AS level Biology Unit 2 Edexcel

Prokaryotic cells do not have nuclei or other membrane-bound cell organelles. Examples of a prokaryotic cell are bacteria and cyanobacteria.

• infolding of cell surface membrane (site of respiration)
• plasmid (small circle of DNA)
• capsule (slimy layer on surface for protection and to prevent dehydration)
• pili (thin, protein tubes allow bacteria to adhere to surfaces)
• flagellum (hollow cylindrical thread-like structure that rotates to move the cell)
• robosome
• circular DNA
• cell surface membrane
• cell wall (contains peptidoglycan, a type of polysaccharide and peptide combined)
• cytoplasm

Most prokaryotics are very small. Their DNA is not associated with any proteins and lies free in the cytoplasm. A cell wall is always present in a prokaryotic cell.

Contain discrete membrane-bound organelles such as nuclei, mitochondria and chloroplasts. Eukaryotic cells are larger than prokaryotic cells. Not all prokaryotic cells have a cell wall.

A generalised animal cell consists of:

• Nucleus - contains chromosomes and a nucleolus, the DNA in chromosomes contains genes that control the synthesis of proteins.

• Nucleolus - A dense body within the nucleus where ribosomes are made

• Rough endoplasmic reticulum (rER) - a system of interconnected membrane-bound flattened sacs. Ribosomes are attached to the outer surface. Proteins made by these ribosomes are then transported through the ER to other parts of the cell.

• Ribosomes - made of RNA and protein, they are found in the cytoplasm or attached to the endoplasmic reticulum. They are the site of protein synthesis.

A generalised animal cell consists of:

• Golgi apparatus - sacks of flattened membrane-bouns sacs formed by fusion of vesicles from the ER. Modifies proteins and packages them in vesicles for transport.

• Lysosome - spherical sacs containing digestive enzymes and bound by a single membrane. Involved in the breakdown of unwanted structures within the cell, and destruction of whole cells when old cells are to be replaced or during development.

• Centrioles - Every animal cell has one pair of centrioles. They are involved in the formation of the spindle during nuclear division and in transport within the cell cytoplasm.

• Cell surface membrane - phospholipid bilayer containing proteins and other molecules forming a partially permeable barrier.

• Smooth endoplasmic reticulum (sER) - like rough ER, but does not have any attached ribosomes. Smooth ER makes lipids and steroids.

In nucleus:

• Transcription of DNA to mRNA
• mRNA leaves the nucleus through a pore in the nuclear envelope

In cytoplasm:

• Protein that is made on ribosomes enters the rough ER
• the protein moves through the ER assuming three-dimensional shape en route
• vesicles pinched off the rough ER contain the protein
• vesicles from rough ER fuse to form the flattened sacs of the Golgi apparatus
• proteins are modified within the Golgi apparatus
• vesicles are pinched off the Golgi apparatus contain the modified protein
• vesicle fuses with the cell surface membrane releasing protein, such as extracellular enzymes.

Gametes are highly specialised, with structures and functions that are different from other body cells. The sperm and ovum (egg) cells are the gametes (sex cells), the gametes are adapted for their roles in sexual reproduction.

Ovum:

• large cell that is incapable of independent movement
• it is wafted along by ciliated cells lining the tubes and by muscular contractions of the tubes.
• the cytoplasm of the ovum contains protein and lipid food reserves for a developing embryo
• surrounding the cell is a jelly-like coating called the zona pellucida

Sperm:

• much smaller and can move by itself
• to enable it to swim the sperm has a long tail powered by energy released by the mitochondria
• the sperm consists of the acrosome, nucleus and the mitochondrion
  

• sperm reach ovum

• chemicals released from cells surrounding ovum triggering the acrosome reaction

• the acrosome swells fusing with the sperm cell surface membrane

• digestive enzymes in the acrosome are released

• the enzymes digest through the follicle cells

• and the jelly-like layer surrounding the ovum

• the sperm fuses with the ovum membrane

• the sperm nucleus enters the ovum

• enzymes released from the lysosomes in the ovum thicken the jelly-like layer, preventing entry from other sperm

• nuclei of the ovum and sperm fuse

• Human cells contain 46 chromosomes made up of 22 homologous pairs and one pair of sex chromosomes.

• Fruit fly cells contain 8 chromosomes made up of 3 homologous pairs and one pair of sex chromosomes.

• White campion plant cells have 24 chromosomes made up of 11 homologous pairs and no sex chromosomes (as separate male and female individuals are not produced)

Gametes have half the number of chromosomes found in normal cells (one chromosome from each homologous pair).

