8a- Nucleic Acids

What are DNA + RNA?

• important information-carrying molecules

• both are polymers of the biological molecule group called nucleic acids (phosphate group in the nucleus)

What is the difference between DNA + RNA?

DNA:

• stores genetic information

RNA:

• transfers genetic information from DNA to ribosomes

What are DNA + RNA polymers of?

nucleotides

What is a nucleotide made up of?

• a pentose sugar

• a nitrogen-containing organic base

• a phosphate group (compromising a phosphate ion)

Why is a pentose sugar so called?

contains 5 carbons

What is a nucleotide?

• monomers that join to form polynucleotide strands

• chain held together because phosphate group is linked to the sugar of the next nucleotide by strong covalent bonds

What bond forms in a nucleotide chain?

phosphodiester bonds

What is the sugar-phosphate backbone?

the phosphate group and sugar are identical throughout the chain

What type of reaction is used to join the nucleotides together?

condensation

What is the only way in which 1 polynucleotide chain can differ from another?

the sequence of nucleotides/ bases in the polynucleotide chain

What is the pentose sugar of a DNA nucleotide called?

deoxyribose

What are the components of a DNA nucleotide?

• deoxyribose

• a nitrogen-containing organic base

• a phosphate group

What are the 4 bases in DNA nucleotides?

• adenine

• guanine

• cytosine

• thymine

What are purines?

adenine and guanine

What are pyrimidines?

thymine and cytosine

What is DNA?

two polynucleotide chains/ strands that twist to form a double helix

> the bases of each strand are held together by hydrogen bonding

Which bases are always paired together?

Adenine + Thymine

Cytosine + Guanine

What term is used to describe the pairing bases?

complementary base pairs

How many bonds does adenine form with thymine?

2

How many bonds does cytosine form with guanine?

3

Some viruses have single stranded DNA. How could an analysis of a piece of DNA tell you if it was single stranded?

• there wouldn’t be equal proportions of A+T or G+C

• there wouldn’t be an equal number of purines and pyrimidines

How does the structure of DNA relate to its function?

• Stable as it is a double helix and there are many H bonds

• Strong due to covalent bonds in the sugar-phosphate backbone

• Can replicate due to complementary base pairing

• Can separate due to weak H bonds

• Compact

• Contain large amounts of coded information as the chain is long

• Allows DNA to fit inside nucleus as it has a double helix shape

• Precise genetic code determined by sequence of bases which controls protein synthesis

How many different bases are there in DNA?

4

How many different amino acids are there in proteins?

20

What is the role of DNA?

• contains the genetic code

• codes for production of proteins

• provides diversity and variety

What is the structure of RNA?

relatively short, single stranded polynucleotide

What is the pentose sugar called in RNA?

ribose

What are the components of RNA nucleotides?

• a ribose

• a nitrogen-containing organic base

• a phosphate group

What are the bases in RNA?

• adenine

• guanine

• cytosine

• uracil

Which base is not found in RNA?

thymine

Is uracil a purine or pyrimidine base?

pyrimidine

What is DNA replication?

new cells have the same genetic code as the parent cells

• cells divide and their DNA is copied

What is the process called for DNA replication?

semi-conservative replication

Why is DNA replication termed as semi-conservative?

• 2 strands of the DNA molecule are separated

• Both strands of the DNA molecule act as a template for the formation of a new complementary strand

• Following replication each new DNA molecule consists of one original (old) strand and one ‘new’ strand

Describe semi-conservative replication

• Enzyme called DNA helicase is required to unwind the DNA double helix by breaking weak hydrogen bonds between complementary bases in the polynucleotide strands. This separates the strands

• Each exposed strand now acts as a template for the formation of a new strand

• New DNA nucleotides are attracted to exposed bases on the template strands and attach by complementary base pairing. New hydrogen bonds form

• Ezyme DNA polymerase joins the new nucleotides together to form a new polynucleotide strand. Phosphodiester bonds are formed during condensation reactions

What is some evidence for semi-conservative replication?

Experiments by Meselson and Stahl in 1958

• used bacteria E.coli grown in a medium containing heavy isotope (15N)

• then transferred to a medium containing normal light isotope of nitrogen (14N) and allowed to grow

• After periods of time, samples were taken and the DNA was extracted and centrifuged

Summarise the generations from the E.coli experiment

• Generation 0= DNA all heavy

• Generation 1= 2 hybrid molecules of DNA

• Generation 2= 2 hybrid molecules and 2 all new light DNA (50% hybrid, 50% light)

• Generation 3= 2 hybrid molecules and 6 all new light DNA (25% hybrid, 75% light)

What is the full name of ATP?

Adenosine triphosphate

What is ATP?

the immediate source of energy

What do ATP molecules consist of?

• the organic base adenine

• ribose sugar

• 3 phosphate groups (ions)

W

What does ATP look like?

Describe the synthesis of ATP

• synthesized by the addition of ADP to an inorganic phosphate

• condensation reaction catalysed by ATP synthase

• requires input of energy from a metabolic process and occurs during respiration

What is phosphorylation?

the addition of a phosphate molecule

Describe the breakdown of ATP

• the covalent bond linking the phosphate groups is unstable and easily broken by the enzyme ATP hydrolase in a hydrolysis reaction

• this occurs when an inorganic phosphate group is removed

• energy is released and ATP becomes ADP

What is the role of ATP?

• active transport

• exocytosis

• synthesis of substances (biosynthesis)

• movement

• activation of other molecules

Describe how ATP is used for active transport

• energy required to move substances against a conc gradient using carrier proteins

• The protein pumps are also ATP hydrolase enzymes catalyse the splitting of ATP to ADP + Pi and use the energy released to change shape and pump the molecule

Describe how ATP is used for exocytosis

• energy required to make vesicles prior to secretion from cells

• energy required for the reverse process (endocytosis) used to bring large molecules into cells

Describe how ATP is used for synthesis of substances

• energy required to make large molecules from smaller ones in condensation reactions

Describe how ATP is used for movement

• energy required for muscle contraction

Describe how ATP is used for activation of other molecules

• transfers it phosphate group to other molecules, phosphorylating them and making them more reactive

• reduce the activation

Suggest how you can make a molecule more reactive

add a phosphate group to phosphorylate and make them more reactive

Describe ATP as an energy source

• not a good long term energy store due to instability of its phosphate bonds

• cells maintain only a few seconds supply

• ATP is an immediate energy source as ATP is rapidly reformed

What is the ATP cycle?

Why is ATP more useful than glucose as an immediate source of energy?

• breakdown is a single reaction making energy immediately available. Breakdown of glucose is a complex reaction

• ATP is soluble and easily moved around but cannot pass through cell membranes

• breakdown of ATP releases a small amount of energy whereas glucose releases more energy than required so is inefficient