Mitosis and DNA Replication

• A1 BIOLOGY Unit 4 focuses on genetic information, variation, relationships between organisms, DNA, RNA, protein synthesis, and cell division.

DNA, Genes, and Chromosomes

• Prokaryotic cells contain short, circular DNA molecules not associated with proteins.
• Eukaryotic cells contain very long, linear DNA molecules associated with proteins called histones.
• A DNA molecule with its associated proteins forms a chromosome.
• Mitochondria and chloroplasts in eukaryotic cells also have short, circular DNA not associated with protein.
• A gene is a DNA base sequence that codes for a polypeptide's amino acid sequence or a functional RNA.
• A gene occupies a fixed position (locus) on a DNA molecule.
• A triplet, a sequence of three DNA bases, codes for a specific amino acid.
• The genetic code is universal, non-overlapping, and degenerate.
• Eukaryotic nuclear DNA includes non-coding multiple repeats between genes.
• Exons are sequences within a gene that code for amino acid sequences, while introns are non-coding sequences separating exons.

DNA and Protein Synthesis

• The genome is the complete set of genes in a cell.
• The proteome includes the full range of proteins a cell can produce.
• Transcription involves producing mRNA from DNA and requires RNA polymerase to join mRNA nucleotides.
• In prokaryotes, transcription directly creates mRNA from DNA.
• In eukaryotes, pre-mRNA is produced during transcription, then spliced to form mRNA.
• Translation involves ribosomes, tRNA, and ATP to produce polypeptides from the mRNA codon sequence.

Structure of DNA and RNA

• DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are information-carrying molecules.
• DNA holds genetic information, while RNA transfers it from DNA to ribosomes; ribosomes are made of RNA and proteins.
• DNA and RNA nucleotides consist of a pentose sugar, a nitrogen-containing organic base, and a phosphate group.
• DNA nucleotides contain deoxyribose, a phosphate group, and the organic bases adenine, cytosine, guanine, or thymine.
• RNA nucleotides contain ribose, a phosphate group, and the organic bases adenine, cytosine, guanine, or uracil.
• A phosphodiester bond forms via a condensation reaction between two nucleotides.

DNA Structure

• DNA comprises two polynucleotide chains forming a double helix, held by hydrogen bonds between complementary base pairs.
• RNA is a relatively short polynucleotide chain.
• Adenine always pairs with Thymine
• Guanine always pairs with Cytosine
• The simplicity of DNA's structure initially led scientists to doubt it carried the genetic code.
• Hydrogen bonds between the bases maintain DNA's stable structure.
• The DNA helix coils further into a super helix for compact storage of genetic information.
• Genes are DNA sections with coded information that determine the nature and development of organisms.
• DNA's ability to self-replicate via complementary base pairing is crucial for cell division.
• Mutations, alterations in the DNA base sequence, provide genetic diversity and are the basis for evolution.

DNA Replication

• Semi-conservative replication ensures genetic continuity between cell generations.
• DNA helicase unwinds the double helix and breaks hydrogen bonds between complementary bases.
• New DNA nucleotides are attracted to exposed bases on template strands, following base pairing rules.
• DNA polymerase joins adjacent nucleotides via a condensation reaction.

The Cell Cycle

• Eukaryotic cells that can divide undergo a cell cycle, with DNA replication during interphase.
• Mitosis produces two daughter cells with identical DNA copies from the parent cell after DNA replication.
• Mitosis phases include interphase, prophase, metaphase, anaphase, and telophase, with spindle fibers separating chromatids.
• Cytokinesis, the division of the cytoplasm, typically follows, producing two new cells.
• Interphase is the non-dividing cell stage where cell growth occurs.
• Mitosis produces cells with the same chromosome number as the parent cell, resulting in two identical diploid cells.
• The stages of mitosis are prophase, metaphase, anaphase, and telophase.
• Mitosis is a controlled process; uncontrolled division can lead to tumors and cancers.
• Treatments often control the rate of cell division to combat cancers.
• Binary fission in prokaryotes involves circular DNA and plasmid replication, followed by cytoplasm division.

Genetic Diversity

• Gene mutations, changes in the chromosome base sequence, can arise spontaneously during DNA replication (base deletion/substitution).
• The degenerate genetic code means not all base substitutions alter the encoded amino acid sequence.
• Mutagenic agents can increase the gene mutation rate.
• Mutations in chromosome number can occur via non-disjunction during meiosis.
• Meiosis produces genetically different daughter cells through two nuclear divisions, forming four haploid cells from a diploid parent cell.
• Independent segregation of homologous chromosomes also contributes to genetically different daughter cells.
• Crossing over between homologous chromosomes further increases genetic variation in daughter cells.

DNA vs RNA

• RNA differs from DNA:
• Ribose sugar instead of deoxyribose.
• Uracil replaces thymine.
• mRNA and tRNA are single-stranded, unlike double-stranded DNA.
• Double-stranded RNA (siRNA) also exists.
• Messenger RNA (mRNA): Linear molecule with codons (mRNA triplets), formed in the nucleus during transcription, involved in protein synthesis.
• Transfer RNA (tRNA): 'Clover leaf' shape due to hydrogen bonds, with an anticodon at one end and a binding site for a specific amino acid at the other.