The sperm and the ovum each contain 23 chromosomes, made up of one of each homologous pair and one sex chromosome. So when the gametes fuse the full number of 46 chromosome sis restored.

Haploid gametes come together to form a diploid zygote.

• Mitosis produces new body cells as an organism grows and develops. This retains the full number of chromosomes, called the diploid number (46 chromosomes).

• Meiosis produces gametes with only half the number of chromosomes, called the haploid number (23 chromosomes).

Gamete production by meiosis:

• starts with homologous pair of chromosomes
  • chromosomes replicate before division. after replication each chromosome is made up of two strands of genetic material, two chromatids
• homologous pair of replicated chromosomes made (chromatids)
  • homologous chromosomes pair up and then separate
• chromatids separate and gametes are formed, each with half the original number of chromosomes.

Meiosis has two important roles in biology. It results in haploid cells, which are necessary to maintain the diploid number after fertilisation. Secondly, it helps create genetic variation among offspring.

The suffling of existing genetic material into new combinations during meiosis is important in creating genetic variation.

This shuffling includes:

• independent assortment
• crossing over

Independent assortment:

• during meiosis only one chromosome from each pair ends up in each gamete
• the independent assortment of the chromosome pairs as they line up during meiosis 1 is a source of genetic variation
• this process is random; either chromosome from each pair could could be in any gamete
• an organism with 6 chromosomes, that is the three homologous pairs **, YY and ZZ could form eight combinations in its gametes,

Crossing over:

During meiosis 1, homologous chromosomes pair. At points where they make contact, called chiasmata, the chromatids break and rejoin exchanging sections of DNA. The non-sister chromatids exchange corresponding sections of DNA.

• Maternal and paternal chromosome
• chiasma, site of crossing over
• they cross over
• and the homologous chromosomes separate
• the chromatids then separate
• crossing over produces chromosomes that contain new combinations of alleles from both parents

In mammals:

To produce a new individual, the nuclei from the gametes have to combine in the process of fertilisation.

• In mammals the nucleus from one sperm enters the ovum
• and the genetic material of the ovum and sperm fuse
• forming a ferlilised ovum called a zygote
• This cell now contains genetic material from both parents.

In plants: In flowering plants, nuclei from the gametes also have to combine in the process of fertilisation.

• fertilisation takes place in the embryo sac within the ovule
• the pollen grain germinates on the style
• a pollen tube grows down through the style towards the ovary, with its growth controlled by the tube nucleus.
• the pollen grain contains two nuclei ( the tube nucleus and the generative nucleus
• on germination of the pollen, the generative nucleus divides to form two haploid gamete nuclei which move down the pollen tube.
• the tube grows through a microscopic pore into the embryo sac and the two male gamete nuclei enter the sac.
• one fuses with the egg cell and forms a diploid zygote
• the second fuses with two nuclei in the embryo sac called polar nuclei to form a triploid cell.

This diploid zygote divides to form the embryo. The triploid cell divides to form the seed's storage tissue, endosperm.

Prophase:

• The chromosomes condense becoming shorter and thicker
• with each chromosome visible as two strands called chromatids.
• The two strands are identical copies of one another, produced by replication.
• They are two chromosomes joined at one rejoin called the centromere.
• microtubules from the cytoplasm form three-dimensional structure called the spindle.
• the centrioles move around the nuclear envelope and position themselves at appropriate sides of the cells.
• these form the two poles of the spindle, and are involved in the organisation of the spindle fibres
• the spindle fibres form between the poles. (the widest part of the spindle is called the equator.

Metaphase:

• the breakdown of the nuclear envelope signals the start of metaphase
• the chromosomes' centromere attach to spindle fibres at the equator

Anaphase:

• the spindle fibres shorten
• pulling the two halves of each centromere in opposite defections
• one chromatid of each chromasome is pulled to each of the poles
• anaphase ends when the separated chromatids reach the poles and the spindle breaks down

Telophase:

• The chromosomes unravels
• nuclear envelope reforms
• the two sets of genetic information become enclosed in the separate nuclei.

Interphase:

• new cell organelles are synthesised and DNA replication occurs
• by the end of interphase the cell contains enough cell contents to produce two new cells.

