AS Edexcel SNAB Biology Revision - Topic 3

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Overview of the biological principles covered in t

Gamete structure and function provide the starting point for this topic. You will look at their role in fertilisation. To understand how a single cell divides

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Prokaryotic Cells & Eukaryotic Cells:

Bacteria and cyanobacteria together make up the Prokaryote Kingdom. Their cells do not have nuclei or other membrane bound cell organelles. This type of cell is called a "prokaryotic cell",  meaning "before the nucleus".

(http://www.arabslab.com/myimages/bacteria/bacteria_cell_wall_structure.jpg)

Most Prokaryotes are extremely small with diameters between 0.5 and 5um. Their DNA is not associated with any proteins and lies free in the cytoplasm. A cell wall is always present is protaryotic cells.

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Eukaryotic Cells:

All other living organisms have cells that contain discreate membrane bound organelles such as nuclei, mitochondria and chloroplasts (Plants only). These are "eukaryotic cells" mean "true nucleus". Eukaryotic cells are larger than prokaryotic cells with a diameter of 20um or more. Unlike the prokaryotes, not all eukaryotic cells have a cell wall. Organisms with eukaryotic cells are eukaryotes.

Inside a eukaryotic animal cells:

The classical child's diagram of a cell resembles a fried egg. Using a simple light microscope this is almost as much detail as it is possible to see. However the use of electron microscopy reveals a wealth of structures within the cell, often described as the cell ultrastructure.

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Generalised Animal Cell:

(http://1.bp.blogspot.com/_207DNIaL-gc/TQkaIFguBKI/AAAAAAAAAWk/X7OPuE0bfnU/s1600/animal_cell.jpg)

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The movement of proteins through the cell involves

The production of proteins and their route through the cell.

1.) Transcription of DNA to mRNA.

2.) mRNA leaves the nucleus through the nuclear envelope.

3.) Protein made on ribosomes on rough ER.

4.) Protein moves through the ER assumin three dimensional shape en route.

5.) Vesicles pinched off the rough ER contain the protein.

6.) Vesicles from rough ER fuse to form the flattened sacs of the Golgi apparatus.

7.) Proteins are modified within the Golgi apparatus.

8.) Vesicles pinched off the Golgi apparatus contain the modified protein.

9.) Vesicle fuses with the cell surface membrane releasing protein,such as extracellular enzymes.

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How the Gametes are adapted:

The Ovum (the large cell incapable of independent movement) is wafted along one of the oviducts from the ovary by muscular contractions of the tubes. The cytoplasm of the ovum contains protein and lipid food reserves for a developing embryo. Surrounging the cell is a jelly like coating called the zona pellucida.

The sperm cell is much smaller  that the ovum and is motile (can move). To enable it to swim, the sperm cell has a long tail (a flagellum) powered by energy released by mitochondria. Males continually produce large numbers of sperm once they have reached puberty.

Sperm, which enter the ****** during intercourse, swim through the uterus, their passage assisted by muscular contractions of the uterus walls. If intercourse takes place at about the time of ovulation, the sperm may meet the the ovum in the oviduct,

The sperm are attracted to the ovum by chemicals released by it. To penetrate the ovum, the acrosome in the head of the sperm releases digestive enzymes, which break down the zona pellucida of the ovum. The acrosome is a type of lysosome. Lysosomes are enzyme filled sacs found in the cytoplasm of many cells. They are involved in the breakdown of unwanted structures within the cell.

Once a sperm fuses with and penetrates the membrane surrounding the egg, chemicals released by the ovum cause the zona pellucida to thicken preventing any further sperm from entering the egg.

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When sperm meets the ovum:

1.) Sperm reach ovum.

2.) Chemicals are released from the cells surrounding the ovum, triggering the acrosome reaction.

3.) The acrosome swells, fusing with the sperm cell surface membrane.

4.) Digestive enzymes in the acrosome are released.

5.) The enzymes digest through the follicle cells...

6.) ...and the jelly like layer surrounding the ovum.

7.) The sperm fuses with the ovum membrane

8.) The sperm enters the ovum

9.) Enzymes releassed from lysosomes in the ovum thicken the jelly like layer, preventing entry of other sperm.

10.) Nuclei of the sperm and ovum fuse.

