Nucleic Acids + Protein Synthesis (Chapter 6) Flashcards Preview

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What are the two types of nucleic acid?



What does DNA stand for?

Deoxyribonucleic acid


What does RNA stand for?

Ribonucleic acid


What are nucleic acids?

Polymers - made up of many nucleotides
(therefore polynucleotides)


What is a polymer?

A molecule made of many similar, smaller molecules joined in a long chain


What are nucleotides?

The smaller molecules from which DNA and RNA are made


Describe the structure of a nucleotide

Made up of 3 smaller components:
1) A nitrogen containing (nitrogenous) base
2) A pentose sugar
3) A phosphate group


What are the 5 different types of bases?

Both: Adenine, Cytosine, Guanine
DNA: Thymine
RNA: Uracil


What is the structure of a purine base?

A double ring structure


What is the structure of a pyrimidine base?

A single ring structure


Which ones are the purine bases?

Adenine and Guanine


Which ones are the pyrimidine bases?

Cytosine, Thymine and Uracil


Which base does Uracil replace in RNA?



What is the pentose sugar in DNA?



What is the pentose sugar in RNA?



What is adenosine?

Adenine with a sugar joined to it


What is ATP?

Adenosine triphosphate - adenosine combined with three phosphate groups (v. similar to adenine nucleotide just with two extra phosphate groups and sugar has an extra OH group)


How are the polynucleotides (DNA and RNA) formed?

Many nucleotides are linked together into a long chain
- they are formed of alternating sugars and phosphates linked together, with the bonds projecting sideways
- the covalent sugar-phosphate bonds (phosphodiester bonds) link the 5-carbon of one sugar molecule and the 3-carbon of the next
- the polynucleotide is said to have 3' and 5' ends


Where and when does the formation of polynucleotides take place?

Inside the nucleus, during interphase


Describe the structure of DNA molecules

- They are made of two polynucleotide strands lying side by side, running in opposite directions (antiparallel)
- The two strands are held together by H-bonds between the bases


Which bases out of purine or pyrimidine are bigger and what does this mean for the DNA molecule?

Purine bases are bigger
- therefore, in a DNA molecule, there is just enough room between the two sugar-phosphate backbones for one purine and one pyrimidine molecule
- therefore, a purine in one strand must always be opposite a pyrimidine in the other


What is complementary base pairing?

Adenine always pairs with thymine, while cytosine always pairs with guanine


What is the 3D shape of the DNA molecule?

A double helix


How can DNA replication and protein synthesis occur?

- The H-bonds linking the bases (and holding the strands together) can be broken relatively easily
- This happens during both of these processes
- The breaking of the H-bonds is a very important feature of the DNA molecule that enables it to perform its role in the cell


Describe the structure of RNA molecules?

They remain as single strand polynucleotides and can form very different 3D structures


Describe semi-conservative replication

Watson and Crick suggested that the two strands of the DNA molecule could split apart
- new nucleotides line up along each strand, opposite their appropriate partners, and join up to from complementary strands along each half of the original molecule
- the new DNA molecules are just like the old ones, because each base only pairs with its complementary one
- each pair of strands then winds up again into a double helix


Why is it called semi-conservative replication?

Because half of the original molecule is kept in each of the new molecules


Explain what happens during semi-conservative replication

1) The DNA double helix unwinds as the H-bonds between the bases break - both strands of DNA act as templates
2) Each of the bases of free activated (phosphorylated) nucleotides in the nucleus pairs up with its complementary base of each of the parent DNA strands - the process continues along the whole of the DNA molecule
3) DNA polymerase catalyses the reaction by linking the sugar and innermost phosphate group of next door nucleotides, forming the sugar-phosphate backbone


How is the DNA molecule copied perfectly many times over?

DNA polymerase will only link an incoming nucleotide to the growing new chain if it is complementary to the base on the old strand (mistake only 1 in every 10^8 base pairs)


What is a gene?

A sequence of nucleotides that forms part of a DNA molecule and codes for on polypeptide


What is a gene mutation?

A change in the sequence of nucleotides that may results in an altered polypeptide


What is an allele?

A different variant of a gene, which originally arose by the process of mutation


How does DNA control protein structure?

By determining the exact order in which the amino acids join together when proteins are made in a cell


What is the sequence of nucleotide bases in a DNA molecule a code for?

