DNA, RNA & Protein Synthesis - A1 Biology Revision - PDF

***A1 BIOLOGY*** **Unit 4: Genetic Information, Variation and Relationships Between Organisms:** **DNA, RNA, Protein Synthesis** **and Cell Division** -- --  **4.1 DNA, genes and chromosomes** In prokaryotic cells, DNA molecules are short, circular and not assoc...

***A1 BIOLOGY*** **Unit 4: Genetic Information, Variation and Relationships Between Organisms:** **DNA, RNA, Protein Synthesis** **and Cell Division** -- --  **4.1 DNA, genes and chromosomes** In prokaryotic cells, DNA molecules are short, circular and not associated with proteins. In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome. The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein. A gene is a base sequence of DNA that codes for: - the amino acid sequence of a polypeptide - a functional RNA (including ribosomal RNA and tRNAs). A gene occupies a fixed position, called a locus, on a particular DNA molecule. A sequence of three DNA bases, called a triplet, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate. In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, non-coding multiple repeats of base sequences between genes. Even within a gene only some sequences, called exons, code for amino acid sequences. Within the gene, these exons are separated by one or more non-coding sequences, called introns. **4.2 DNA and protein synthesis** The concept of the genome as the complete set of genes in a cell and of the proteome as the full range of proteins that a cell is able to produce. The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA). Transcription as the production of mRNA from DNA. The role of RNA polymerase in joining mRNA nucleotides. - In prokaryotes, transcription results directly in the production of mRNA from DNA. - In eukaryotes, transcription results in the production of pre-mRNA; this is then spliced to form mRNA. Translation as the production of polypeptides from the sequence of codons carried by mRNA. The roles of ribosomes, tRNA and ATP. Students should be able to: - relate the base sequence of nucleic acids to the amino acid sequence of polypeptides, when provided with suitable data about the genetic code - interpret data from experimental work investigating the role of nucleic acids. Students will not be required to recall in written papers specific codons and the amino acids for which they code. **1.5.1 Structure of DNA and RNA** Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes. Ribosomes are formed from RNA and proteins. Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: - The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. - The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. - A condensation reaction between two nucleotides forms a phosphodiester bond. A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs. An RNA molecule is a relatively short polynucleotide chain. Students should be able to appreciate that the relative simplicity of DNA led many scientists to doubt that it carried the genetic code. **1.5.2 DNA replication** The semi-conservative replication of DNA ensures genetic continuity between generations of cells. The process of semi-conservative replication of DNA in terms of: - unwinding of the double helix - breakage of hydrogen bonds between complementary bases in the polynucleotide strands - the role of DNA helicase in unwinding DNA and breaking its hydrogen bonds - attraction of new DNA nucleotides to exposed bases on template strands and base pairing - the role of DNA polymerase in the condensation reaction that joins adjacent nucleotides. Students should be able to evaluate the work of scientists in validating the Watson--Crick model of DNA replication. **2.2 All cells arise from other cells** Within multicellular organisms, not all cells retain the ability to divide. Eukaryotic cells that do retain the ability to divide show a cell cycle. - DNA replication occurs during the interphase of the cell cycle. - Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication. The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis. The role of spindle fibres attached to centromeres in the separation of chromatids. Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells. Meiosis is covered in section 3.4.3 Students should be able to: - recognise the stages of the cell cycle: interphase, prophase, metaphase, anaphase and telophase (including cytokinesis) - explain the appearance of cells in each stage of mitosis. Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and of cancers. Many cancer treatments are directed at controlling the rate of cell division. Binary fission in prokaryotic cells involves: - replication of the circular DNA and of plasmids - division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids. Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles. **4.3 Genetic diversity can arise as a result of mutation or during meiosis** Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. Mutagenic agents can increase the rate of gene mutation. Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis. Meiosis produces daughter cells that are genetically different from each other. The process of meiosis only in sufficient detail to show how: - two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell - genetically different daughter cells result from the independent segregation of homologous chromosomes - crossing over between homologous chromosomes results in further genetic variation among daughter cells. Students should be able to: - complete diagrams showing the chromosome content of cells after the first and second meiotic division, when given the chromosome content of the parent cell - explain the different outcome of mitosis and meiosis - recognise where meiosis occurs when given information about an unfamiliar life cycle - explain how random fertilisation of haploid gametes further increases genetic variation within a species. **DNA Structure** **Nucleic Acids** DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) are both nucleic acids. They are **polymers** of sub-units called **nucleotides**. Each nucleotide consists of three different molecules joined by **condensation reactions.** These are: a five carbon sugar (Pentose); a phosphoric acid molecule; a nitrogen containing base. **Structure of DNA** It consists of **two** polynucleotide strands joined together by **hydrogen bonding** to form an **alpha double helix**. Each DNA nucleotide has: the Pentose sugar Deoxyribose a phosphoric acid molecule. one of nitrogen containing bases : - **Adenine, Thymine,** **Cytosine** and **Guanine** The DNA nucleotides of **each** polynucleotide strand are joined together by condensation reactions between the pentose and the phosphate group forming a **phosphodiester** bond. The sugar and phosphate groups form the backbone of the polynucleotide strands. The organic bases are orientated towards the centre of the helix, protecting them from reacting with other chemicals. The bases on one of the strands have a specific complementary pairing with bases on the other strand i.e. **Adenine** (Purine) always pairs with **Thymine** (Pyrimidine) **Guanine** (Purine) always pairs with **Cytosine** (Pyrimidine) -----------------------------  ----------------------------- **\ ** The bases are joined by **hydrogen bonds** which, although individually weak, due to their large number collectively maintain a stable structure. The DNA helix is further coiled to produce a super helix, providing a **compact** store of genetic information. -- -- **Base Calculations** +-----------------------------------------------------------------------+ | **1.** If DNA has 46% of organic bases which are thymine, what is the | | percentage of adenine present? | | | | **2.** If DNA has 30% of organic bases which are adenine, what is the | | percentage of guanine present? | | | | **3.** If DNA has 28% of organic bases which are thymine, what is the | | percentage of cytosine present? | +-----------------------------------------------------------------------+ **DNA has several important functions**: **Genes** are sections of DNA that contain coded information as a specific sequence of bases. Genes code for polypeptides that determine the nature and development of organisms. DNA has the ability to **self-replicate** (due to complementary base pairing) which is essential for cell division (mitosis and meiosis). Although DNA is a relatively stable molecule, alterations in its base sequence (**mutations**) can occur providing **genetic diversity** (variation) and via natural selection, the basis for evolution. **How is the structure of DNA adapted for its function?** +-----------------------------------+-----------------------------------+ | **Structure** | **Function** | +===================================+===================================+ | Sugar-phosphate backbone | - Gives strength | +-----------------------------------+-----------------------------------+ | Coiling of DNA | - Gives compact shape | +-----------------------------------+-----------------------------------+ | Double helix | - Each strand serves as a | | | template in replication | | | | | | - Protects sequence of bases | | | | | | - Makes molecule more stable | +-----------------------------------+-----------------------------------+ | Large molecule | - Large amount of information | | | can be stored | +-----------------------------------+-----------------------------------+ | Many hydrogen bonds | - Gives stability | | | | | | - Allows chain to unzip easily | | | for replication | +-----------------------------------+-----------------------------------+ | Sequence of bases | - Codes for synthesis of | | | specific proteins | +-----------------------------------+-----------------------------------+ | Complementary base-pairing | - Enables information to be | | | replicated accurately | +-----------------------------------+-----------------------------------+ **Historical Doubts about the Structure of DNA** Before the structure and function of DNA was discovered it was thought that proteins were most likely to be the genetic material passed from generation to generation. This was because proteins are much more varied, the 20 amino acids can be arranged in a wide variety of ways. Although DNA was discovered in the 1800's it was thought to be too simple a molecule. A range of experiments were carried out that eventually proved that DNA carried genetic information from generation to generation. The detailed structure was finally published by Watson and Crick in 1953 using X-ray diffraction studies carried out by Rosalind Franklin. **Structure of RNA** RNA differs in structure from DNA in that: - the Pentose is **Ribose** not Deoxyribose; - the organic base **Uracil** replaces Thymine; (the four bases found in RNA are **Adenine, Guanine, Cytosine** and **Uracil** ) - mRNA and tRNA are **single** stranded in comparison to **double** stranded DNA. **Messenger RNA (mRNA)** It has a linear structure and contains **codons** (= mRNA triplets). It is involved in protein synthesis, being formed in the nucleus during transcription and then moving to the ribosomes in the cytoplasm. **Transfer RNA (tRNA)** There are at least twenty different types of tRNA molecule found in the cytoplasm. Each molecule has a \'clover leaf\' shape due to the presence of hydrogen bonds between complementary base pairs. (see diagram). At one end of the molecule, there is an **anticodon** (3 unattached tRNA nucleotide bases) and at the other end, a **binding site** for the attachment of a specific amino acid. -- -- **\ ** **DNA Replication** DNA replication occurs during the process of cell division and has an important role in the growth and reproduction of organisms. **The semi-conservative mechanism of DNA replication** - When DNA replicates its double helix uncoils (unzips) as the hydrogen bonds between the two polynucleotide strands are broken by the enzyme **DNA helicase** - Each of the strands acts as a template for the formation of two new complementary strands. - Individual **DNA nucleotides** align next to the exposed bases of each template strand according to **specific complementary base** pairing. - Adenine pairs with Thymine and Cytosine pairs with Guanine. - [Adjacent] DNA nucleotides of each new polynucleotide strand are joined together by the enzyme **DNA polymerase** to form **complementary strands** to the original polynucleotide strands. - DNA polymerase catalyses the formation of phosphodiester bonds in condensation reactions between adjacent nucleotides. - The original and new polynucleotide strands of each daughter DNA molecule are joined together by **hydrogen bonds.** - The two daughter DNA molecules are **identical** to each other and to the original parent DNA molecule. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Each newly formed daughter DNA molecule contains one of the original polynucleotide strands and one new polynucleotide strand, hence the term **semi-conservative replication**. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Meselson and Stahl Experiments- Evidence for semi-conservative replication** - Cells of the bacterium E coli were grown on a medium in which normal isotope ^14^N was replaced with the heavy isotope ^15^N. - The cells were allowed to divide until it was certain that heavy nitrogen had been incorporated into the entire DNA. - The bacteria were then transferred to a medium containing only the normal isotope ^14^N, and allowed to divide. - Samples of bacteria were then taken after each division (generation) and the DNA was extracted and spun in a centrifuge. - DNA containing the heavy isotope ^15^N is slightly heavier than DNA containing the normal isotope ^14^N. **In the 1st generation;** - Each DNA molecule contains one strand of the heavy isotope '^15^N and one strand of the normal isotope ^14^N. **In the 2nd generation;** - 50% of the cells contain DNA molecule with one strand of the heavy isotope ^15^N and one strand of the normal isotope ^14^N; - 50% contain DNA molecule with both strands consisting of the normal isotope ^14^N. **In the 3rd generation;** - 25% of the cells contain DNA molecule with one strand of the heavy isotope ^15^N and one strand of the normal isotope '^14^N; - 75% contain DNA molecule with both strands consisting of the normal isotope ^14^N.  **Genes** Genes are sections of DNA that contain coded information as a specific sequence of bases. Genes code for polypeptides that determine the nature and development of organisms. A gene occupies a fixed position, called a **locus,** on a particular strand of DNA or chromosome. **Non-coding DNA** **In eukaryotes, much of the DNA does not code for polypeptides. DNA not coding for polypeptides includes;** - **Introns - These are base sequences present within genes but do not code for amino acids. (The base sequences in genes that do code for amino acids are known as exons).** - **Multiple Repeats -- These are some of the base sequences present between genes. Often this non-coding DNA consists of the same base sequence occurring again and again, known as multiple repeats.** **Alleles** A gene can exist in different forms called **alleles** that code for different types of the same characteristic. For example, the alleles B (brown eyes) and b (blue eyes) are different forms of the gene for eye colour. Alleles arise due to mutations of a gene. Alleles of a gene differ in their base sequence and code for different sequences of amino acids, i.e. different polypeptides are produced. Sometimes this change in the polypeptide makes the protein produced non-functional. This occurs due to a change in the tertiary structure of the protein. The change in amino acid sequence affects hydrogen and ionic bonds (and if they are present may affect disulfide bonds) which disrupts the tertiary structure. If the protein coded for is an enzyme, the change in its tertiary structure may alter the shape of the active site, and the enzyme is non-functional as the substrate is not complementary. Below is a diagram showing a section of DNA in two alleles of a single gene. Differences in the base sequence result in a different sequence of amino acids being coded for. **[Allele 1:]** DNA base sequence T A T G G C G T G T C T A C T A T A --------------------- ------- ------- ------- ------- ------- ------- Amino acid sequence Tyr Gly Val Ser Thr Ile **[Allele 2]** DNA base sequence T A T G G C G **A** G T C T A **G** T A T A --------------------- ------- ------- ----------- ------- ----------- ------- Amino acid sequence Tyr Gly **Glu** Ser **Ser** Ile **DNA and Chromosomes** In **eukaryotes,** DNA is **linear** and is associated with **histone** proteins. It is present in a nucleus surrounded by a nuclear membrane. In **prokaryotes,** DNA molecules are **smaller**, **circular** and are **not** associated with **histone** proteins. Prokaryotes do not form chromosomes. **Chromosomes** During cell division in eukaryotes, DNA and protein is organised into thread-like structures called chromosomes. Genes are located on chromosomes. The **genome** is the complete set of genes in a cell. The **proteome** is the full range of proteins that a cell is able to produce. Alleles of a particular gene are located in the same relative position (**locus**) on homologous chromosomes. A **homologous** pair of chromosomes are the same length and carry genes controlling the same characteristics but not necessarily the same alleles. **Protein Synthesis** **The Genetic Code** A sequence of three nucleotide bases codes for **one** amino acid at the ribosome. Therefore: **The base sequence of a gene determines the amino acid sequence in a polypeptide.** A sequence of three nucleotide bases is known as a **base triplet.** Base triplets in **mRNA** are specifically referred to as **codons**. **Features of the code:** - Each amino acid in a protein is coded for by a sequence of three nucleotide bases on mRNA (i.e. a codon). - A few amino acids have a single codon. - The code is a **degenerate code**. This means that most amino acids have more than one codon. For example, the amino acid leucine has six different codons. - The **start** of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If this first methionine does not form part of the final polypeptide, it is later removed. - Three codons do not code for any amino acid. These are called **stop codons** and mark the end of a polypeptide chain. - The code is **non-overlapping**, that is, each base in the sequence is read only once. - It is a **universal code**, that is, the same codon codes for the same amino acid in all organisms.  **Control of Protein Synthesis** Protein synthesis can be divided into two main processes: **transcription** and **translation**. Transcription occurs in the **nucleus** and involves '**rewriting**' (transcribing) part of the DNA code into a strand of messenger RNA. Translation occurs in the cytoplasm and involves ribosomes synthesising proteins using the information provided by messenger RNA. protein synth overview **Transcription** - Occurs in the nucleus and involves \'rewriting\' (transcribing) part of the DNA code into a strand of messenger RNA. - The relevant section of the DNA molecule uncoils and the two strands are separated as hydrogen bonds are broken by the enzyme **DNA helicase**. - One of these two strands acts as the template (blueprint). - Individual RNA nucleotides line up alongside the DNA nucleotide bases on the template strand due to specific (complementary) base pairing. - Uracil (RNA) lines up alongside Adenine (DNA). - The sequence of bases in the messenger RNA strand is the same as in the non-template strand, except that uracil replaces thymine. - The individual RNA nucleotides are then joined together using the enzyme **RNA polymerase,** to form a strand of messenger RNA. - The mRNA strand leaves the nucleus through the nuclear pore and following **splicing** (see following notes) attaches to a ribosome in the cytoplasm, where translation occurs. - The strands of DNA in the nucleus will recoil when sufficient mRNA has been produced. **Splicing of pre-mRNA** In Eukaryotes the DNA of a gene is made up of sections called **exons** that code for proteins and sections called **introns** that do not. The mRNA formed during transcription using DNA containing both exons and introns is known as **pre-mRNA.** The splicing of **pre-mRNA** involves the removal of introns and the joining together of the exons to form **mRNA**. This is known as post transcriptional processing. **Note:** Once the introns have been removed the remaining exon sections can be rejoined in a variety of different combinations. This means that a gene can code for a variety of different proteins depending on the order in which the exons are combined.  **\ Translation** During translation the sequence of codons on the mRNA strand is used to determine the sequence of amino acids in a polypeptide. This is carried out by ribosomes in the cytoplasm. In the cytoplasm there are twenty different types of tRNA, a specific type for each of the twenty amino acids. The process is summarised in the diagram below translation - Each tRNA molecule has three exposed bases known as an **anticodon**. - A tRNA molecule with the complementary anticodon to the first **codon** on the mRNA strand moves to the ribosome bringing its specific amino acid. - Other tRNA then join in the order determined by the codons on the mRNA strand. The amino acid on the first tRNA molecule is attached to the amino acid on the second tRNA molecule by a peptide bond. - This requires ATP and the action of an enzyme. - The first tRNA molecule then moves away from the ribosome leaving the amino acid behind. - It collects another molecule of the same amino acid from the 'amino acid pool' in the cytoplasm. - This process continues along the mRNA strand until all the codons have been 'read' and the specific polypeptide has been produced. - The polypeptide folds itself into its secondary and tertiary structure. - The sequence of amino acids in this polypeptide has been determined by the codons on the mRNA strand. - As this mRNA strand has been transcribed from the DNA template strand, it is the sequence of DNA nucleotides that ultimately determines which specific polypeptide is produced. **Translation Exercise** Q1. Complete the table below. **Amino Acid** **DNA triplet code** **mRNA codon** **tRNA anti-codon** ------- ---------------- ---------------------- ---------------- --------------------- **1** Glycine CCT CCU **2** Methionine TAC AUG **3** Leucine GAA CUU **4** Serine AGC UCG **5** Arginine TCT UCU **6** Histidine CAC GUG **7** Tyrosine ATA UAU **8** Tryptophan ACC ACC **9** Phenylalanine AAA Q2. What are the names of the amino acids in the polypeptide below? U A C G U G A C C U C G G A A Q3a. Complete the diagram below (with both amino acids and anti-codons). U C U A A A U C G Q3b.What is the DNA sequence which would code for the amino acids sequence above? **Cell Division: Mitosis** Mitosis is a type of nuclear division which produces cells that are **genetically identical**. During mitosis the parent cell divides to produce two daughter cells, each containing an identical copy of the DNA in the parent cell. (**Meiosis** is the other type of nuclear division in which four genetically different cells are produced from the parent cell). - Mitosis increases cell number for growth and repair of tissues. - During mitosis, the nuclear material becomes organised into structures called chromosomes. - In a normal body cell, the chromosomes can be grouped into homologous pairs of chromosomes. - **Diploid number** (2n) represents the total number of chromosomes in a normal body cell. In humans this is 46 - i.e. 23 homologous pairs. - - Mitosis produces cells with the same number of chromosomes as the parent cell so that a diploid parent cell will divide to produce two identical diploid cells. **The Cell cycle** Mitosis is part of the **cell cycle** in which cells undergo a regular cycle of nuclear and cell division separated by periods of cell growth. The cell cycle has three stages. 