Mitosis ensures genetic consistency by:

• making daughter cells genetically identical to each other
• and identical to the parent cell
• each daughter cell containing exactly the same number and type of chromosomes as their parent cell
• this is achieved by:
  • DNA replication prior to nuclear division
  • the arrangement of the chromosomes on the spindle and the separation of chromatids to the poles

This genetic consistency is important in growth and repair, and also in asexual reproduction.

Growth and repair:

• Mitosis occurs in the growth of any organism
• it insures that a multicellular organism has genetic consistency (all the cells in the body have the same genetic information)
• some organisms can regenerate lost parts of their body using mitosis
• mitosis also allows old and damaged cells to be replaced with identical new copies

Asexual reproduction:

• many organisms grow copies of themselves by mitosis producing offspring that are genetically identical to each other, and to their parent.
• asexual reproduction occurs when bacteria undergo binary fission; the bacterial cell grows and then divides into new cells. An outgrowth from the parent detaches to form a new individual.
• it takes place in plants too (vegetative reproduction) E.g. bulbs and tubers.
• some organisms such as mosses, liverworts and all plants that reproduce both asexually and sexually at one time another.

Cells in the early embryo:

After as human zygote has undergone three complete cell cycles:

• it consists of eight identical cells
• cells are totipotent (as each cell can develop into a human being)
• five days after conception a hollow ball of cells called the blastocyst has formed
• the outer blastocyst cell layer goes on to form the placenta
• the inner cell mass of around 50 cells goes on to form the tissues of the developing embryo
• these 50 cells are known as pluripotent embryonic stem cells
• each of these 50 cells can potentially give rise to most cell types

Cells become more differentiated:

as the embryo develops into a multicellular body the cells from which it is made become increasingly differentiated. These cells are known as multipotent stem cells, e.g. neural stem cells can develop into cells found in the nervous system. Differentiation is irreversible in animal cells.

In a plant:

Most plant cells remain totipotent throughout the life of the plant.

Totipotency of plant cells allows plants to be reproduced using plant tissue culture. Small pieces of plant (explants), are sterilised and then placed on a solid agar medium with nutrients and growth regulators.

The cells then divide to form a mass of undifferentiated cells called a callus. By altering the growth regulators in the medium they can be grown into a full plant.

Potential use of human stem cells in medicine:

• They could produce universal human donor cells which would provide new cells, tissues, or organs for treatment or repair by transplantation.
• pluripotent stem cells for medicine and research can be taken from 'spare embryos'
  • one problem with this approach is that the tissue might be rejected by the immune system
  • there are also many ethical concerns with this approach
• therapeutic cloning
  • a diploid cell is removed from the patient (e.g. from the base of a hair)
  • this cell or its nucleus would then be fused with an ovum from which the haploid nucleus had been removed
  • the result would be a diploid cell like a zygote
  • this process is known as somatic cell nuclear transfer (a somatic cell is any diploid body cell)
• tissue typing
  • it has helped in the past to find a suitable donor for a blood transfusion

People agree that there are no ethical objections to using multipotent stem cells taken from adults.

But the problem is that scientists believe that these stem cells are less valuable to their research than pluripotent stem cells which can only be found in human embryos.

Arguments:

• some people believe that an embryo is a human being from the moment it is created so it should have full rights as a human being

• some people argue that an embryo requires and deserves no particular moral attention whatsoever

• others accept the special status of an embryo as a potential human being, but argue that the respect should increase as it develops. They believe that the importance the embryos have in stem cell research outweighs the embryos importance.

The nucleus has a role in controlling the development of the individual cell and the whole multicellular organism's phenotype. This was first shown in classic experiments using giant algal cells.

The Acetabularia mediterranea and the Acetabularia crenulata have:

• a hat
• a stalk
• and a rhizoid (bottom) containing the nucleus

If the hats are removed and the stalks swapped, the plant develops hats with features of both species. (intermediate hats)

If the intermediate hats are then removed, new ones grow that correspond to the nucleus in the rhizoid.

This shows the importance of the nucleus and chemical messengers in the development of the cell.

• sheep 1: mammary cell donor sheep. mammary cells are grown in culture.

• sheep 2: egg cell donor sheep. the nucleus from an egg cell found in the ovary is removed.

• cells are fused together

• the nucleus from the mammary cell inside egg cell minus its nucleus

• it is grown in a culture

• an early embryo forms

• it is implanted into the uterus of a third sheep

• the embryo develops

• and a lamb is born that is chromosomally identical to mammary cell donor

As the embryo develops, cells differentiate: they become specialised for one function or a group of functions. Structure and function of each cell type is dependent on the proteins it synthesises.