DNA Replication Mechanism

• Enzymes called DNA helicase break the hydrogen bonds between two polynucleotide strands
• Each strand acts as a template for producing a new complementary strand.
• Individual DNA nucleotides align according to specific base pairing (A with T, C with G).
• DNA polymerase joins adjacent DNA nucleotides to form complementary strands through condensation reactions.
• Original and new polynucleotide strands are joined by hydrogen bonds.
• Each daughter molecule contains one original and one new polynucleotide strand resulting in identical daughter DNA molecules.

Meselson and Stahl Experiment on DNA Samples

• E. coli bacteria cells contain the heavy isotope 15N instead of the normal isotope 14N
• After the first division, each DNA molecule contained one strand of heavy 15N and one strand of normal 14N.
• In the second generation, 50% of cells contained DNA with one strand of each isotope, and 50% contained DNA with both strands of the normal 14N isotope.
• In the third generation, 25% had DNA with one strand of each isotope, and 75% had DNA with both strands of the normal 14N.

Genes and Non-Coding DNA

• Genes are sections of DNA with coded information determining organism development.
• Much of eukaryotic DNA is non-coding; it includes:
• Introns are base sequences within genes that do not code for amino acids but introns that do code for amino acids are called exons).
• Multiple Repeats are base sequences between genes.
• Alleles are different forms of a gene coding for different variations caused by mutations with different base sequences coding for different sequences of amino acids.

DNA and Chromosomes Makeup

• Eukaryotes contain linear DNA associated with histone proteins, within a nucleus surrounded by a nuclear membrane.
• Prokaryotes contain smaller, circular DNA not associated with histone proteins, without forming chromosomes.
• The genome is the complete set of genes, and the proteome is the range of proteins a cell can produce.
• Alleles of a gene are located at the same relative position (locus) on homologous chromosomes where a homologous pair of chromosomes are the same length and control characteristics.

Protein Synthesis

• A sequence of three nucleotide bases codes for one amino acid where the sequence is known as a base triplet that determine the polypeptide base sequence.
• Base triplets in mRNA are codons.
• Most amino acids have more than one codon.
• The start of a DNA sequence coding for a polypeptide is always the same triplet and three codons do not code for specific amino acids are known as stop codons which mark the polypeptide chain.
• A universal code, where the same amino acid sequences across all organisms in order to determine relationships and evolutionary information.

Transcription Process

• Involves rewriting part of the DNA code into a strand of messenger RNA
• Occurs in the nucleus
• A relevant section of the molecule splits through DNA helicase breaking the strands

Translation

• The sequence of codons on the mRNA strand is used to determine the sequence of amino acids in a polypeptide.
• 20 different types of tRNA exist
• Each tRNA has three exposed bases known as an anticodon.
• New tRNA molecules with complementary anticodons can be attached to other tRNA
• A polypetide folds into a secondary or tertiary structure
• mRNA is transcribed from a polypeptide.

Cell Division: Mitosis

• Mitosis produces cells that are genetically identical for growth, repair, and during the division of the other nuclear material, the material becomes structured into chromosones.
• The total number of chromosones is represented by diploid , while a single set of chromosomes is represented by haploid.
• The phases of mitosis include interphase, prophase, metaphase, anaphase, and telophase along with nucleur and cell division.
• Interphase often represents the longest time in the cycle.
• Prophase: Sister chromatids join together by a centromere and then thicken.
• Metaphase: The centromeres form a spindle across the cells.
• Anaphase: The centromere spits and the sister chromatids split.
• Telophase: The chromatids are at opposite poles in the cell.

Mitosis Summary

• Spindle Fibres and DNA Polymerase are active during Prophase.
• DNA Replication and cell organelles are replicated during interphase
• Sister chromatids pull apart and move to opposite poles during anaphase.
• Chromosones shorten and thicken in metaphase.
• Centrioles move to the poles during telophase.

Cancers

• Cancers are diseases from uncontrolled growth and rapid division from mitosis regulated injuries.
• Cell Division can stop the spread of cancer.
• Prokaryotic division occurs during asexual reproduction known as binary fisson, rather than mitosis.
• Circular Molecules replicated through the cell membrane start to grow between the two DNA molecules where original cells grow to identical daughter cells.

Cell Division: Meiosis

• A type of asexual division produces genetically different cells for gamates.
• Halving chromosones is called haploid cells.
• Chromosomes duplicate through DNA Replication in two pairs to form chromatids.
• Non-disjunction seperation increases risk of less number of genes or more in an offspring where Downs Syndrom is the additional chromosone 21 trait.
• Gamates are produced from a cell which will prosess one member from an orginal homologuous trait.
• To calculate the possibilities , times the number of chromosomes of traits with each other, using 2" where N = number of chromosonal pairs.
• DNA Replication mutations can arise and cause incorrect pairings of mutation due to other genes mutating causing more mutations
• Mutagens such as X-rays and Gama Rays cause mutations.

Explore the vital processes of mitosis and DNA replication. Understand RNA's function, DNA/RNA nucleotide differences, and bond formation in DNA/RNA strands. Learn about DNA replication enzymes, semi-conservative replication, and the significance of mitosis phases.