1\. **Interphase -** this represents the non-dividing cell when cell growth occurs. **2. Nuclear division - the nucleus divides into two (mitosis) or four (meiosis).** **3. Cell division --the cell divides into two (mitosis) or four (meiosis) separate cells.** ------------------------- --  ------------------------- -- As shown above, **interphase** often represents the longest time period of the cell cycle. However, in rapidly dividing cells (e.g. those lining the intestines) the time a cell spends in interphase is short enabling rapid multiplication and replacement of cells. **Interphase** The cell is carrying out its normal cellular functions, but during late interphase it prepares for nuclear division in the following ways. +-----------------------------------+-----------------------------------+ | 1. Increase in protein | | | synthesis. | | | | | | 2. DNA content is doubled via | | | DNA replication. | | | | | | 3. Cell organelles are | | | replicated e.g. mitochondria | | | and ATP content is increased, | | | as cell division is an active | | | process. | | +-----------------------------------+-----------------------------------+ **Mitosis** Although mitosis is a continual process, biologists divide it into four stages, prophase, metaphase, anaphase and telophase. **Prophase** Due to DNA replication during interphase each chromosome consists of two identical **sister chromatids joined together by a centromere.** Each chromosome then shortens and thickens. The centrioles move to opposite poles (sides) of the cell. The nuclear membrane breaks down. -- ------------------------  -- ------------------------ **\ ** **Metaphase** The centrioles form a spindle across the cell. The spindle consists of protein microtubules. Each chromosome moves to the equator (centre) of the spindle and attaches to it via its centromere. Sister chromatids are orientated towards opposite poles of the cell. -- -- -- -- **Anaphase** The centromere splits and the sister chromatids separate. Sister chromatids are pulled to opposite poles of the cell by the spindle microtubules. -- ------------------------  -- ------------------------ **Telophase** The chromatids are at opposite poles of the cell and begin to uncoil. The nuclear membrane reforms. The two cells are **genetically identical** to each other and to the original parent cell. -- -------------------------------------------------------------------------------------------------------------------------------------------------------- ![C:\\Documents and Settings\\MCrowe\\Local Settings\\Temporary Internet Files\\Content.IE5\\MYB23WYP\\Early%20Telophase\[1\].jpg](media/image27.jpeg) -- -------------------------------------------------------------------------------------------------------------------------------------------------------- **\ ** **Cell Division** **This follows nuclear division and involves the splitting of the cytoplasm into two (mitosis) or four (meiosis). Cell surface membranes (and in plant cells, cellulose cell walls) form to separate the cells.** **Summary of Mitosis** Put a tick in the box to indicate at which stage of the cell cycle each of the following events occur: **Event** **I** **P** **M** **A** **T** ---- ------------------------------------------------ ------- ------- ------- ------- ------- 1 Spindle fibres form 2 DNA replication occurs 3 Sister chromatids are pulled to opposite poles 4 Nuclear membrane breaks down 5 Nuclear membrane reforms 6 Chromosomes shorten and thicken 7 Chromosomes move to the equator of the cell 8 Cell organelles are replicated 9 DNA polymerase is active 10 Centrioles move to the poles **Cancer and the cell cycle** Cancer is a group of diseases caused by uncontrolled growth and rapid division of cells. This often results from damage to the genes that regulate **mitosis** and the **cell cycle**. Uncontrolled cell division and growth results in a group of abnormal cells, called a **tumour**. Cells may break away and move to other areas of the body leading to spread of a cancer. Cancer treatments often use drugs to stop cancerous cells dividing. Drugs may be used to inhibit the enzymes, DNA helicase or DNA polymerase (both important in DNA replication) or to inhibit the formation of the spindle. The problem is with such drugs is that they disrupt the cell cycle of healthy cells, however they are more effective against rapidly dividing cancer cells. Normal cells that also divide rapidly are heavily affected, e.g. hair producing cells, hence hair loss during treatment. **\ ** **Do prokaryotic cells divide by mitosis?