Nucleic Acids, DNA, and Protein Synthesis

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What are the components of a nucleotide?
A phosphate group, pentose sugar, and organic base
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What are the monomers of nucleic acids?
Nucleotides
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What are the complementary organic bases in DNA?
Adenine & Thymine, Cytosine &Guanine
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What are the organic bases in RNA?
Adenine & Uracil, Cytosine & Guanine
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What is the arrangement of complementary polynucleotides in the double helix?
Antiparallel (opposite directions)
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What is the function of DNA?
Passing genetic material from cell to cell and to code for proteins
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Give 3 reasons how DNA is adapted to do its job
Phosphate backbone provides strength and protects bases, helix and large molecule so can store lots of info in a small space, double stranded so semi-conservative replication, weak hydrogen bonds so strands separate, many hydrogen bonds- it's strong
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How many hydrogen bonds are there between C and G?
3
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How many hydrigen bonds are there between A and T?
2
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Define semi-conservative replication of DNA
The means by which DNA makes exact copies of itself by unwinding the double helix so each strand acts as a template for the next. The new molecules have 1 new and 1 original strand of DNA.
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What are the 4 conditions for DNA replication?
1- A pool of the 4 types of nucleotides must be present, 2- Both strands of DNA being copied must act as a template, 3- DNA Polymerase present, 4- Energy (ATP) is required
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Step 1 of DNA Replication
DNA Helicase breaks the hydrogen bonds between the bases of the 2 strands and the double helix separates
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Step 2 of DNA Replication
Each exposed strand acts as a template
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Step 3 of DNA Replication
Complementary free nucleotides are attracted to the exposed bases on the strands. Energy is required to activate the free nucleotides
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Step 4 of DNA Replication
Once the nucleotides are in place, their 'backbone' is joined together by DNA Polymerase. 2 new DNA molecules are produced, each containing 1 original and 1 new strand of DNA.
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What is a gene?
A section of DNA, in the form of specific sequences of bases, which contains the coded information for making polypeptides
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What is the triplet code?
The term used to describe that 3 bases code for 1 amino acid
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How many amino acids regularly occur in proteins?
About 20
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Define locus
The position of a gene on a chromosome
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Define degenerate code
A term used to describe the fact that most amino acids are coded for by more than one triplet
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Define non-overlapping
Each base in the sequence is read only once
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Define universal
Each triplet usually codes for the same amino acid in all organisms (indirect evidence for evolution)
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What are exons?
The coding sequences within genes
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What are introns?
The non-coding sequences within genes
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Give 3 features of DNA in eukaryotes
Large molecule, linear molecule, associate with proteins forming chromosomes
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Give 4 features of DNA in prokaryotes
Smaller molecules, form circles (plasmids), do not associate with proteins so don't form chromosomes, no introns
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What is an allele?
A form of a gene
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Name the pentose sugar in DNA
deoxyribose
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Name the pentose sugar in RNA
ribose
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How many strands is RNA?
1
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What is the function of mRNA?
To carry the genetic message from DNA in nucleus to ribosomes where proteins are synthesised
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What is the function of tRNA?
To collect the specific amino acid and take to ribosome for assembly into proteins
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Give 2 structural features of mRNA
Long molecule, not folded
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Give 2 structural features of tRNA
Small molecule, folded (clover-leaf shape)
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Explain the structure of tRNA
It is folded lots and one end carries a specific amino acid, the other has an anticodon- triplet of bases which attatches to a codon on mRNA and detremines the amino acid carried
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What are the 3 stages of protein synthesis/gene expression?
1- Transcription, 2- Amino acid activation, 3- Translation
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What happens in transcription?
DNA Helicase unzips the DNA helix and RNA Polymerase attaches to the DNA template (sense strand). As RNA polymerase moves along the sense strand, RNA nucleotides are assembled due to complementary base pairing. Pre-mRNA is produced
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How is mRNA produced from pre-mRNA?
The introns are removed and exons are spliced together
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What happens after mRNA is produced?
It peels off the DNA and leaves the nucleus via nuclear pore, DNA rewinds
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What happens in amino acid activation?
tRNA combines with a specific amino acid using ATP- the anticodon determines which specific amino acid is attatched
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What happens in translation?
Ribosome binds to mRNA near the start codon and tRNA with complementary anticodon binds to start codon. The ribosome slides along mRNA to read next codon. New tRNA brings second amino acid and a peptide bond is formed between the 2 amino acids.
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What happens in translation? (cont.)
The first tRNA is released, leaving its amino acid behind. The process repeats itself and one by one each codon is read as the ribosome moves along mRNA. Polypeptide is complete when ribosome reads stop codon and polypeptide is released
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Define polyribosomes
Several ribosomes slide alond mRNA molecule simultaneously
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What happens to tRNA after translation?
They are recycled
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What happens to mRNA after translation?
It is used again or recycled into nucleotides
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What are the purine bases?
Adenine and guanine
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What are the pyrimidine bases?
Thymine/uracil and cytosine
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Is there hydrogen bonding in tRNA?
Some due to the folding
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Is there hydrogen bonding in DNA?
Yes
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Is there hydrogen bonding in mRNA?
No
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What mutagens increase rate of gene mutation?
High energy radiation, e.g. UV, Ionising radiation, e.g. X-rays, Chemicals, e.g. mustard gas
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How many nucleotides are in a chain of DNA?
Millions
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How many nucleotides are in mRNA?
100s-1000s
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How many nucleotides are in tRNA?
About 80
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What are the 2 types of mutation?
Substitution and deletion
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What is substitution?
When one base is substituted with another, e.g. ATCCGT to ATCTGT
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What is deletion?
When one base is deleted, e.g. ATCCGT to ATCCT
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What effect could a mutation have on an enzyme?
Different amino acid sequence so different folding and different tertiary structure so bonds in different places which changes the shape of the active site. As a result, the enzyme and substrate are no longer complemenatry so no ESC formed
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Why do some mutations have no effect on the sequence of amino acids?
Because of the degenerate nature of genetic code- most amino acids are coded for by more than one triplet so in a substitiution mutation, it is possible that another codon is produced which codes for the same amino acid as the original codon
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What is a gene mutation?
When there is a change to one or more bases in DNA
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Why do the middle and end parts of chromosomes have different percentages of bases?
The different parts have different genes and therefore different base sequences
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What is the maximum number of DNA that a sequence of 27 bases could code for? Why is it actually likely to be less than this?
9, there are introns and start/stop codes
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