Higher Human Biology

Unit 1 Human Cells1 Division and differentiation in human cells

(a) Following the formation of the zygote formed by the joining of the gametes the cells divide by mitosis.

Cellular differentiation is the process by which a cell develops more specialised functions by expressing the genes characteristic of that type of cell.

Cells may be grouped into stem cells, somatic cells and germline cells.

(b) Stem cells: These may be subdivided into embryonic and tissue stem cells. Stem cells are unspecialised cells that can continue to divide and can differentiate into

specialised cells of one or more types. During embryological development the unspecialised cells of the early embryo differentiate

into cells with specialised functions. These include nervous tissue, muscle and the various types of blood cells. Embryonic stem cells can become any type of human cell – they are pluritipotent. Tissue (adult) stem cells replenish differentiated cells that need to be replaced and give rise

to a more limited range of cell types – they are multipotent. Once a cell becomes differentiated it only expresses the genes that produce the proteins

characteristic for that type of cell, e.g. the genes for haemoglobin in a red blood cell.The cells in bone marrow are tissue (adult) stem cells and are capable of differentiating into a variety of different types of blood cell, e.g. red blood cells, platelets, lymphocytes and phagocytes.

(c) Somatic cells divide by mitosis to form more somatic cells. During cell division the nucleus of a somatic cell divides by mitosis to maintain the diploid

chromosome number. Diploid cells have 23 pairs of homologous chromosomes.

(d) Germline cells divide by mitosis to produce more germline cells or by meiosis to produce haploid gametes.

(e) There is a great deal of research being carried out into the use of stem cells to repair diseased or damaged organs or tissues, as disease models or for drug testing.

Stem cell research provides information on how cell processes such as cell growth, differentiation and gene regulation work.

As once source of stem cells is embryonic tissue their use can be controversial as the embryos are potential human lives which are destroyed.

(f) Cancer cells divide excessively to produce a mass of abnormal cells (a tumour) that do not respond to regulatory signals and may fail to attach to each other. If the cancer cells fail to attach to each other they can spread through the body to form secondary tumours.

Higher Human Biology

2 Structure and replication of DNA

(a) Structure of DNA

DNA is made of two strands of nucleotides held together by hydrogen bonds between complementary bases on the nucleotides.Each nucleotide consists of a deoxyribose sugar molecule joined to a phosphate group and one of four nitrogen-containing “bases”, adenine, thymine, guanine or cytosine.The nucleotides join sugar to phosphate to sugar, etc, forming a sugar-phosphate backbone.

All cells store their genetic information in the base sequence of DNA. The genotype is determined by the sequence of DNA bases. DNA is the molecule of inheritance and can direct its own replication.

The bases pair in a particular way, adenine pairs with thymine (A-T) and guanine pairs with cytosine (C-G). The base pairs are held together by hydrogen bonds and the cumulative effect of all the base pairs holds the two DNA strands together.

The two strands run in opposite directions, an antiparallel arrangement.

Each strand finishes with a phosphate at one (5’) end and the sugar at the other (3’) end.The two strands wind round one another to form a double helix.

Each cell requires a complete set of DNA so, prior to cell division, the DNA is replicated (copied) using an enzyme called DNA polymerase. The replication process occurs at several locations on a DNA molecule. At these points one strand of the DNA molecule is cut and the DNA unwinds and unzips, i.e. the strands separate. A short strand of complementary RNA nucleotides (a primer) attaches to the exposed bases and further bases are added by DNA polymerase. Bases can only be added to the 3’ (deoxyribose) end of the DNA strand so one side is replicated continuously and the other is replicated in fragments which are joined by DNA ligase.

Higher Human Biology

Polymerase chain reaction (PCR)

The polymerase chain reaction (PCR) is a technique for the amplification of DNA in vitro (i.e. in a test tube). It is used to create large numbers of DNA molecules from a small original sample, a process called amplification.

The DNA sample is heated to 92-98oC to separate the DNA strands; the strands are cooled (to 50-65oC) and DNA primers added which bind to a specific target sequence of bases. A heat tolerant bacterial DNA polymerase is then used to extend the DNA strands from the primers (at 70-80oC) until complete DNA molecules are obtained. The cycle is then repeated a number of times and the number of DNA molecules obtained increases exponentially. This can be carried out automatically in a machine which performs the necessary heating and cooling cycles and has a supply of nucleotides. The amplified DNA can then be used for further analyses.