The sequence of amino acids in a polypeptide


Describe the triplet code

- The code is a 3-letter (triplet) code - each sequence of 3 bases stands for one amino acid
- The sequence always reads in the same direction and from only one of the two strands of the DNA molecule (the sense strand) - the other complementary strand is called the anti-sense strand
- There are often several different triplet codes that code for one amino acid (degenerate code)


What is one example of a mutation that has a significant effect?

The mutation involved in sickle cell anaemia, an inherited blood disorder


What is haemoglobin made up of?

4 polypeptide chains (2 alpha, 2 beta), each with one Fe-containing haem group at the centre


Explain the mutation in people with sickle cell anaemia

- In the gene which codes for the amino acid sequence in the beta-polypeptides, the base sequence CTT is replaced by CAT
- This changes the HbA (normal) allele of the gene to into the HbS (sickle cell) allele of the gene
- The first 'Glu' amino acid in the beta polypeptide is replaced by a 'Val' amino acid


What is the type of mutation called that causes sickle cell anaemia?

A substitution


What does the sickle cell mutation result in?

The small difference in the amino acid sequence results in the genetic disease sickle cell anaemia in individuals with two copies of the HbS allele


What are the two stages of protein synthesis?

Transcription and translation


What happens during transcription?

1) In the nucleus, a complementary copy of the code from a gene is made by synthesising messenger RNA (mRNA), using the sense strand of a DNA molecule that has unwinded as a template
2) Free activated RNA nucleotides pair up with the exposed bases of this strand - this process copies the DNA code onto the mRNA strand
3) As the RNA nucleotides pair up with their complementary bases, their sugar-phosphate groups are bonded together by RNA polymerase, forming the backbone
4) The mRNA molecule leaves the nucleus via a pore in the nuclear envelope and attaches to the small subunit on a ribosome in the cytoplasm - 6 bases at a time are exposed to the large subunit


What happens at the end of transcription?

Transcription of a gene ends when the enzyme has reached a terminator sequence
- at this point, the enzyme stops adding nucleotides to the mRNA
- H-bonds holding the mRNA and DNA are broken and the DNA reforms
- the last triplet transcribed onto mRNA is one of the DNA triplets coding for 'stop'


What is translation?

When the DNA code is translated into an amino acid sequence


What are transfer RNA molecules?

They are in the cytoplasm (with free amino acids)
- they are molecules which have a triplet of bases at one end (anticodon) and a region where amino acids can attach at the other end


What happens during translation?

1) The tRNA picks up their specific amino acids (under the control of the specific enzyme and with energy from ATP) from the cytoplasm and brings them to the mRNA on the ribosome
2) The triplet of bases (an anticodon) of each tRNA links up with a complementary triplet (a codon) on the mRNA molecule
3) A second tRNA molecule binds with the next 3 bases on the mRNA, bringing a second amino acid
4) This brings 2 amino acids side by side and a peptide bond is formed between them
5) The ribosome now moves along the mRNA, 'reading' the next 3 bases
6) A third tRNA brings a third amino acid and the first tRNA leaves
7) The polypeptide continues to grow until a 'stop' codon is exposed on the ribosome


The triplet of bases on the tRNA molecule is called the...?



Th triplet of bases on the mRNA molecule is called the...?



How many tRNA molecules can fit onto the ribosome at any one time?



Which enzyme binds the two amino acids together?

Peptidyl transferase


What are polyribosomes?

Several ribosomes working on the same mRNA strand at the same time


Therefore, how does the DNA sequence determine the sequence of the amino acid?

- The base sequence on the DNA molecule determines the base sequence on the mRNA, which determines which tRNA molecule can link up with them
- Because each type of tRNA molecule is specific for just one amino acid, this determines the sequence in which the amino acids are linked together as the polypeptide molecule is made


What is ATP?

A phosphorylated nucleotide


Explain the experimental evidence for semi-conservative replication of DNA shown by Meselson and Stahl

1) They grew populations of E.coli in two different types of ammonium chloride - one containing heavy nitrogen (15N) and the other containing normal nitrogen (14N)
2) Therefore the bacteria in the heavy nitrogen used 15N to make their DNA and were left to divide many times, so that nearly all of their DNA contained 15N atoms
3) They were then put in 14N and in 1 generation, in a centrifuge the DNA were in an intermediate stage of heaviness, which shows that the DNA molecules were a combination of heavy and normal nitrogen
4) In further generations there is a split - some of them are fully light and some are half heavy half light - but it becomes increasingly lighter as more light strands match up


What were the three possible mechanisms for DNA replication?

1) Conservative
2) Dispersive
3) Semi-conservative

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