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Plant Gametes:

Male part of the plant: stamen - this is made up of the anther attatched to a stalk known as the filament. Inside the anther, cells divide to produce pollen grains, which contain the male gamete nuclei. Inside the female ovary, one of more ovules develop which contain the female gametes, the ova.

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Gamete cells are unsual:

Human cells contain 46 chromosomes made up of 22 homologus pairs and one pair of sex chromosomes; fruit fly cells contain 8 chromosomes made up of three homologous pairs and one pair of sex chromosomes.

The fundamental difference between gametes and other cells is the number of chromosomes they contain. Gametes have half the number of chromosomes found in normal cells - one chromosome from each homologous pair. Gametes contain half the full number of chromosomes,23 in humans, made u pof one of each homologous pair and one sex chromosome.

human ovum n=23 human sperm n=23 zygote 2n= 46

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How do gametes form:

Mitosis: produces new body cells as an organism grows and develops. This retains the full number of chromosomes, called the diploid number (2n) (46 in humans). The other type of cell division involved meiosis and produces gametes with only half the number of chromosomes, called the haploid number (n) (23 in humans). This form of cell divion occurs in the ovaries and testes of animals, and te ovaries and anthers of flowering plants.

Meiosis:

Chromosomes replicate before division. After replicaiton, each chromosome is made up of two strands of genetic genetic material, two chromatids.

Meiosis I:

Homologous chromosomes pair up and then seperate.

Meisosis II:

Second division

In female plants, only one of the four nuckeu at the end of meiosis forms an ovum - the others disintegrate. In male plants the four haploid cells produced at the end of meiosis underfo another division, this time by mitosis, where each nucleus divides to give two haploid nuclei within the pollen grain.

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Fertilisation in Mammals & Plants

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 enter the ovum, and the genetic material of the ovum and sperm fuse, forming a fertilised ovum called a zygote. This cell now contains genetic material from both parents.

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 and a pollentube grows down through the style towards the ovary, with its growth controlled by the tube 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 and the second fuses with two nuclei in the embro sac called polar nuclei to form a triploid cell.

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

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How does meiosis result in genetic variation?

The shuffling of existing genetic material into new combinations during meiosis is important in creating genetic variation. This shufflin includes: indepentent assortment and 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 I is a source of genetic variation. This process is random; either chromosome from each pair could be in any gamet. This way of sharing out shromosomes produces genetically varied gametes.

Crossing over:

During the first meiotic division, homologous chromosoems come together as pairs and all four chromatids come into contact. At these contact points the chromatids break and rejoin, exchanging sections of DNA. The point where the chromatids break is called a "chiasma" and several of these often occur along the length of each pair of chromosoe, giving rise to a large amount of variation.

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The cell cycle:

Interphase is a time of intesne and organised activity during which the cell synthesises new cell components such as organelles and membranes, and new DNA.The formation of new cellular proteins occurs throughout interphase, whereas DNA synthesis occurs during the S (for synthesis of DNA) phase. The S phase separates the first gap or G1 phase from the second gap or G2 phase.

The length of interphase differes depending on the role of the cell.

The S and G2 phaes of most cells remain relatively constant in duration. The length of the G1 phase is more variable; some cells can take weeks, months or even years to complete this phase.

During interphase, the individual chromosomes are unravelled. This allows access to the genetic material enabling new proteins to be synthesis. In preparation for cell division, the cell synthesises additional cytoplasmic protein and organelles. The cell must also produce copies of DNA for the two new cells. It is vital that this DNA is identical in both structure and quality to the DNA in the original cell.

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Cell Divison

By the end of interphase, the cell conatins enough cytoplasm, organelles and DNA to form two new cells. The newxt step is to share out both the DNA and the contents of the cytoplasm so that each new cell can function independently. The DNA is separated in nuclear division (mitosis). Cytoplasmic division follows.

Cell division is a continous process, where a single cell with double the usual amount of cell contents becomes two new cells. However, it is possible to describe 4 stages during nuclear division by the behaviour of the chromosomes and other structures within the cell.

Prophase, metaphase, anaphase and telophase.