3 Gene expression.

(a) In any cell, only a fraction of the genes are expressed (translated into proteins). The phenotype (physical appearance and function) of an organism is determined by the proteins produced as the result of gene expression. Gene expression is controlled by the regulation of both transcription and translation.

(b) Structure and functions of RNA

RNA is formed by a string of nucleotides, similar to DNA, but the thymine in DNA is replaced by another base, uracil, in RNA and the deoxyribose sugar is replaced by ribose. RNA is also single stranded compared to the double stranded DNA. There are three types of RNA:

Messenger RNA, or mRNA, carries a copy of the DNA code from the nucleus to the ribosome.

Ribosomal RNA, or rRNA, and proteins form the ribosome(s). Each transfer RNA, (tRNA), carries a specific amino acid.

(c) A mRNA molecule is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm. RNA polymerase moves along DNA unwinding and unzipping the double helix and synthesising a primary transcript of RNA by complementary base pairing.

Genes have introns (non-coding regions of genes) and exons (coding regions of genes). The introns of the primary transcript of RNA are removed. The remaining exons are coding regions and are then joined together to form the mature transcript. This process is called RNA splicing.

Higher Human Biology

(d) Once transcribed, the mRNA is translated into a protein structure on a ribosome. Transfer RNA

molecules fold into a particular shape, forming a triplet anti-codon site and an attachment site for a specific amino acid.

The mRNA attached to the ribosome where there are two attachment sites for tRNA molecules. A set of three bases (a codon) on the mRNA determines which tRNA attaches by complementary base-pairing (A-U, G-C, etc.) When two tRNA molecules are in place on the ribosome their attached amino acids are joined by peptide bonds, the first tRNA detaches from the ribosome, the second tRNA with the growing polypeptide chain attached moves to position 1, a third tRNA with another amino acid attached moves into position 2, joins to the peptide chain, moves to position 1, etc until the whole mRNA molecule has been translated into a polypeptide or until a stop codon is reached.

There are specific “start” and “stop” codons on the mRNA which indicate where translation should begin and terminate.

(e) Different mRNA molecules can be produced from the same primary transcript depending on which RNA segments are treated as exons and introns.

Higher Human Biology

4 Genes and proteins in health and disease

Proteins have a large variety of structures and shapes resulting in a wide range of functions. The polypeptide chains fold to form a particular three dimensional shape held together by peptide bonds, hydrogen bonds and other interactions between different amino acids.

Mutations, (alterations to the DNA), result in no protein, or a faulty protein, being produced. Genetic disorders are caused by changes to genes or chromosomes that result in proteins not being expressed or the proteins being expressed not functioning correctly.

Mutations can be classed as mutations within a single gene, gene mutations, where the DNA nucleotide sequence is altered by substituting, inserting or deleting nucleotides.

Single nucleotide substitutions include missense (replacing one amino acid codon with another), nonsense (replacing an amino acid codon with a premature stop codon — no amino acid is made and the process stops) and splice-site mutations (creating or destroying the codons for exon-intron splicing).

Nucleotide insertions or deletions result in frame-shift mutations or an expansion of a nucleotide sequence repeat where all the amino acids after the insertion or deletion are altered.

Gene mutations tend to affect the structure and function of one particular protein which affects the health of the individual affected.

Condition Type of mutation Effect on health

Sickle cell anaemia missenseMalformed red blood cells, impaired oxygen carrying capacity.

Phenylketonuria (PKU) missenseCannot metabolise phenyl alanine – leads to mental impairment.

Duchenne muscular dystrophy nonsense Causes muscle wasting + early death.

Higher Human Biology

Tay-Sachs syndrome frameshift insertion Deterioration of nerve cells, early death (by 4 years old).

Cystic fibrosis frameshift deletion Impaired lung and digestive function.