Some people demonstrated that different genes are expressed in different cells. They extracted mRNA from differentiated and undifferentiated frog cells.

• The mRNA is extracted from undifferentiated cells in early frog blastula
• the mRNA in differentiated cells in later development (gastrula) is extracted
  • complimentary DNA (cDNA) is made using reverse transcriptase
  • mRNA is then digested
  • the cDNA and the mRNA are combined
• any mRNA that is also produced in the differentiated cells will combine with cDNA to form double strands
• free cDNA is from mRNA produced in the differentiated cells

Cells become specialised because only some genes are switched on and produce active mRNA which is translated into proteins within the cell.

Some French geneticists studied the control of genes in the prokaryote Escherichia coli.

These bacteria only produce the enzyme β-galactosidase to break down the carbohydrate lactose when it is present in the surrounding medium.This enzyme converts the disaccharide lactose to the monosaccharides glucose and galactose.

If lactose is not present in the environment:

A lactose repressor molecule binds to the DNA and prevents the transcription of the β-galactosidase gene.

If lactose is present in the environment:

The repressor molecule is prevented from binding to the DNA, and the β-galactosidase gene is expressed. mRNA coding for β-galactosidase is transcribed and traslation of this mRNA produces the enzyme.

Genes in uncoiled, accessible regions of the eukaryote DNA can be transcribed into messenger RNA.

The enzyme RNA polymerase binds to a section of the DNA adjacent to the gene to be transcribed. This section is known as the promoter region.

Only when the enzyme is attached to the DNA will transcription proceed.

The gene remains switched off until the enzyme attaches to the promoter region successfully. The attachment of a regulator protein is usually also required to start transcription.

Transcription of a gene can be prevented by protein repressor molecules attaching to the DNA of the promoter region, blocking the attachment site.

Protein repressor molecules can attach to the regulator proteins themselves preventing them from attaching.

Growing bones in the wrong places: FOP (fibrodysplasia ossificans progressiva)

FOP is characterised by the growth of bones in odd places, such as within muscles and connective tissue.

What causes it?:

FOP is an inherited condition caused by a gene mutation. In FOP one of the genes that are used to produce proteins that make bone cells is not switched off in white blood cells. So when the tissue is damaged white blood cells move to the site of damage and produce the protein that makes bone cells.

How are cells organised into tissues?:

Specialised cells can group themselves into a cluster working together as a tissue. Cells have specific recognition proteins, also known as adhesion molecules, on their surface membranes. Adhesion molecules help to recognise other cells like themselves and stick to them.

Cell:

In multicellular organisms cells are specialised for a particular function. For example,muscle cells and epithelial cells.

Tissue:

A group of specialised cells working together to carry out one function. For example, muscle cells combining to form muscle tissue, and epithelial cells forming epithelial tissue.

Organ:

A group of tissues working together to carry out one function. For example, muscle, nerve and epithelium work together in the heart.

Organ systems:

A group of organs working together to carry out a particular function. For example, the circulatory system.

The precise sequence of transcription and translation of genes determines the sequence of changes during development.

Master genes:

Master genes control the development of each segment.The master genes produce mRNA which is translated into signal proteins. These proteins switch on the genes responsible for producing the proteins needed for specialisation of cells in each segment.

The ABC of flowering plants:

When a plant starts to flower, cells in a meristem become specialised to form the organs that make up the flower.Most flowers contain the organs: sepals, petals, male stamens and female carpels. These are arranges in concentric whorls.

The expression of genes in cells across the meristem determines which structures will form. When only gene A is expressed sepals form, when only gene C is expressed carpels form. Petals form when A and B are expressed, and stamens when B and C are expressed.

The characteristics of an organism (e.g. height) are known as its phenotype. Differences in phenotype between the members of a population are caused by differences in:

• genetic make-up or genotype
• the environment in which an individual develops

Some characteristics are controlled by the organism's genotype, with the environment having little or no effect (e.g. blood group). Such characteristics are controlled by genes at a single locus and show discontinuous variation. They have phenotypes that fall into discrete groups with no overlap.

Characteristics that are affected by both genotype and environment often show continuous variation (e.g. human height). If a graph is drawn showing the frequency distribution of the different height categories it will be bell-shaped.

Characteristics that show continuous variation are controlled by genes at many loci, known as polygenic inheritance, and also by the environment, either directly or by influencing gene expression.

There are many example of genes and environment interacting together to produce an organism's phenotype. Some examples are:

• height
• hair colour
• MAOA
• causes of cancer

Height: