis- during this stage, the cytoplasm divides, thus producing two daughter cells
• interphase (involving three seperate stages)

Question 18

The three stages of interphase:

Answer 18

G1- during this stage, the cell grows (cell enlargement- more cytoplasm, protein synthesis)
S- chromosomes are replicated (DNA replicates) and begin to condense to form chromatin
G2- the cell prepares to divide (proof-reading enzymes check if DNA has been copied correctly), replicating organelles for a full set in each new cell

Question 19

How does meisosis ensure that the cell is genetically varied?

Answer 19

-Crossing over- the exchange of sections of DNA between homologous chromosomes
-Independent Assortment- there are various combinations of ways maternal and paternal chromosomes can be sistributed between the two daughter cells
-Possible mutations in DNA
-random fertilisation
-mate selection

Question 20

Stages of Meiosis:

Answer 20

Interphase- in the germ line cell with single pair of homologous chromosomes, the DNA is doubled but the number of chromosomes don’t change.
Meiosis I- responsible for the variation of offspring. Involves the pairing up of homologous chromosomes (to form a tetrad) and the recombination of genes (crossing over) using enzymes. Independent assortment also occurs during this stage.
Anaphase I- responsible for forming 2 haploid cells
Meiosis II and anaphase II- sister chromatids split. 2 haploid cells split into 4 haploid, non-identitcal cells that only have one copy of each chromosome.

Question 21

Chromosome mutations:

Answer 21

• Changes to the structure of chromosomes or changes to the number of chromosomes.
• Translocation- swapping of genes, not between alleles in a homologous pair. Involves genes breaking off one chromosome and joining to another.
  -duplication, deletion or inversion
  -aneuploidy (changes in number of chromosomes due to mutation)- happens as a result of non-disjunction- where homolous chromosomes or sister chromatids fail to seperate. (can result in more thsn two chromosomes in a pair- polysomy or less than two chromosomes in a pair-monosomy)

Question 22

Purpose of cell cycle and mitosis:

Answer 22

producing identical daughter cells for growth and a sexual reproduction.

Question 23

Prophase of Mitosis:

Answer 23

-Occurs after interphase stage.
Prophase (preparation):
1. DNA shortens, thickens and condenses (becomes visible)
2. Nuclear membrane breaks down
3. Spindle fibres develop between centrosomes (organelles on poles of cell)
4.Centrosomes migrate to opposite poles

Question 24

Metaphase stage of mitosis:

Answer 24

Metaphase (middle):
1.spindle fibres attach to centromeres on chromosomes
2. chromosomes line up on the ‘equator’ of the cell

Question 25

Anaphase stage of mitosis:

Answer 25

Anaphase (apart): 1. Spindle fibres shorten (some lengthen- outer fibres to stabalise the cell) 2. centromeres (centre point of homologous chromosomes) split. 3. Sister chromatids get pulled apart to opposite poles.

Question 26

Telophase stage of mitosis:

Answer 26

Telophase (two): 1. nuclear membranes reform 2. DNA lenthens and decondenses (disappears from view) 3. Spindle fibres break down

Question 27

Cytokinesis stage (including in plant cell) of the cell cycle:

Answer 27

-microtubules and protein filaments involved -2 genetically identical daughter cells produced ‘asexual reproduction’ Plant cell cytokinesis: 1. Golgi apparatus produced molecules for a new cell wall 2. Vesicles move along microtubules and fuse 3. New cell wall forms from inside a.k.a. cell plate.

Question 28

Stem cells definition:

Answer 28

undifferentiated cells which can keep dividing to give rise to other cell types.