There are also larger scale mutations where the structure of a chromosome can be altered; these are called chromosome mutations. These mutations can take the form of a deletion (loss of a segment of a chromosome), duplication (repeat of a section of a chromosome) or translocation (rearrangement of chromosomal material involving two or more chromosomes).

The substantial changes in chromosome mutations often make them lethal.

Some examples of chromosome mutations are given below.

Condition Chromosome mutationCri-du-chat syndrome Deletion of part of short arm of chromosome 5.

Chronic myeloid leukaemia Translocation of a gene from chromosome 22 fused with a gene on chromosome 9.

Familial Down’s syndrome Part of chromosome 21 translocated to chromosome 14.

5 Human genomics

Advances in analytical techniques and faster and cheaper computer processing has made it possible to determine the sequence of DNA bases for individual genes and entire genomes (complete DNA of an organism).

Bioinformatics is the use of computer technology to identify DNA sequences.

The enormous amount of data produced by DNA and protein sequencing can be managed and analysed using computer technology and shared over the internet. Computer programs can be used to identify gene sequences by looking for coding sequences similar to known genes, start sequences or sequences lacking stop codons. Computer programs can be used to identify base sequences that correspond to the amino acid sequence of a protein.

Personalised medicine is based on an individual’s genome. Analysis of an individual’s genome may lead to personalised medicine through understanding the genetic component of risk of disease. For example, it may be that individuals carrying a particular allele of a gene have an increased risk of developing a particular disease like breast cancer. The information gained from DNA studies can provide information on the structure of the genes and proteins involved in disease. Rational drug design synthesises specific drugs that will bind to these proteins or prevent their synthesis by binding to a specific region of DNA preventing transcription or by binding to mRNA preventing translation, for example interfering RNA (RNAi).

There are minor variations in DNA base sequence between individuals due to mutations; these may be neutral, i.e. cause no ill-effects or harmful. Mutations in non-coding regions of DNA will be more likely to be neutral than mutations in a region of DNA coding for a crucial gene. It may become possible in the near future to tailor medicines to interact directly with DNA where the base sequence in an individual is different from the majority of the population. 6 Metabolic pathways

Higher Human Biology

Metabolism encompasses the integrated and controlled pathways of enzyme catalysed reactions within a cell. Metabolic pathways involve biosynthetic processes (anabolism), e.g. the building of new proteins, and the breakdown of molecules (catabolism) to provide energy and building blocks. Synthetic pathways require the input of energy; pathways that break down molecules usually release energy. Metabolic pathways may exist that can bypass steps in a pathway. These can sometimes cause the creation and accumulation of unwanted metabolic products. Metabolic pathways are controlled by the presence or absence of particular enzymes in the metabolic pathway and through the regulation of the rate of reaction of key enzymes within the pathway. Regulation can be controlled by intra- and extracellular signal molecules. Genes for some enzymes are continuously expressed. These enzymes are always present in the cell and their control involves regulation of their rate of reaction. These would include, for example, the enzymes involved in the respiration pathways.Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction. Some bacteria will produce an enzyme which breaks down the sugar in milk, but only if exposed to the sugar. This ensures that the enzyme is only made when needed, saving energy and resources.

The lock and key hypothesis of enzyme action did not really explain how the enzyme catalysed the reaction and has been superseded by the induced fit hypothesis. In the induced fit hypothesis the enzyme and substrate still bind at the active site but there are particular amino acids at the active site which have an affinity for the substrate, i.e. they attract by means of hydrogen and ionic bonds.

The attractive forces of these bonds ensure that the substrate(s) are orientated correctly, the enzyme then closes around the substrate, the “induced fit”. Once held in this position the bonds in the substrate are put under stress, making it easier for the desired reaction to occur, whether anabolic or catabolic. By pushing together or pulling apart substrate molecules, the activation energy (energy input needed for the reaction to occur) is lowered. Enzymes speed up reactions by holding the reactants close together and by reducing the activation energy required for a reaction to occur. The end products of the reaction have a lower affinity for the enzyme than the substrate and are released from the active site.

The direction and speed of an enzyme reaction can be affected by the concentrations of the substrate and end products. Increasing the substrate concentration will speed up a reaction as the chances of the enzyme and substrate coming together are increased. The presence of high concentrations of the end product can slow down or reverse a reaction. End products are normally passed on to another reaction (where they become the substrate) and so do not normally accumulate.

