What bond joins the pentose sugar to the nitrogenous base?
A glycosidic bond
Which carbon of the sugar is the phosphate group joined to?
5 or 3
What is a phosphodiester bond?
The linkage between the 5’ carbon of one pentose sugar and the 3’ carbon of another pentose sugar. The hydroxyl group on the phosphate group on 5’ carbon of one nucleotide lines up the hydroxyl group on the 3’ carbon on the adjacent deoxyribose sugar, a water molecule is produced as a condensation reaction occurs
On the non-rotated chain of the DNA molecule in which direction does the chain run?
5’ to 3’
On the rotated strand of the DNA molecule, which way does the chain run?
3’ to 5’
What are the monomers for nucleic acids?
Nucleotides
When does a nucleotide become phosphorylated?
When it contains more than one phosphate group
What are the functions of nucleotides?
- Act as monomers for nucleic acids
- Help regulate many metabolic pathways (cAMP)
- Component of many coenzymes, adenine nucleotides are the components of NADP and NAD and FAD, coenzymes used in photosynthesis and respiration respectively
Describe the structure of DNA?
- DNA is comprised of two strands each running parallel to each other but in opposite directions, hence the structure f DNA is referred to as anti-parallel
- Each DNA nucleotide is composed of a nitrogenous base, phosphate group and a deoxyribose sugar
- Phosphodiester bonds link adjacent deoxyribose sugars together, the phosphate group is attached to the 5’ carbon on one sugar and the 3’ carbon on another sugar
- DNA molecules are long and therefore can carry a lot of encoded genetic information
- Hydrogen bonds are formed between complementary nitrogenous bases opposite each other on the strands of DNA
How is DNA organised in eukaryotic cells?
- The majority of DNA, or the genome, is in the nucleus
- Each large molecule of DNA is wound tightly around histone proteins into chromosomes, each chromosome is therefore one molecule of DNA
- There is a loop of DNA in the mitochondria and the chloroplasts without the histone proteins, therefore DNA doesn’t exist as chromosomes in the mitochondria and the chloroplasts
How is DNA organised in prokaryotes?
- DNA is in a loop and is within the cytoplasm, it is not enclosed in a nucleus, prokaryotes don’t have a nucleus
- It is not wound around histone proteins and therefore doesn’t exist as chromosomes in prokaryotes
- Viruses that contain DNA have it in a loop of naked DNA
What is the function of DNA?
The genome within every cell of an organism contains the coded instructions to make and maintain that organism
Why does DNA need to be replicated?
So that each new daughter cell receives the full set of instructions for making and maintaining the organism
Describe the process of DNA replication
- DNA molecule unwinds, the double helix untwists, this is catalysed by a gyrase enzyme
- The DNA unzips, the hydrogen bonds are broken between the nucleotide bases, this is catalysed by DNA helicase and results in two single strands of DNA with unpaired nucleotide bases
- Free phosphorylated nucleotides present in the nucleoplasm are complementarily bonded with the exposed bases
- DNA polymerase catalyses the addition of the new nucleotides in the 5’ to 3’ direction
- The leading strand which is the 3’ to 5’ strand is synthesised continuously whereas the lagging strand which is the 5’ to 3’ strand is synthesised in fragments which have to be joined together in reactions catalysed by ligase enzymes
- The free nucleotides are triphosphate nucleotides, these need to be hydrolysed into monophosphate nucleotides so they can be added to the strand. This releases energy that is used to from phosphodiester bonds that link the 5’ carbon on one nucleotide and the 3’ carbon on the next carbon
- 2 new DNA molecules are produced that are identical to each other and the parent molecule
- Each DNA molecule produced contains one new strand and one original strand, this is semi-conservative replication
What is the product of DNA replication?
2 DNA molecules each identical to each other and the parent molecule that they were replicated from
Why is DNA replication described as semi-conservative?
Each new DNA molecule contains one new strand and one original strand
What were the three theories for DNA replication in the 1950s?
Conservative- original molecule acts as a template and a new molecule is formed
Semi-conservative- new molecule consists of one newly formed strand and one original strand
Dispersive- Each molecule breaks up into nucleotides and each one joins to a complementary nucleotide and a new strand is formed
How does DNA replication occur in prokaryotes?
A bubble sprouts out from the loop and unwinds and unzips, then complementary nucleotides join to the exposed nucleotides and the loop is eventually copied
Why is the 3’ to 5’ the leading strand and the 5’ to 3’ the lagging strand
- The 3’ to 5’ strand can unwind and have free nucleotides added continuously as nucleotides are added in the 5’ to 3’ direction to make the strands anti-parallel which is essential to allow the bases to be in the right orientation to form hydrogen bonds. DNA polymerase adds bases in the 5’ to 3’ direction which means that new nucleotides can be added as the strand unwinds
- The 5’ to 3’ strand has to have its complementary strand running in the 3’ to 5’ direction to make the strands anti-parallel, this means that DNA polymerase has to add the new nucleotides from bottom to top as the complementary strand runs from 3’ to 5’. This means that as the strand unwinds, DNA polymerase adds the nucleotides in fragments as part of the DNA strand unwinds, the nucleotides are added from the bottom of the unwound section of the strand to the top. Once these have been added the next part of the strand unwinds and nucleotides are added from the bottom of the unwound section to the top again. This leaves unjoined fragments that have to be joined together, catalysed by ligase enzymes
Which enzymes are involved in DNA replication?
Gyrase enzyme- catalyses the unwinding of the DNA strands
DNA helicase- catalyses the breaking of the hydrogen bonds between nucleotide bases
DNA polymerase- catalyses the addition of new nucleotide bases in the 5’ to 3’ direction to the single strands of DNA, using each of these single strands as a template
Ligase enzymes that catalyse the joining of fragments in the DNA molecule containing the lagging strand
How is an RNA molecule structurally different to DNA?
- It has a ribose sugar instead of a deoxyribose sugar, this has an OH on carbon 2 instead of an H
- It has the nitrogenous base uracil instead of thymine
- RNA is single stranded
- There are three forms of RNA: mRNA, tRNA, rRNA
What is a gene?
A specific length of DNA on a chromosome, each gene codes for a protein
What percentage of an organism’s dry mass is protein?
75%
How should amino acids be arranged in a channel protein?
Hydrophilic amino acids should be on the inside of the channel protein so that the ion that is transporting can pass through as the ion will be aqueous. It needs to have lipophilic (hydrophobic) amino acids on the outside of the channel protein, this enables the channel protein to embed itself into the membrane
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Membranes52
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Cell Structure67
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Key Concepts11
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Enzymes55
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Biological Molecules105
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Practical Skills16
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Communicable Diseases81
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Cell Division49
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Transport in Plants24
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Photosynthesis23
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Respiration28
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Excretion88
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Nucleic Acids41
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Cellular control33
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Communication and Homeostasis39
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Exchange Surfaces and Breathing47
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Transport in Animals82
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Transport in Plants66
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Patterns of Inheritance12
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Hormonal Communication49
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Neuronal Communication3
