TOPIC 3

Nucleus: large organelle surrounded by a nuclear envelope, which contains many pores. Contains chromatin, which is made from DNA and proteins, and a structure called the nucleolous. The nucleus controls the cells activities and contains instructions to make proteins. The pores allow for substances to pass between the nucleus and the cytoplasm and the nucleolous makes ribosomes.

Lysosome: round organelle surrounded by a membrane, no clear internal structure. Contains digestive enzymes which are kept seperate from the cytoplasm and can be used to digest invading cells or to break down worn out components of the cell.

Ribosome: very small organelle that either floats free or is attached to the rough endoplasmic reticulum, made up of proteins and RNA, but not surrounded by a membrane. Proteins are made at the ribosome.

Rough Endoplasmic Reticulum: a system of membranes enclosing a fluid-filled space, surface is covered with ribosomes. Folds and processes proteins that have been made at the ribosome.

Smooth Endoplasmic Reticulum: similar to RER but with no ribosomes, synthesises and processes lipids.

Golgi Apparatus: group of fluid-filled, membrane-bound, flattened sacs, vesciles are often seen at the edges of the sacs. It processes and packages new lipids and proteins and makes lysosomes.

Mitochondria: usually oval shaped with a double membrane, the inner one is folded to make cristae and inside is the matrix which contains enzymes involved in respiration. Site of aerobic respiration, where ATP is produced. They are very active and require a lot of energy.

Centriole: small, hollow cylinders made of microtubules, found in animal cells but only some plant cells. Involved with the seperation of chromosomes during cell division.

1. The ribosomes on the RER make proteins that are excreted or attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.

2. New proteins produced at the RER are folded and processed, e.g sugar chains are added, in the RER.

3. They are then transported from the RER to the golgi apparatus in vesicles.

4. At the golgi apparatus, the proteins may undergo further processing, e.g sugar chains are trimmed or more are added).

5. The proteins enter more vesicles to be transported around the cell. Extracellular enzymes move to the cell surface to be secreted.

Prokaryotic cells are simpler and smaller than eukaryotic cells. Bacteria are examples of prokaryotic cells.

• A prokaryotic cell has no membrane-bound organelles. It has ribosomes but they are small.
• Some prokaryotic cells have a flagellum in order to move. Some have more than one flagellum.
• A prokaryotic cell doesn't have a nucleus. The DNA floats free in the cytoplasm as one long coiled up strand.
• Plasmids are small loops of DNA that aren't part of the main DNA but contain genes for things such as anti-biotic resistance and can be passed between prokaryotes.
• The plasma membrane controls the movements of substances in and out of the cell.
• The cell wall supports the cell and prevents it from changing shape. It's made of a polymer called murein.
• Some prokaryotes have short hair-like structures called pili. This helps them stick to other cells and can be used in the transfer of genetic material.
• The capsule helps protect bacteria from attack.
• Mesosomes are inward folds in the plasma membrane and people believe they play a role in cellular processes.

A tissue is a group of similar cells that are specially adapted to work together to carry out a particular function.

Squamous Epithelium: a single layer of cells lining a surface. It is found in many places including the lungs.

Ciliated Epithelium: a layer of cells covered in cilia. It is found on surfaces where things need to be moved such as in the trachea, where the cilia waft mucus along.

Xylem Tissue: a plant tissue with two jobs, which transports water around the plant and supports the plant. It contains xylem vessel cells and parenchyma cells.

Cartilage: is a type of connective tissue found in the joints. It shapes and supports the ears, nose and windpipe.

An organ is a group of different tissues that work together to perform a particular function.

The leaf is an example of a plant organ and is made up of the following tissues: lower epidermis - contains stomata to let air in and out for gas exchange, spongy mesophyll - full of spaces to let gas circulate, palisade mesophyll - most photosynthesis occurs here, xylem - carries water to the leaf, phloem - carries sugar away from the leaf, upper epidermis - covered in a waterproof waxy cuticle to reduce water loss.

The lungs are an example of animal organs, they are made up of the following tissues: squamous epithelium tissue - surrounds the alveoli where gas exchange occurs, fibrous connective tissue - helps to force air back out of the lungs when exhaling, endothelium tissue - makes up the wall of the capillaries, which surround the alveoli, and lines the larger blood vessels.

