Stage 2 Biology 1.1 DNA Structure 2025 PDF

Stage 2 Biology 1.1 DNA Structure Slides Green boxes - an indication of the key points. This does not mean anything outside of the box is irrelevant. You should be writing your notes in your own words. Questions! Always be trying to answer questions. Science Understanding DNA stores and transmi...

Stage 2 Biology 1.1 DNA Structure Slides Green boxes - an indication of the key points. This does not mean anything outside of the box is irrelevant. You should be writing your notes in your own words. Questions! Always be trying to answer questions. Science Understanding DNA stores and transmits genetic information; it functions in the same way in all living things. DNA is a helical double-stranded molecule. DNA is unbound in and circular in the cytosol of prokaryotes and in the mitochondria and chloroplasts of eukaryotes. Content Checklist Compare chromosomes in prokaryotes and eukaryotes. - Replication of DNA allows for genetic information to be inherited. - Base-pairing rules and method of DNA replication are universal. Describe the structural properties of the DNA molecule: - Nucleotide composition and pairing. - The weak bonds between strands of DNA that allow for replication. Explain the importance of complementary base pairing (A-T, C-G). Recognise that DNA strands are direction are read 5’ to 3’. Describe and represent the process of semi-conservative replication in DNA.  Nucleotides Nucleotides are monomers containing: Phosphate group Deoxyribose/ribose sugar Nitrogenous base They are held together by covalent bonds. Nucleotides form polynucleotides (many nucleotides) which results in a nucleic acid being formed, DNA or RNA. Nucleic Acids Nucleic acids are macromolecules that store and transmit the genetic information that determines the structure and function of proteins. Nucleic acids are made from chains of smaller repeating units called nucleotides. DNA & RNA are different types of nucleic acids. This means they are made up of nucleotides with a different composition Proteins We will discuss proteins in more detail later but for now: Proteins are macromolecules that carry out life processes (hormones, enzymes). DNA stores the genetic information RNA builds the proteins based on the stored genetic information Proteins are the functional unit to carry out life processes.  DNA - prokaryotes and eukaryotes DNA is a macromolecule found in all living cells. In eukaryotes, DNA is located in the nucleus. In prokaryotes, DNA is located in the cytosol - fluid part of the cell. This is referred to as the nucleoid region. Both chloroplasts and mitochondria contain DNA. This is because they were once ‘free-living’ unicellular organisms. In your book, complete the table from page 4 (1.1). DNA - prokaryotes and eukaryotes Prokaryotic DNA Mitochondria/Chloroplast Eukaryotic DNA DNA # DNA Strands Two Two Two # of chromosomes One One 2 or more in pairs Shape of Circular Circular Linear chromosomes Location in cell Nucleoid region Mitochondria matrix, stroma Nucleus  DNA - structure & function DNA is a double stranded molecule consisting of repeating nucleotides result in a double helix structure. DNA stores and transmits genetic information. The double helical structure of DNA allows for it to: Make identical copies of itself so that genetic information can passed from cell to cell and be inherited. Provide a code that can be used to manufacture protein molecules. 1. Name the pentose sugar that makes up DNA. 2. Name the four nitrogenous bases found in DNA.  DNA - structure & function (cont) The phosphate of one nucleotide is attached to the sugar of the next nucleotide. This results in a ‘backbone’ of alternating phosphates and sugars. 5’ strand of DNA ending with a phosphate (PO43-) 3‘ strand of DNA ending with a sugar molecule that has a hydroxyl group (OH). This gives the DNA directionality. DNA is read from 5’ to 3’. (5’/3’ = prime). The phosphate is negatively charged giving DNA an overall negative charge. This can help separate DNA molecules. DNA - structure & function (cont)  DNA - structure & function (cont) The end of the DNA strand with the phosphate (PO43-) is the 5’ end. The end with the sugar molecule exposed with a hydroxyl group is the 3’ end.  DNA - structure & function (cont) Each base forms weak hydrogen bonds with its complementary base on the opposing strand. Weak bonds are beneficial as they are easy to break and reform. This is critical in DNA replication due to the breaking of these bonds. A = T (two hydrogen bonds). C ≡ G (three hydrogen bonds). This is referred to as complementary base pairing. Chromosomes Chromosomes DNA in cells is found in structures called chromosomes. Chromosomes are the structural unit of information in cells as DNA stores and transmits genetic information. The number of centromeres refers to the number of chromosomes. Chromosomes vs chromatid Chromosomes are the structural unit containing DNA and proteins (hereditary information). When DNA divides, a chromosome makes a copy of itself (another DNA molecule). The chromosome is made up of two identical DNA molecules called sister chromatids.  