nucleic acids

  • Created by: Abi9ai1
  • Created on: 04-09-19 13:33

Nucleotides

  • Form the monomers of nucleic acid
  • Become phosphorolated nucleotides when they contain more than one phosphate group
  • Help regulate metabolic pathways
  • May be components of many coenzymes

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What is DNA?

  • DNA is found in the nuclei of all eukaryotic cells, within the cytoplasm of prokaryotic cells and is also inside some viruses.
  • It is hereditary material and carries coded instructions used in the development and functioning of all known living organisms.
  • DNA is one of the important macromolecules that make up the structure of living organisms, the others being proteins, carbohydrates and lipids. 
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Structure of DNA

  • DNA is a polymer, its monomer is nucleotides.
  • A molecule of DNA consists of 2 polynucleotide strands
  • The 2 stands run in opposits directions so the are antiparallel
  • Each DNA nucleotide has a phosphate group, deoxyribose and one of 4 nitrogenous bases
  • Adenine, guanine, thyamine or cytosine
  • The covalent bonds between these molecules are called phosphodiester bonds
  • These bonds are brokent when polynucleotides are broken; they are formed when polynucleotides are synthesised.
  • DNA molecules are long so they carry a lot of encoded genetic infomation
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The importance of hydrogen bonds

The 2 antiparallel DNA strands are joined to eachother by hydrogen bonds between nitrogenous bases.

  • Adenine always pairs with thyamine, using 2 hydrogen bonds
  • Guanine always pairs with cytosine , using 3 hydrogen bonds
  • A purine (adenine or guanine) always pairs with a pyrimidine (thyamine or cytosine), giving eval sized "rungs" on the DNA laddder. These can then twist into the double helix, giving the molecule stability.
  • Hydrogen bonds allow the molecule to unzip for transcriptionn and replication.
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How DNA is organised in cells

Eukaryotic

  • The majority of DNA content/ genome is in the nucleus.
  • Each large molecule is tighly wound around special histone proteins into chromosomes.
  • Each chromosome is one molecule of DNA.
  • There is also a loop of DNA, without the histone proteins, inside mitochondria and chloroplasts.

Prokaryotic

  • DNA is in a loop and is in the cytoplasm, not in a nucleus.
  • It is not wound around histone proteins, ita described as "naked".

Viruses that contain DNA have a loop of naked DNA

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DNA definitions

Double helix - Shape of DNA molecule, due to coiling of the two sugar phosphate backbone strand into a right handed spiral configuration.

Monomer- Molecule that when repeated makes up a polymer.

Nucleotide- Molecule consisting of a 5 carbon sugar, a phosphate group and a nitrogenous base.

Polynucleotide- Large molecule containing many nucleotides.

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How DNA replicates definitions

DNA polymerase- enzyme that catalyses formation of DNA from activated deoxyribose nucleotides, using single-stranded DNA as a template.

Helicase- Enzyme that catalyses the breaking of hydrogen bonds between the nitrogenous pairs of bases in a DNA molecule.

Semi-conservative replication- How DNA replicates, resulting in 2 new molecules, each of which contains one old strand and one new strand. One old strand is conserved in each new molecule.

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DNA is a self-replicating molecule

  • All the DNA within a cell (genome), and within every cell of the organism, carries the coded instructions to maintain the organism.
  • Every time a cell divides, the DNA has to be copied so each new daughter cell receives the full set of instructions
  • Each molecule replicates; this takes place during interphase, meaning that the copy is identical.
  • At first they joined together, at the centromere, forming 2 sister chromatids.
  • The DNA in the mitochondria and chloroplasts also replicates each time the organelles divide, just before the cell divides. 
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Semi-conservative replication

To make a new copy of itself, each DNA molecule:

  • Unwinds- the double helix is untwisted, a bit at a time, catalysed by a gyrase enzyme.
  • Unzips- hydrogen bonds between the nucleotide bases are broken. This is catalysed by DNA helicase, and results in 2 single strands of DNA with exposed nucleotide bases. 

