biology module 2 nucleotides and nucleic acids

what are nucleotides

monomers from which DNA and RNA are built.

structure of a nucleotide

• pentose sugar

• nitrogenous base

• a phosphate base

difference between a DNA and RNA nucleotide

DNA has a deoxyribose sugar while RNA has a ribose sugar

what are the two structural forms of nitrogenous base molecules

• purines

• pyrimidines

what is the structural form of adenine and guanine

purines - double ring structure

what is the structural form of cytosine, thymine, and uracil

pyrimidines - single ring structure

how are separate nucleotides joined together

via condensation reactions - the reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide

what does a condensation reaction between nucleotides form

a phosphodiester bond

what is ATP

a nucleic acid that is the energy carrying molecule that provides energy to drive many processes inside living cells

structure of DNA

• made up of 2 polynucleotide strands lying side to side (anti parallel)

• each polynucleotide strand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone (phosphodiester bonds - covalent bonds)

phosphodiester bonds in DNA

they link the 5 carbon of one deoxyribose sugar molecule to the phosphate group from the same molecule which is linked by another phosphodiester bond to the 3 carbon of the deoxyribose sugar molecule of the next mole in the strand

DNA polynucleotide strands

each strand is said to have a 3’ end and a 5’ end. as the strand are anti parallel, one is known as the 5’ to 3’ strand and one is known as the 3’ to 5’ strand

how are the 2 anti parallel strands held together

by hydrogen bonds between the nitrogenous bases

why is DNA referred to as a double helix

its the 3d shape formed by the twisting of the DNA molecule

why does a cell need to copy its DNA before it divides

to ensure the two new daughter cells will both receive a complete set of genetic information

how is dna copied

semi conservative replication

process of semi conservative replication

• occurs during s phase of cell cycle

• DNA helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two anti parallel strands

• free nucleotides are attracted to the exposes bases

• these free nucleotides contain extra phosphates

• adjacent nucleotides are joined together by DNA polymerase to form a new sugar - phosphate backbone

• polymerase catalyses condensation reactions and forms a phosphodiester bond

• hydrogen bonds form between complementary bases to form the ladder of the DNA double helix

why is it important to conserve one DNA strand

retaining one original strand maximises accuracy

each new cell will contain the same genetic information as parent, so genetic continuity is ensured between generations of cells

what is a gene

a sequence of nucleotides that forms part of a DNA molecule

what is the triplet code

the sequence of DNA nucleotide bases is determined by a triplet code. each sequence of 3 bases codes for one aminoe acid

what are start and stop signals

some triplets of bases code for start and stop signals, these tell the cell where individual bases stop and start.

what does it mean when we say the genetic code is non overlapping

each base is only read once in which codon it is part of

what does it mean when we say the genetic code is universal

almost every organism uses the same code

what is a codon and anticodon

• codon - each triplet within the mrna code

• the trna molecules possess anti codons that are complementary to codons on mrna

what are the two stages of protein synthesis

• transcription (dna transcribed and mrna is produced0

• translation (mrna translated and an amino acid sequence is produced)

where does transcription take place

the nucleus

the stages of transcription

• part of a DNA molecule unwinds (hydrogen bonds break)

• the exposed gene can be transcribed

• a complementary copy of the code from the gene is made by building a single stranded nucleic acid molecule (mRNA)

• free RNA molecules pair up with their complementary bases on one strand of the unzipped DNA molecule

• the sugar phosphate groups of these RNA nucleotides are bonded together by the enzyme RNA polymerase to form the sugar - phosphate backbone of the mRNA molecule

• when the gene has been transcribed, the hydrogen bonds between the mRNA and DNA strands break and the double stranded DNA molecule reforms

• the mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

where does translation occur

in the cytoplasm

stages of translation

• after leaving the nucleus, the mRNA molecule attaches to a ribosome

• in the cytoplasm, there are free molecules of tRNA

• these tRNA molecules have a triplet of unpaired bases at one end and a region where a specific amino acid can attach at the other

• the tRNA molecules bind with their specific amino acids and bring them to the mRNA molecule on the ribosome

• the triplet of bases on each tRNA molecule pairs with a complementary triplet on the mRNA molecule

• two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid theyre carrying side by side

• a peptide bond is formed between the two amino acids

• this reaction is catalysed by the rRNA subunits of the ribosomes

• the process continues until a stop codon on the mRNA molecule is reached

• the amino acid chain then forms the final polypeptode