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Cell Divison

By the end of interphase, the cell conatins enough cytoplasm, organelles and DNA to form two new cells. The newxt step is to share out both the DNA and the contents of the cytoplasm so that each new cell can function independently. The DNA is separated in nuclear division (mitosis). Cytoplasmic division follows.

Cell division is a continous process, where a single cell with double the usual amount of cell contents becomes two new cells. However, it is possible to describe 4 stages during nuclear division by the behaviour of the chromosomes and other structures within the cell.

Prophase, metaphase, anaphase and telophase.

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Cell cycle

During prophase the chromosomes condense,becoming shorter and thicker, with each chromosome visible at two strands called chromatids. They are effectively two chromosomes joined at one region called the centromere.

The spindle is formed. The centrioles move around the nuclear envelope and position themselves at opposite sides of the cell. These form the two poles of the psindle, and are involves in the organisation of the spindle fibres.

The break down of the nuclear envelope signals the end of prophase and the start of metaphase.

Metaphase: the chromosomes centromeres attatch to spindle fibres at the equator. When this has been completed the cell has finished metaphase.

Anaphase: the centromeres split. The spindle fibres shorten, pulling the two halves of each centromere in opposite direction. One chromatid of each chromosome is pulled to each of the poles. Anaphase ends whn the sparated chromatids reach the poles and the spindle breaks down.

Telophase: This is the reverse of prophase. The chromosomes unravell and the nuclear envelope reforms, so that the two sets of genetic information become encolsed in seperate nuclei.

The cytokineis occurs. Animal cells: two new cells formed Cleavage furrow.

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Why is mitosis important?

Mitosis ensures genetic consistency, with daughter cells genetically identical to each other and to the parent cell, each 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

Growth and repair: 

Mitosis occurs in the growth of any organism as it develops from a single cell into a multicellular organism. It ensures that a multcellular organism has genetic consistency; all the cells in the body have the same genetic information. Some organisms can regenerate lost or damaged parts of their bodies using mitosis. Mitosis allows damaged cells to be replaced with identical new copies.  

Asexual reproduction:

Many organisms reproduce without producing gametes. They grow copies of themeselves by mitosis, producing offspring that are genetically identical to each other and to their parent. This form of reproduction is asexual, meaning without sex.

Asexual reproduction occurs hen bacteria undergo binary fission; the bacterial cell frows and then divides into two new cells. Asexual reproduction is commonin plants foo, where it is sometimes known as vegetative reproduction. The growth of new plants from tubers and bulbs are familiar examples.

They grow copies of themselves

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Cells in the Early Embryo:

Afer a human zygote has undergone three complete cell cycles, it consists of 8 identical cells. Each of hese cells is said to be Totipotent as it can develop into a complete human being. This is what happens when identical twins form. These cells are called totipotent as the have the potential to develop into a total individual.

By 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 cells mass, of 50 or so cells, goes on to form the tissues of the developing embryo. These 50 cells are knwon as pluripotent embryonic stem cells. Each of these cells can potentially give rise to most cell types, though they cannot give rise to all cell types that make up the human body.

As the embroy develops into a multicellular bosy, the cells from which itt is made becomes increasingly differentiated. Most of them lose the capacity to develop into a wide  range of cells. Instead they become increasingly specialised, functioning as a red blood cell, one of the cell types in bone. However, even in adults some cells remain a certain capacity to give rise to a variety of different cell types. These are known as multipotent stem cells. For example, neural stem cells can develop int othe various types of cell found in the nervous sysyem, white blood stem cells, located in the bone marrow, can develop into red blood cells, platelets and the various sorts of white blood cells.

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

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Potential use of human stem cells in medicine:

Stem cells offer great hope to medicine. They may one day produce universal human donor cells which would provide new cells, tissues or organs for treatment and repair by trans-plantation. Embryonic stem cells may be the most suitable type of stm cells for this sort of treatment. Their potential to develop into any cell type offers the greatest flexibility for development, unlike adult stem cells, which are committed to developing only into ceratin cell types.

Pluriopotent stem cells for research can be isolated from so called "spare embryos". These are produced in fertility clinics that carry out in vitro fertilisation, wheere the ovum is fertilised outside the body. Women undergoing this treatment are given drugs to make them superovulate, producing more eggs than are needed for fertility treatment. Many of the embryos end up being placed in women's womb in the hope of enabling infertile couples to have children, but any additional embryos could be a source of embryonic stem cells. This is controversial.