Next:

  • Free phosphorylated nucleotides, present in the nucleoplam in the nucleus, bond to the exposed bases, following complementary base pairing rules.
  • DNA polymerase catalyses the addition of the new nucleotide bases.
  • The leading strand is synthesised continuously, whereas the lagging strand is in fragments that are later joined, catalysed by ligase enzymes.

The product is 2 moleculaes that are identical. Each strand has an old strand and a new strand, this is why it is called semi-conservative replication.

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Mutations

  • During DNA replications there is a 1 in one hundred million chance that errors may occur and the wrong DNA nucleotide is inserted. This could change the genetic code and results in a point mutation.
  • During the replication process there are enzymes that can proof read and edit out incorrect nucleotides, reducing the number of mutations.
  • However, many genes have changes to their nucleotide sequence; these are called alleles.
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How DNA codes for polypeptides definitions

Gene- a length of DNA that codes for a polypeptide or for a length of RNA that is involved in regulating gene expression.

Polypeptide- A polymer made of many amino acid units joined together by peptide bonds.

Protein- A large polypeptide

Transcription- the process of making messenger RNA from a DNA template.

Translation- Formation of a protein, at ribosomes, by assembling amino acids into paticular sequence according to the coded instrustions carried from DNA to the ribosome by mRNA.

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RNA

  • The sugar molecule in each nucleotide is ribose
  • The nitrogenous base is uracil
  • The polynucleotide chain is usually single-stranded
  • The polynucleotide chain is dhorter than DNA
  • There are 3 forms of RNS: messenger (mRNA), transfer (tRNA), ribosomal (rRNA).
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Genes

  • Each chromosome in a eukaryotic nucleus has a molecule of DNA. Each chromosome has specific lengths of the DNA called genes. Each gene contains a code that determines the sequence of amino acids in a polypeptide or protein.
  • Within each gene there is a sequence of DNA base triplets that determines the amino acid sequence (primary structure) of a polypeptide. If the primary structure of a polypeptide is correct, it will fold correctly and be held in it's tertiary structure, enabling it to carry out its function.
  • Genes are inside the cell nucleus but proteins are made in the cytoplasm, at ribosomes.
  • As the instructions inside the genes, on a chromosomes, cannot pass out of a nucleus, a copy of the gene has to be transcribed (copied) into a length of mRNA. In this form, the sequence of base triplets (codons) can pass out of the nucleus to the ribosome, ensuring that the coded instructions are translated and the protein is assembled correctly from amino acids.
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Genetic code

  • The gentic code is near universal, because in almost all living organisms the same triplet of DNA bases codes for the same amino acid.
  • The genetic code is degenerate because for all, except methionine and tryptophan, there is more than one base triplet. 
  • This may reduce the effect of point mutations, because a change in one base triplet could produce another base triplet thst still codes for the same amino acid.
  • The genetic code is also non-overlapping,and it is read starting from a fixed point. So if a base is added it causes a frame shift, meaning that every amino acid coded for is changed. 
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Transcription

  • A gene unwinds and unzips.
  • Hydrogen bonds between complementary nucleotide bases break.
  • RNA polymerase catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpaired DNA base.
  • A bonds with T, C pairs with G, G pairs with C and U pairs with A on one strand of the unwound DNA. This is called the template strand.
  • A length of RNA that is complimentary to the template strand of the gene is produced. It is therefore a copy of the DNA strand - the coding strand
  • The mRNA now passes out of the nucleus, through the nuclear envelope and attaches to a ribosome.
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Translation

  • Transfer RNA molecules brings the amino acids and find their place where the anticodon binds by temporary hydrogen bonds to the complementary codon on the RNA molecule.
  • As the ribosome moves along the length of mRNA, it reads the code, and when 2 amino acids are adjacent to eachother a peptide bond forms between them.
  • ATP is needed for protein synthesis.
  • The amino acid sequence for the polypeptide is therefore determined by the sequence of bases on the length of DNA - the gene
  • After the polypeptide has been assembled, the mRNA breaks down and its components are recycled into new mRNA, with different codon sequences.
  • The newly synthesised polypeptide is helped, by chaperone proteins in the cell, to fold correctly into its tertiary structure (3D shape), in order to carry out its function.
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