Chromosomes & DNA - prokaryotes Prokaryotes are unspecialised cells and do not contain membrane bound organelles. Key features of prokaryotes: DNA is located in the cytosol. DNA is circular, consisting of one double stranded DNA. This means the DNA is continuous with no ends. The cytosol contains proteins and RNA. Contains plasmids which are small, circular pieces of DNA separate to the cell’s chromosomal DNA. Because their DNA is circular, they lack telomeres.  Chromosomes & DNA - eukaryotes In eukaryotes, the double stranded DNA molecule is bound to histone proteins into condensed linear chromosomes found in the nucleus. Linear means that each chromosome has two ends. Telomeres are short lengths of DNA that ‘cap’ the tip of each end of the chromosome with protects them from breaking down or binding to one another. Chromosomes are only visible under a light microscope during cell division - this means the DNA is coiled around histone proteins (condensed).  Chromosomes & DNA - eukaryotes (cont) When chromosomes are decondensed it is referred to as chromatin. This is not visible under a light microscope. Chromatin is present in the nucleus throughout the cell cycle (with the exception of cell division). Chromatin is critical as it makes the genetic code accessible during growth and development. When cell division occurs, the tightly coiled chromosomes protects the code to ensure exact copies are made. Cell nucleus  Chromosomes - Mitochondria & Chloroplasts Two membrane bound organelles in eukaryotic cells contain DNA, mitochondria and chloroplasts. mtDNA = mitochondria cpDNA = chloroplasts The DNA is double stranded, circular and not bound to histone proteins. Quiz - recall the table you completed earlier Prokaryotic DNA Mitochondria/Chloroplast Eukaryotic DNA DNA # DNA Strands # of chromosomes Shape of chromosomes Location in cell  Chromosomes - genes Chromosomes carry genetic information in the form of genes. Genes: Consist of unique segments of DNA with a specific sequence of bases that code for an RNA molecule or protein. Are inheritable factors that control specific characteristics (e.g. blood type) have a specific location on a particular chromosome (locus). can have more than one alternative expression for a given characteristic (allele)  Summary Essentially, DNA & chromosomes are universal in all living organisms - the structure of DNA allows it to: Make identical copies of itself Provide a code that can be used by cells to synthesise proteins Therefore, store & transmit genetic information DNA differs between species in: 1. The number of chromosomes found in cells (Humans – 46, Koala – 16, Sunflower – 34). 2. The length of DNA molecules and the number of bases 3. The base sequences of genes found of the DNA 4. DNA stores genetic information in the number of bases, sequence and types of organic bases.  DNA Replication DNA Replication Genetic information can be inherited by daughter cells and descendants. This is enabled by DNA replication. DNA is the only molecule capable of producing an identical copy of itself because each single strand stores the information necessary to make a complementary strand following the base pairing rule. DNA Replication location: Prokaryotes: nucleoid region. Eukaryotes: nucleus. DNA replication is semi-conservative because each double strand of DNA consists of one old strand (parent DNA) and a newly synthesised strand. DNA Replication - enzymes involved Helicases: unwind the double helix Primases: initiate replication Polymerases: catalyse the synthesis of the new complementary DNA strands – adding new nucleotides to the growing DNA strand (Note: DNA polymerase cannot initiate – only add – and only in 5’ → 3’ direction) Ligases: joins DNA fragments together How does DNA Replication occur? https://www.youtube.com/watch?v=TNKWgcFPHqw P.7 - 8 – Note the steps of DNA replication or annotate a diagram. Complete the table from page 7 of Essentials summarising DNA replication. Leading Strand Lagging Strand Strand synthesized in the same direction as the Strand synthesized in the opposite direction as the movement of the replication fork (towards) movement of the replication fork (away from) Synthesised continuously (5’ → 3’) Synthesised discontinuously - in fragments, Okazaki (5’ → 3’) Requires only one primer to initiate replication Each fragment requires a separate RNA primer to initiate DNA ligase is not required DNA ligase is required to join the fragments DNA - aftermath Following DNA replication, the DNA is doubled and all chromosomes in the cell consists of two sister chromatids. DNA can be in three forms: Decondensed chromatin Condensed chromosome After DNA replication as two identical sister chromatids The number of times replication can occur is limited because after each replication process, the ends of the telomeres shorten to the point that they are too short. DNA replication occurs prior to cell division (mitosis or binary fission). DNA Replication Karyotype and genome A genome is used to describe the total number of genes of an organism. In 2001, after ten years of research, scientists sequenced every gene across all 46 chromosomes of a human. Humans carry roughly 21,000 genes. Karyotype and genome Karyotype and genome  CONCEPT CHECK Answer the following questions.