Organs work together to form organ systems and each system has a particular function. Some examples are:

• The respiratory system is made up of all the organs, tissues and cells involved in breathing. The lungs, trachea, larynx, nose, mouth and diaphragm are all part of the respiratory system.
• The circulatory system is made up of the organs involved in blood supply. The heart arteries, veins and capillaries are all parts of this system.

In mitosis a parent cell divides to produce two genetically identical daughter cells, and they contain an exact copy of the DNA of the parent cell).

Mitosis is needed for the growth of multicellular organisms, for repairing damaged tissues and for asexual reproduction.

In multicellular organisms not all cells keep their ability to divide. The ones that do follow a cell cycle.

The cell cycle consists of a period of cell growth and DNA replication called interphase. Miitosis happens after that. Interphase is subdivided into three seperate growth stages.

1) Prophase: the chromosomes condense, getting shorter and fatter. Centrioles start moving to opposite ends of the cell, forming a network of protein fibres called the spindle. The nuclear envelope breaks down and chromosomes lie free in the cytoplasm.

2) Metaphase: the chromosomes line up along the middle of the cell and become attached to the spindle by their centriole.

3) Anaphase: the centromeres divide, seperating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite ends of the spindle with the chromatid first, making the chromatids appear V shapes.

4) Telophase: the chromatids reach the opposite poles on the spindle, and uncoil to become long and thin again. A nuclear envelope forms around each group of chromosomes which forms two nuclei. The cytoplasm divides and there are now two daughter cells that are genetically identical to the original cell. Mitosis is finished and the daughter cell starts the interphase start of the cell cycle again.

The mitotic index is the proportion of cells undergoing mitosis.

You can calculate the mitotic index of cells using this formula:

                          mitotic index = number of cells with visible chromosomes

                                                     total number of cells observed

This lets you work out how quickly the tissue is growing.

DNA is passed to new offspring by gametes.

Gametes are male and female sex cells, found in all organisms that reproduce sexually.

They join at fertilisation to form a zygote, which divides and develops into a new organism.

In animals, the male gametes are sperm and the female gametes are egg cells or ova.

Normal body cells contain the full number of chromosomes. Humans have two sets of 23 chromosomes, one from the mother and one from the father, giving each body cell 46 chromosomes.

Gametes only contain half the number of chromosomes as body cells.

Since each gamete contrains half the full number, fertilisation creates a zygote with the full number of chromosomes, as the nuclei fuse.

Combining genetic material from two individuals makes offspring that are genetically unique.

Mammilian gametes are specialised for their function, whilst also containing the same organelles as other eukaryotic cells.

Egg cells are much larger than sperm. They have a plasma membrane and a nucleus, but also have follicle cells that form a protective coating and a zona pellucida, which is a protective glycoprotein layer that sperm have to penetrate. They also have huge food reserves that nourish the developing embryo.

Sperm cells are much smaller than an egg cell. They have a lot of mitochondria at the start of their tail (flagellum), where respiration occurs to produce a lot of energy as ATP so the sperm can swim. Much like the egg, they contain a nucleus and plasma membrane. At the tip of their head, sperm have acrosome, which contains digestive enzymes to break down the egg cells zona pellucida and allows the sperm to penetrate the egg.

In mammals, sperm have to make their way through the cervix and uterus and into one of the oviducts. Once the sperm is in the oviduct, fertilisation may occur.

1) The sperm swim towards the egg cell in the oviduct and once the sperm make contact with the zona pellucida of the egg cell, the acrosome reaction occurs, where the digestive enzymes are released from the acrosome of the sperm.

2) These enzymes digest the zona pellucida and allow the sperm to move through to the cell membrane of the egg.

3) The sperm head fuses with the cell membrane of the egg cell, which triggers the cortical reaction where the egg releases the contants of the vesicles into the space between the cell membrane and the zona pellucida. 

4) The chemicals from the cortical granules make the zona pellucida thicken, which makes it impenetrable to other sperm, so only one sperm fertilises the egg.

5) The nucleus of the egg and sperm fuse - this is fertilisation.

Meiosis is a type of cell division that happens in the reproductive organs to produce gametes. Cells that divide by meiosis have half the number of chromosomes.

1) The DNA replicates so there are two identical copies of each chromosome, called chromatids.

2) The DNA condenses to form double-armed chromosomes, made from two sister chromatids.

3) The chromosomes arrange themselves into homologous pairs - pairs of matching chromosomes.

