Human Biology

• a somatic cell is any cell in the body other than cells involved in reproduction. Somatic stem cells divide by mitosis to form more somatic cells. Each somatic cell contains 46 chromosomes arranged in 23 pairs and is described as diploid.
• Germline cells are gametes (sperm and egg) and the stem cells that divide to form gametes. Germline cells can divide by mitosis to produce more germline cells to maintain the diploid chromosome number. However, they can also divide by meiosis to produce haploid gametes. Haploid gametes only contain 23 single chromosomes.
• During meiosis the nucleus of a germline stem cell undergoes two divisions. The first being the separation of the homologous chromosome pairs and the second being the separating of the chromatids. This results in 4 haploid gametes being produced.
• Cellular differentiation if the process by which a cell expresses certain genes to produce proteins characteristic for that type of cell. This allows a cell to carry out specialised functions. Examples of differentiated cells are sperm cells, nerve cells and red blood cells.
• Stem cells are cells that are unspecialised and have the ability to self-renew and differentiate into specialised cells. There are two types of stem cell: Tissue stem cells and embryonic stem cells.
• Tissue stem cells are involved in the growth, repair and renewal of the cells found in that tissue. They are multipoint- this means they can only differentiate into all types found in a particular tissue.
• All of the genes in embryonic stem cells can be switched on. This allows these cells to differentiate into any type of cell that makes up an individual. Because these cells in the very early embryo can differentiate into all the cell types that make up the individual they are described as pluripotent.
• therapeutic uses of them cells is when stem cells are involved in the repair of damaged or diseased organs/tissues. For example, corneal repair.
• research uses of stem cells involves stem cells being used as model cells to study how diseases develop as well as how they respond to drug treatment. For example, regeneration of damaged skin.
• When a cell becomes cancerous it begins to grow and divide uncontrollably because they do not respond to regulatory signals. New cells are produced even if the body does not need them. A group of cancerous cells produces a growth called a tumour. Cells within the tumour may fail to attach to each other and can spread through the body to other issues and organs where they may form secondary tumours. Metastasis is the spread of cancer from its primary site to other places in the body.
• DNA is the molecule that holds the instructions for growth and development in every living thing. Its overall structure is described as a double-helix held together by complementary base pairs. These base pairs are held together by weak hydrogen bonds.
• DNA is made of repeating units called nucleotides. Each nucleotide consists of a deoxyribose sugar, phosphate and a base. The base which can be 1 of 4: Adenine, Thymine, Cytosine and Guanine.
• There are chemical bonds between the phosphate of one nucleotide and the deoxyribose sugar of the next. This creates the sugar-phosphate backbone.
• The 5' end is located at the phosphate end and the 3' end is the strand that ends with a deoxyribose sugar. The two strands are anti-parallel which means they run in opposite directions. One strand runs from 5' to 3' and the other strand runs from 3' to 5'. The strands coil together to form a double helix structure.
• The requirements for DNA replication are: DNA, primers, free DNA nucleotides, enzymes and ATP.
• DNA replication: Stage 1- DNA is unwound and hydrogen bonds between bases are broken to form two template strands. Stage 2- A primer is always needed to start replication. DNA polymerase will start at the primer and add free nucleotides using complementary base pairing. However, DNA polymerase can only work in a 5' to 3' direction. Therefore, DNA replication happens differently on the leading strand (5' to 3') compared to the lagging strand (3' to 5'). Stage 3- The two new strands then twist together to form a double helix. Each is identical to the original strands.
• Replication of the Leading strand- DNA polymerase can only work in a 5' to 3' direction. The DNA primer binds to the 3' end of the template DNA. The leading strand is synthesised continuously. DNA polymerase adds free nucleotides to the primer at the 3' end. This allows the new strand to be made in a 5' to 3' direction.
• Replication of the lagging strand- DNA polymerase can only work in a 5' to 3' direction. Therefore nucleotides cannot be added in a 3' to 5; direction. This results in the lagging strand being synthesises in fragments. Each fragment starting with a new primer. The fragments are then sealed together by an enzyme called ligase.
• The genome of an individual is the total genetic information encoded into the base sequence of DNA. Polymerase Chain Reaction (PCR) is a laboratory technique used to produce billions of copies of a specific sequence of DNA.
• Requirements for PCR- DNA, Complementary primers, thermal cycler, heat tolerant DNA polymerase and a supply of nucleotides.