In a labatory, embryos bing used for research are allowed to grow to form a blastocyst. At this point, the embryos are cultured for a further period of time to see if stem cells are formed. The stem cells are isolated from each embryo and the rest of the embryo is discarded. The stem cels are then cultured and used in research. In the future they may be used to develop tissues that could be used for transplantation.

One problem with this approach is that even if scientists manage to get the stem cells to develop into the right sort of tissue, the tissue may end up being rejected by te immune system.

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Ethical Concerns about the use of stem cells:

It's okay to use multipotent stem cells derived from adults, but these cells are likely to be less valuable for research and for the development of new treatments than the pluripotent stem cwells, which can only be derived from human embryos. Even if adult stem cells are used for new research, they may be of little use unless a better understanding is gained of how they specialise. This understanding may only come from embryonic stem cells.

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Different Genes Are Expressed

As the embryo develops, cell differentiate: they become specialised for one function or a group of functions. The structure and function of each cell type is dependent on the proteins that it synthesises. Salivary gland cells make salivary amylase, an enzyme for digesting starch, while red blood cells contain the protein haemoglobin, an oxygen carrying pigment.

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Switching Genes On:

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

Switching on B-Galactosidase synthesis:

The genes in the prokaryote Escherichia Coli only produce the enzyme B-Galactosidase to break down the carbohydrate lactose when it is present in the surrounding medium. This enzyme converts the disaccharise lactose into the monosaccharides glucose and galactose.

When lactose is not present in the environment, a lactose repressor molecule binds to the DNA and prevents the transcription of the B Galactosidase gene being expressed. If lactose is present in the envrionment, the repressor molecule is prevented from binding to the DNA, and the B'Galactosidase gene is expressed. mRNA coding for B-Galactosidase is transcibed and translation of this mRNA produces the enzyme.

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What switches an individual gene on or off in euka

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 has attatched to the DNA will transcription proceed. The gene remains switched off until the enzyme attaches to the promoter region succcessfully. The attatchment of a regular protein is usually also required to start transcription.

The transcription of a gene can be prevented by protein repressor molecules attatching to the DNA of the promoter region, blocking the attatchment site. In addition, proein reprewssor molecules can attach to the regulator proteins themselves, preventing them from attatching. In either case the gene is switched offl it is not transcribed within this cell.

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When gene expression goes wrong:

Growing bones in the wrong places:

FOP is characterised by the growth of bones in odd places, such as within muscle and connective tissue. By early adulthood, the abnormal bone growth often leads to freezing of all major joints of the backbone and limbs so they cannot move.

What causes it?

It is an inherited condition caused by a gene mutation. Bone cells are normally only produced in growing limbs and in other places where the skeleton develops. Here, genes are expressed that produce all the protein needed to become specialised bone cell. In people with FOP, one of the genes is not switched off in white blood cells. When tissue is damaged, for example when having an injection, the body's white blood cells move to the site of damage. The white cells produce the protein, which diffuses into the surrounding muscle cells. The protein causes the muscle cells to start expressing other genes that turn them into bone cells.

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How are cells organised into tissues?

Specialised cells can group themselves into clusters, working together as a tissue. Cells have specific recognition proteins, also known as adhesion molecules, on their cell surface membranes. Adhesion molecules help cells to recognise other cells like themselves and stick to them. A small part of each recognition protein is embedded in the cell surface membrane; a larger part extends from the membrane. This exposed section binds to complementary proteins on the adjacent cell. The particular recognition porteins synthesised by a cell determine which cells it can and cannot attatch to. If cells from different tissues are separated and then mix together, they reform into the tissues as the recognition proteins bind.

In the human embryo, cells begin to form tissues as they start their specialised functions. At this point, the genes coding for the recognition proteins are switched on. Reorganisation of tissues during development occurs through changes in the type of recognition proteins produced by the cells.

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Gene expression and development:

The precise sequence of transcription and translation of genes determines the sequence of changes during development. By studying the embryonic development of model animals and plants, researchers have shown how genes determine the structures produced duribng development.