4) First division - the homologous pairs are seperated, halving the chromosome number.

5) Second division - the pairs of sister chromatids are seperated.

6) Four new daughter cells that are genetically different from each other are produced, these are gametes.

1) Before the first division of meiosis, homologous pairs of chromosomes come together and pair up.

2) Two of the chromatids in each homologous pair twist around each other.

3) The twisted bit breaks off their original chromatid and rejoin onto the other chromatid, recombining their genetic material.

4) The chromatids still contain the same genes but they now have a different combination of alleles.

5) This means that each of the four new cells formed from meiosis contains chromatids with different alleles.

The four daughter cells formed from meiosis have completely different combinations of chromosomes.

All your cells have a combination of chromosomes from your parents, half from your mother (maternal) and half from your father (paternal).

When the gametes are produced, different combinations of the chromosomes go into each cell.

This is called independent assortment (seperation) of the chromosomes.

The position of a gene on a chromosome is called a locus (loci) and independent assortment means that genes with loci on different chromosomes end up randomly distributed in the gametes.

Genes with loci on the same chromosome are said to be linked because the genes are on the same chromosome, they'll stay together during independent assortment and their alleles will be passed on to the offspring together. They will only be seperated if crossing over occurs.

The closer together the loci of two genes on a chromosome, the more closely they are linked. This is because crossing over is less likely to split them up.

A characteristic is sex-linked when the locus of the allele that cose for it is on a sex chromosome.

In mammals, females have two X chromosomes and males have one X and one Y chromosome.

The Y chromosome is smaller than the X chromosome and carries fewer genes, so most genes are carried on the X chromosomes.

As males only have one X chromosome, they often only have one allele for sex-linked genes, so because they only have one copy, they express the characteristic of this allele even if it's recessive. This makes more likely than females to show recessive phenotypes for genes that are sex-linked.

Genetic disorders caused by faulty alleles on sex chromosomes include colour blindness and haemophilia. The faulty alleles for these disorders are carried on the X chromosome and are called X-linked disorders.

Multicellular organisms are made up from many different cells that are specialised for their function, and all of them originally came from a stem cell.

Stem cells are unspecialised cells that develop into other types of cell. Stem cells divide by mitosis to become new cells, which then become specialised.

The process by which a cell become specialised is called differentiation.

In humans, some stem cells are found in the embryo where they become specialised to form a fetus.

The ability of stem cells to differentiate into specialised cells is called potency and there are 2 main types: 

• Totipotency - the ability to produce all cell types, including all the specialised cells in an organism and extraembryonic cells (cells of the placenta and umbilical cord), these are only present in mammals in the first few cell divisions of an embryo.
• Pluripotency - the ability of a stem cell to produce all the specialised cells in an organism.

A cells genome is its entire set of DNA, including all the genes is contains. A cell doesn't express (make proteins from) all the genes in its genome. Stem cells become specialised because different genes in their DNA become active and get expressed.

Stem cells all contain the same genes, but not all of them are expressed because not all of them are active. 

Under the right conditions, some genes are activated and others are inactivated. mRNA is only transcribed from the active genes, which is then translated into proteins.

These proteins modify the cell and determine the cell structure and control cell processes, including the activation of more genes, which produces more proteins.

Changes to the cell produced by these proteins cause the cell to become specialised, these changes are difficult to reverse, so it will always remain specialised.

Gene expression can be controlled by altering the rate of transcription of genes - increased transcription produces more mRNA which can be used to make more protein.

This is controlled by transcription factors - proteins that bind to DNA and activate or deactivate genes by increasing or decreasing the rate of transcription.

Factors that increase the rate of transcription are called activators and those that decrease the rate are called repressors. Activators often work by helping RNA polymerase bind to the DNA and begin transcription. Repressors often work by preventing RNA polymerase from binding which stops transcription.

Transcription factors bind to specific DNA sites near the start of their target genes - the genes they control the expression of. In prokaryotes, control of the gene expression often involves transcription factors binding to operons.

An operon is a section of DNA that contains a cluster of structural genes, that are transcribed together, as well as control elements and sometimes a regulartory gene:

• The structural genes code for useful proteins, such as enzymes.
• The control elements includes a promoter - a DNA sequence located before the structural genes that RNA polymerase binds to. An operator is a DNA sequence that transcription factors bind to.
• The regulatory gene codes for an activator or repressor.