• PCR Process- Stage 1- DNA is heated to between 92 and 98°C. This causes the hydrogen bonds to break- separating the two strands. Strand 2- DNA is then cooled to between 50 and 65°C. This allows primers to bind to target sequences. Stage 3- DNA is then heated to between 70 and 80°C. This allows heat-tolerant DNA polymerase to replicate the region of DNA. Repeated cycles of heating and cooling amplify the target region of DNA.
• A practical application of PCR is solving crimes. PCR helps to rapidly identify people. Specific areas of DNA known to vary between individuals is amplified. Giving different sized fragments in different people. Can be used to forensically amplify DNA at a crime scene to identify criminals/victims. It can also be used for diagnosing genetic disorders. Dan sequences that are known to indicate certain genetic disorders or diseases are amplifies using PCR for the purposes of diagnosis. Can be used to settle paternity suits which can help determine the father of a child.
• A gene is a section of DNA which codes for a protein.
• RNA is single stranded, uses ribose sugar and has a uracil base.
• DNA is double stranded, uses deoxyribose sugar and a thymine base.
• Messenger RNA (mRNA) is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm. It carries a complementary copy of the DNA code from the nucleus to the ribosome. Each triplet of bases on the mRNA molecule is called a codon. Each codon codes for a specific amino acid.
• Transfer RNA (tRNA) fold due to complementary base pairing. Each tRNA molecule carries its specific amino acids to the ribosome. A tRNA molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end.
• Ribosomal RNA (rRNA) and proteins form the ribosome. The ribosome is the site of protein synthesis.
• Protein synthesis is the process by which the genetic code in DNA is used to create a protein. Protein synthesis consists of two stages- transcription and translation. Transcription occurs in the nucleus and results in the formation of mRNA. Translation occurs at the ribosome and results in the formation of a polypeptide.
• Transcription is the synthesis of mRNA from a section of DNA. This occurs in the nucleus and allows a complementary copy of the DNA code to move out of the nucleus in search of a ribosome. Stage 1- RNA polymerase enzyme moves along the DNA, unwinding the double helix by breaking the hydrogen bonds between the bases. Stage 2- free nucleotides align with the complementary DNA base pairs. RNA polymerase then joins the RNA nucleotides together by forming bonds between the ribose sugar of one nucleotide and the phosphate of the next.
• Stage 3- once the terminator sequence has been reached, the mRNA becomes separated from the DNA template and is called the primary mRNA transcript. Stage 4- DNA can be wound back up again into a double helix.
• RNA splicing- not all the regions in a gene are required to produce the final protein. These non-coding regions are called introns. The coding regions are called exons. In order to make the final mature mRNA transcript, the introns must be removed and the exons spliced together. This forms the mature mRNA transcript. This process is called RNA splicing.
• One gene can make many proteins by alternative RNA splicing. different proteins can be expressed from one gene, as a result of alternative RNA splicing. Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained.
• Translation: Stage 1- the mRNA molecule enters the ribosome. Stage 2- the first codon of the mRNA is a start codon which signals the start of translation. The first anti-codon matches with the first codon on the mRNA via complementary base pairing. Stage 3- the next tRNA moves along onto the mRNA producing a chain of amino acids. Each amino acid is joined together with a peptide bond. Stage 4- the tRNA molecules break away leaving the amino acid chain. Stage 5- when the polypeptide (chain of amino acids) is complete, it breaks away from the ribosome.
• Once the amino acid chain has been released from the ribosome, it folds into a complete 3D shape depending on the protein that has been created. During folding, different regions of the polypeptide chain can come into contact with one another. This allows interaction between one or more chains- resulting in cross connections. Proteins are held in a 3D shape by hydrogen bonds and other interactions between individual amino acids. An organism's phenotype is determined by the proteins produced as a result of gene expression.
• Mutations are changes in the DNA that can result in no protein or an altered protein being synthesised. A change in the normal base pair sequence of a DNA molecule can result in altered proteins being synthesised during transcription and translation that alter protein function.
• Single gene mutations involve the alteration of a DNA nucleotide sequence as a result of substitution, insertion or deletion of nucleotides.
• insertion is when an extra nucleotide is added into the base sequence.
• deletion is when a nucleotide is removed from the base sequence.
• Insertions and deletions result in frame-shift mutations. These cause all the codons and therefore all the amino acids after the mutation can be changed. This has a major effect on the structure of the protein produced. Diseases caused by insertion and deletion mutations: Insertion can result in Tay Sachs and Deletion can result in Cystic Fibrosis.