** Cell division in prokaryotic cells takes place by a process called **binary fission** i.e. Prokaryotic cells do not carry out mitosis as they do not have chromosomes. Binary fission takes place as follows: - The circular DNA molecule replicates and both copies attach to the cell membrane. - The plasmids also replicate. - The cell membrane begins to grow between the two DNA molecules and begins to pinch inward, dividing the cytoplasm into two. - A new cell wall forms between the two molecules of DNA, dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids. http://classes.midlandstech.edu/carterp/courses/bio225/chap06/Slide16.JPG **Cell Division: Meiosis** Meiosis is a type of nuclear division that produces cells that are **genetically different**. During meiosis, a single cell divides twice but DNA replication only occurs once. Thus four cells are produced which are varied and possess half the number of chromosomes (haploid) of the original cell. These cells are gametes, i.e. reproductive cells. - Meiosis is important in the production of **haploid** gametes. - The production of haploid gametes will result in the diploid number being restored when the gametes fuse at **fertilisation** to produce a zygote. **The process of Meiosis** Meiosis occurs as part of the cell cycle i.e. 1\. Interphase 2. Nuclear division (meiosis) 3. Cell division During late interphase, the following events occur: - DNA replication - Build-up of ATP - Protein synthesis and replication of cell organelles Then two successive nuclear divisions occur - i.e. meiosis 1 and meiosis 2, to produce 4 haploid, genetically different cells. - The first meiotic division separates of the members of each homologous pair. - The second meiotic division separates the chromatids of each chromosome. This is shown in the diagram.  **\ ** **Meiosis and Variation** An important feature of meiosis is the variation produced in the haploid cells (gametes). The following two processes in meiosis produce cells which are genetically different. 1. Independent segregation of homologous chromosomes 2. Genetic recombination by crossing over **1**. **Independent Segregation of Homologous Chromosomes** During the first meiotic division, homologous chromosomes pair together and then separate so that **one member from each pair** enters the gamete. The pairing and subsequent separation of the two members of a pair is completely independent from the separation of another pair. Therefore, the chromosomes randomly associate within a gamete as shown below. The diagram shows a cell with **a diploid number of 6** i.e. **3 homologous pairs of chromosomes.**  From 3 homologous pairs of chromosomes there are 8 **different** **combinations of paternal and maternal chromosomes** that can be produced following independent segregation (assortment). These combinations are shown below.   -- ------------------------ -- ------------------------   - Gametes produced from this cell will possess **one member from each original homologous pair** due to the independent segregation of homologous chromosomes in meiosis. - To calculate the possible number of varied combinations 2^n^ is used where n is the number of homologous pairs. In the above example: i.e. 3 pairs of homologous chromosomes → 2^3^ = 2x2x2 = 8 possible combinations. **2. Crossing over** This only occurs in meiosis during prophase of the first meiotic division. The two members of each homologous pair lie side by side forming a structure known as a **bivalent:** The chromatids of the homologous chromosomes then intertwine  Sometimes the chromatids break and equivalent portions of the chromatids are exchanged. This results in the **exchange of alleles** of the same genes and can produce **new combinations of alleles**, known as **recombinants**. The process is known as **genetic recombination**. The chromosomes then separate. The number of recombinants formed is relatively low as crossing over is relatively rare. ------------------------------------------- -- -------------------------------------------- ------------------------ --   **Parental gametes occur in high number** **Cross-over gametes occur in low number** ------------------------------------------- -- -------------------------------------------- ------------------------ -- **Possible Chromosome combinations** Homologous pairs of chromosomes line up along the equator of the cell during Meiosis I. Either of the pair can pass to either daughter cell during independent segregation. You can calculate the possible number of combinations of chromosomes using: 2^n^ where ^n^ = the number of pairs of homologous chromosomes. So an organism with 4 homologous pairs of chromosomes can produce 24 or 16 different possible combinations in the daughter cells. Variation is further increased by the random pairing of male and female and gametes. This can be shown mathematically by: (2^n^)^2^ where n = the number of homologous chromosomes. e.g. in an organism with 4 homologous pairs of chromosomes. (2^n)2^ = 256 different chromosome combinations. All the above calculations do not take into account the fact that variation is further increased by crossing over and recombination. **Gene Mutations** Gene mutations are changes in the **quantity of DNA** or **changes in the sequence of nucleotide bases** in the DNA resulting in the formation of a different polypeptide. This is due to the altered base sequence coding for a different sequence of amino acids. New alleles of genes are produced by mutations. **Causes of mutations** - Mutations may arise as a result of incorrect pairing during DNA replication - Mutations occur spontaneously naturally at random and the rate at which genes mutate varies from gene to gene. **Mutagenic** **agents** increase the frequency of mutations, e.g. e.g. X-rays, gamma rays, U.V. light, chemicals such as nitrous oxide, benzene. **Types of Gene mutations** There are several ways the sequence of nucleotide bases in a gene can be altered including substitution and deletion. **Substitution** Substitution is the replacement of one or more bases by one or more different bases. The substitution of **a single base** may result in: - a new triplet coding for a different amino acid in the polypeptide chain which **may** result in a **non-functional protein** being formed - **one** different amino acid in the polypeptide change but a **functional protein** is still produced - the same amino acid may be coded for due to the **degeneracy** of the DNA code e.g. CGA and CGG both code for the same amino acid, alanine so that the **polypeptide remains unchanged** - the formation of a **stop code** which terminates the polypeptide chain so that a non-functional protein is produced **Deletion** Deletion is the removal of one or more bases. - This results in a **frame shift**, which is the alteration in the codons from the point of deletion. - The sequence of amino acids is altered from the point of deletion and the protein formed is non-functional. ------------------------  ------------------------ **Chromosome mutations** Changes in structure or number of whole chromosomes is called a chromosome mutation. -Changes in whole sets of chromosomes occur when an organism has three or more sets of chromosomes rather than the usual two. This is called **polyploidy** and is mainly in plants. -Changes in the number of individual chromosomes. If homologous pairs of chromosomes fail to separate in meiosis, known as non-disjunction. Results in a gamete having one more or one less chromosome. This will result in the resultant offspring having one more or one less chromosome in all their cells. E.g. Downs Syndrome is the effect of and additional chromosome 21. **[DNA and Chromosomes Mastery Questions]** 1. Draw and label a nucleotide. 2. Convert the answer to question one into a dinucleotide. 3. What is a polynucleotide? 4. Which base pairs with cytosine? 5. Which base pairs with thymine? 6. Name the complementary base pairs and state how many hydrogen bonds hold the bases together. 7. Describe the DNA double helix. 8. What is the sugar-phosphate backbone? 9. If a gene had the following sequence, what would the complementary sequence be; 10. Describe eukaryotic DNA. 11. Describe prokaryotic DNA. 12. What is a gene? 13. Describe and explain the genetic code. 14. What is an intron? 15. What is an exon? 16. How does the order of bases affect the proteins produced by a cell? 17. What is an allele? 18. Why is DNA replication described as semi-conservative? 19. Describe conservative replication. 20. Fully describe the process of semi-conservative replication. 21. Describe and explain the evidence for semi-conservative replication. **[Protein Synthesis Mastery Questions]** 22. Describe DNA in Eukaryotes. 23. Describe DNA in Prokaryotes. 24. What is a gene? 25. What is an allele? 26. What is protein synthesis? Which organelle is needed? 27. Draw and label a DNA nucleotide 28. Where is nitrogen found in the nucleotide? 29. What bonds hold the DNA bases together? 30. What are the rules of base pairing? 31. Define 'codon' 32. Define Genome. 33. Define Proteome 34. What is transcription? 35. Where exactly does it take place? 36. What does mRNA do? 37. Compare mRNA and DNA. 38. What base differences are there between RNA and DNA? 39. Which enzyme is needed for transcription to take place? 40. Describe the process of transcription 41. How is mRNA edited in eukaryotic cells (introns and exons) 42. Where does translation take place? 43. Draw and label a tRNA molecule 44. If there are 20 different amino acids how many different types of tRNA are needed? 45. Distinguish between anticodon and codon 46. Describe the process of translation 47. What is a 'stop signal' 48. Why is the genetic code described as degenerate, universal and non overlapping? Discuss the significance