E. coli is a bacterium that respires glucose, but it can use lactose if glucose isn't available.

The genes that produce the enzymes needed to respire lactose are found on an operon called lac operon. The lac operon has three structural genes - lacZ, lacY, and lacA which produce proteins that help the bacteria digest lactose (including beta-galactisidase and lactose permease).

Lactose NOT present: the regulatory gene produces the lac repressor, which is a transcription factor that binds to the operator site when there's no lactose present. This blocks transcription because RNA polymerase can't bind to the promoter. 

Lactose present: when lactose is present, it binds to the repressor, changing the repressor's shape so that it can no longer bind to the operator site. RNA polymerase can now begin transcription of the structural genes.

Stem cells can develop into any specialised cell type, so scientists think they could be used to replace damaged tissues in a range of diseases. 

Some stem cell therapies already exist such as the treatment of leukaemia. Leukaemia kills all the stem cells in the bone marrow so bone marrow transplants can be given to replace them.

There is research in the use of stem cells as a treatment for lots of conditions such as spinal cord injuries, heart disease and damage caused by heart attack. The stem cells can be used to repair the damaged tissues.

Stem cells could save many lives - stem cells could be used to grow organs for those people awaiting transplants.

They could improve the quality of life for many people - they could be used to replace damaged cells in the eyes of people who are blind.

These are obtained from the body tissues of an adult, such as the bone marrow.

They can be obtained in a relatively simple operation. The donor is anaesthetised and a needle is inserted into the center of the bone (ususally the hip) and a small quantity of bone marrow is removed.

Adult stem cells aren't as flexible as embryonic stem cells - they can only develop into a limited range of cells.

However, if a patient needs a stem cells transplant and their own adult stem cells can be used, there's less risk of rejection - which is when the patients immune system attack the foreign cells.

These are obtained from early embryos.

Embryos are created in a laboratory using in vitro fertilisation, where the egg cells are fertilised outside of the womb. Once the embryos are approximately 4 to 5 days old, the stem cells are rmoved and the rest of the embryo is destroyed.

Embryonic stem cells can develop into all types of specialised cells.

Obtaining stem cells from embryos raises ethical issues because the procedure results in the destruction of embryo that could become a fetus if places in the womb.

Many people believe that the moment of fertilisation a genetically unique individual is formed, which has the right to life. They believe it is wrong to destroy embryos.

Continuous variation: This is when the individuals in a population vary within a range - there are no distinct categories. For example: height, mass or skin colour.

Discontinuous variation: This is when there are two or more distinct categories and each individual falls into only one of these categories. For example: blood type.

Variation in Genotype: This can result in a variation in phenotype. For example: there are six different combinations for blood group alleles, which can produce one of four blood groups. Some characteristics are controlled by only one gene and tend to show discontinuous variation. Most characteristics are controlled by a number of genes and tend to show coninuous variation.

Environment: Most characteristics are controlled by genotype and the environment. Some examples include: height, which depends on nutrition.

Epigenetic control doesn't alter the base sequence of DNA but works by attaching or removing chemical groups from the DNA, which alters how easy it is for enzymes and other proteins needed for transcription to interact with and transcribe genes.

Methylation of DNA: This includes a methyl group being attached to the DNA coding for a gene. Increased methylation changes the DNA structure, so that the proteins and enzymes needed for transcription can't bind to the gene so it cannot be expressed.

Modification of Histones: Histones are proteins that DNA wraps around to form chromatin. Chromatin can be highly condensed or less condensed. How condensed it is effects the accessibility of the DNA. Epigentic modifications to histones include the addition or removal of acetyl groups.

• When histones are acetylated, the chromatin is less condensed, which means that the proteins involved in transcription can bind to the DNA, allowing genes to be transcribed.
• When acetyl groups are removed from the histones, the chromatin becomes highly condensed and genes can't be transcribed because the proteins cannot bind to them.

When a cell divides and replicates, epigentic changes to its gene may be passed on to the daughter cells. For example, methyl groups are usually removed from DNA during the production of gametes, but some can end up in the sperm or egg cells.

If epigenetic changes get passed on, it means that certain genes that are activated or deactivated in the original cell will also be activated or deactivated in the daughter cells.

If an epigenetic change occured in response to a change in the environment, this means that the daughter cells with be equpped to deal with the changed environment in the same way the original cell was.