Higher Human Biology

Control of metabolic pathways can be achieved by competitive, non-competitive and feedback inhibition.Competitive inhibition occurs when the inhibiting substance has a similar shape and charge to the substrate and competes with the substrate to bind to the active site. The effect of competitive inhibition can be reduced by increasing the concentration of the substrate.Non-competitive inhibition occurs when the inhibitor binds to another part of the enzyme, away from the active site, changing the shape of the active site and preventing the substrate from binding.Heavy metals, like lead, inhibit in this way.Feedback inhibition occurs when the end product of a series of reactions binds to an enzyme that catalyses a reaction early in the pathway, reducing that enzyme’s activity. This slows down the series of reactions, preventing the production of more of the end product until its concentration falls and the inhibition is removed.

7 Cellular respiration

The metabolic pathways of cellular respiration are central to metabolism. They yield energy and are connected to many other pathways.

ATP (Adenosine Tri-Phosphate) is used to transfer energy to synthetic pathways and other cellular processes where energy is required.

At the beginning of the respiration pathway glucose is phosphorylated (has a phosphate group added from ATP) this occurs in the cytoplasm. This is regarded as an “investment” phase where ATP is consumed. In the latter part of glycolysis ATP is generated, this is regarded as an energy pay-off phase. At the end of the glycolysis stage pyruvate is formed which progresses to the citric acid cycle if oxygen is available.

Pyruvate is broken down to an acetyl group that combines with coenzyme A to be transferred to the citric acid cycle as acetyl coenzyme A. Acetyl coenzyme A combines with oxaloacetate to form citrate followed by the enzyme mediated steps of the cycle. This cycle results in the generation of ATP, the release of carbon dioxide and the regeneration of oxaloacetate in the matrix of the mitochondria.

Dehydrogenase enzymes remove hydrogen ions and electrons which are passed to the coenzymes NAD to form NADH in glycolysis and citric acid pathways. NADH release the high-energy electrons to the electron transport chain on the mitochondrial membrane and this results in the synthesis of ATP.

The electron transport chain is a collection of proteins attached to the folded inner membranes of the mitochondria. NADH release the high-energy electrons to the electron transport chain where they pass along the chain, releasing energy. The energy is used to pump H ions across the inner mitochondrial membrane. The return flow of H ions rotates part of the membrane protein ATP synthase and produces the bulk of the ATP generated by cellular respiration.

Higher Human Biology

The final electron acceptor is oxygen, which combines with hydrogen ions and electrons to form water.

8 Energy systems in muscle cells

During vigorous exercise, the muscle cells do not get sufficient oxygen to support the electron transport chain. Under these conditions, pyruvate is converted to lactate. This conversion involves the transfer of hydrogen from the NADH produced during glycolysis to pyruvic acid to produce lactate. This regenerates the NAD needed to maintain ATP production through glycolysis. Lactate accumulates in muscle causing fatigue. The oxygen debt is repaid when exercise is complete; this allows respiration to provide the energy to convert lactate back to pyruvic acid and glucose.

There are different types of skeletal muscle fibres:

Slow twitch (Type 1) muscle fibres contract more slowly, but can sustain contractions for longer and so are good for endurance activities. These muscle fibres are good for endurance activities like long distance running, cycling or cross-country skiing. Slow twitch muscle fibres rely on aerobic respiration to generate ATP and have many mitochondria, a large blood supply and a high concentration of the oxygen storing protein myoglobin. The major storage fuel of slow twitch muscles fibres is fats.

Fast twitch (Type 2) muscle fibres contract more quickly, over short periods, so are good for bursts of activity. These muscle fibres are good for activities like sprinting or weightlifting. Fast twitch muscle fibres can generate ATP through glycolysis only and have few mitochondria and a lower blood supply than slow twitch muscle fibres. The major storage fuels of fast twitch muscles fibres are glycogen.

Most human muscle tissue contains a mixture of both slow and fast twitch muscle fibres. Athletes show distinct patterns of muscle fibres that reflect their sporting activities.