Master Genes:

Master genes control the development of each segment of the body, e.g. of a fruit fly. These genes were discovered by looking at mutations which cause the development of the wrong appendage for that 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.

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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. Flowering of Arabidopsis thaliana has been used to model what is happening when plants change from begative growth to reproductive development. Most hermaphrodite flowers, those that have male and female structures, contain four sets of floral organs: sepals, petals, male stamens and female carpels. These are arranged in concentridc whorls.

The expression of genes in cells aross the meristem determines which structures will form. Three genes determine which type of specialised organ will be produced in each area of the meristem. Where only gene A is expressed, sepals form, and where only gene C is expressed carpels form. Petals form where A and B genes are expressed, and staments form where B and C genes are expressed. As with the master genes in fruit flies, these genes produced mRNA that codes for signal proteins which switch on appropriate genes. Synthesis of proteins coded for by these genes results in development of specialised cells in each floral organ.

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Genes in the envrionment:

The characteristics of an organism, such as its height, shape, blood group or sex, are known as its phenotype. Differences in phenotype between the members of a population are caused by differences in:

  • Genetic make up (Genotype)
  • The Environment in which an individual develops.

Some characteristics are controlled almost completely by the organisms genotype, with the environment having little or no effect. For example, a person's bloo group (group A,B,AB or O) is controlled by the genes they inherit. The genes code for protein antigens on the surface of their red blood cells. A person's blood group is not affected by the envrionment in which they develop. Such characteristics are controlled by genes at a single locus and show discontinuous variation.

Characteristics that are affected by both genotype and environment often show continuous variation. Human height is a characteristic showing continuous variation. This means that a person can be any height within the human range. The most common height will be somewhere mid way between the extremes of the range. 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 conteolled by genes at many loci, known as polygenic inheritance, and also by the environment, either directly or by influencing gene expression.  

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Polygenetic inheritance:

In monohybrid inheritance, each locus is responsible for a different heritable feature. For example, one gene might be coding for the colour of a flower with another gene coding for the shape of the petals. However, inheritance of most characteristics does not follow simple Mendelian rules of inheritance, but involves  interaction of alleles at many loci. When a number of genes are involved in the inheritance of a characteristic, rather than just one, we call the pattern of inheritance polygenic.

In many human characteristics that showe polygenic inheritance, such as height and skin colour, the alleles clearly have additive effects. With conditions such as diabetes, coronary heart disease, Alzheimer's disease and some cancers, several genes may confer a susceptibilityto the condition, with environmental factors also contributing. Two people who inherit the same susceptibility may not both develop the illness; it will depend on environmental factors, such as diet, exposure to toxins and stress. Such factors act as triggers to bring about the symptoms of disease. Conditions where several genetic and one or more environmental factors are involved are said to be multifactorial.

The degree of similarity between identical twins is a measure of the influence of the genes on that characteristic. 99% of identical twins have the same eye colour, and 95% have the same fingerprint ridge count. Where the envrionemnt has a greater effect, the similarity falls. One study found that if you are an identical twin and you have Alzheimer's disease, your twin sibling (brother or sister) has a 40% chance of having Alzheimer's disease. However, it you have Alzheimer's disease as a non identical twin, your sibling has only a 10% chance of having it. This suggests that there is a significant but not inescapable genetic basis to Alzheimer's disease. Both genes and the environment influence the development of the disease.

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How Polygenic inheritance works:

In any introductory course on genetics, it is common for eye colour to be used as an example for monohybrid inheritance - a single locus with brown dominant to blue. This is not entirely the case. Eye colour is an example of polygenic inheritcane; alleles at several loci control eye colour. Eye colour ranges from blue to brown, depending on the amount of pigment in the iris. The pigment absorbs lights so brown eyes appear dark. Blue eyes have little pigment, so light reflects off the iris.

Let us say three loci are involved in the inheritance of this characteristic, each with alleles B and b. B adds pigment to the iris and b does not. If all three loci were homozygous for the allele B, the person's genotype would be BB BB BB. The attitive effect would not produce a dark brown iris, whereas bb bb bb would add no pigment to the iris, maknig it pale blue. A range of possuble genotypes and phenotypes are possibly between these two extremes, accoriding to how many alleles add brown pigment, as shown below.

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