Question 29

Multipotent cells:

Answer 29

can give rise many different cell types

Question 30

Pluripotent stem cells:

Answer 30

cells that can give rise to many types of specialised cells, but not placental cells

Question 31

Totipotent stem cells:

Answer 31

can give rise to all cells including specialised cells and placental cells. All genes are active and can differentiate)

Question 32

Specialisation of cells through differential gene expression:

Answer 32

- a stimulus acts on unspecialised cells - Activator and repressor molecules can bind to promoter regions on the DNA sequence switching genes on (active) and off. In order for the gene to be switched on, transcription factors must be present. -the active genes are transcribed to produce RNA -mRNA is then translated on ribosomes and used to produce protein -the protein has the ability to change the structure and function of cells -plants cells can dedifferentiate (animal cells cannot)

Question 33

Undifferentiated cells can be obtained from…

Answer 33

-embryos (e.g. unused ones from IVF) - bone marrow -umbilical chord -placenta -addition of adult nucleus to enucleated egg (remove egg nucleus and add adult diploid nucleus, shock the cell to create the stem cell)

Question 34

Pros of using embryonic stem cells to treat disease:

Answer 34

-treats disease to reduce suffering of the patient -spare embryos from IVF are destroyed/discarded so instead, they can be put to use to help patients -embryos shouldn’t be considered as humans unless they are viable (implanted in the nucleus)

Question 35

Cons of using embryonic stem cells to treat disease:

Answer 35

- ES cells have potential to become people - could pressure women to produce a surpluss of embryos - could give rise to cancer in treated patients (the stem cells have no hayflick limit)

Question 36

iPSCs:

Answer 36

- induced pluripotent stem cells. - taking a somatic cell and making it pluripotent by activating DNA (e.g. transcription factors leading to RNA polymerase on promoter/ operator to produce mRNA)

Question 37

Pros of iPSCs:

Answer 37

- less chance of immunorejection if used for theraputic clonong because your own iPSCs are used- genetically identical to patient- same antigens- no immunorejection drugs needed. - no embryos used- no ethical issues - lower proliferation (growth) rate, less risk of cancer developing

Question 38

Cons of iPSCs:

Answer 38

They are only pluripotent- less useful than totipotent cells as some genes have been deactivates/ switched off

Question 39

phenotype:

Answer 39

the characteristics of an organism which result from the interaction of the genes of the organism with the environment in which it lives.

Question 40

Two types of variation within a phenotype:

Answer 40

continuous- variation within a range (e.g. mass or height) discontinuous (catagoric)- variation can only take particular values (e.g. gender and shoe size)

Question 41

Characteristics influenced by one gene only:

Answer 41

monogenic inheritance (discontinuous variation)

Question 42

Polygenic inheritance:

Answer 42

-several genes at different loci -continuous variation

Question 43

Characteristics influenced by both the genotype and the environment

Answer 43

-height- polygenic but can be limited by environmental factors such as nutrition -some people are genetically predisposed to lung cancer due to the presence of proto-oncogenes which regulate the cell cycle. Smoking exposes them to chemicals which convert these genes into active oncogenes in lung cells, thus leading to uncontrolled cell division (cancerous tumours) -animal hair colour (e.g. Siamese cats have a gene coding for enzyme tryosinase which darkens fur which is active only below 31 degrees- extremities of siamese cats are dark)

Question 44

Epigenome

Answer 44

-within the complete set of DNA in a cell (genome), all of the modifications hat regualte the activity (expression) of genes. -Epigenetic modifications are modifications to DNA which do not change the base sequence -modify the activation of certain genes

Question 45

Epigenetic modifications:

Answer 45

DNA methylation: methyl groups are added to the DNA. Suppresses gene transcription as DNA winds more tightly around histone proteins so RNA polymerase cannot access the DNA. DNA acelation: addition of acetyl groups- activates gene transcription as DNA unwinds from the histone proteins so that RNA polymerase can access the DNA.

Question 46

Lac Operon:

Answer 46

The gene in bacteria is suppressed by repressor molecules in the absense of lactose-preventing energy expenditure in transcribing the gene for lactose digesting proteins when lactose is absent. When lactose is present, it prevents the repressor molecule from binding to the operator region of the DNA, allowing the gene to be transcribes, thus the protein (B-galactosidase) is produced. - when B-galactosidase is produced, ONPG causes solution to turn yellow.

Question 47

What does the transcription factor complex do?

Answer 47

-helps RNA polymerase bind- gene expression initiated