[Y1] Genetic Information, Variation, and Relationships Between Organisms Flashcards
What is a gene?
A section of DNA that contains the coded information for making polypeptides and functional RNA.
What is a locus?
A particular position of a gene on a section of DNA or a chromosome.
(pl: Loci)
What can a base sequence of DNA code for?
- The amino acid squence of a polypeptide.
- A functional RNA, including rRNA and tRNAs
Why must there be a minimum of three bases that code for each amino acid?
- Only 20 differnet amino acids regularly occur in proteins.
- Each amino acid must have it’s own code of bases on the DNA.
- Only four different bases are present in DNA.
- If each base coded for a different amino acid, only four different amino acids could be coded for.
- If a pair were used, 16 (4²) different codes are possible, which is still inadequate.
- If three bases were used, 64 (4³) different codes, more than enough to satisfy the 20 amino acid requiremnt.
In fact some amno acids have more than one triplet.
How does code for a polypeptide bengin and end?
- The start of a DNA sequence that code for a polypeptide is always the same triplet coding for the amino acid methionine. If this isn’t in the polypeptide then it is removed later.
- There are three non coding triplets, these are stop triplets and make the end of a polypeptide chain.
What is degenerate code?
When an amio acid can be coded for by more than one different triplet. (can be 2-6 different triplets.
(although a few are coded by one codon)
How is the nature of code in DNA indirect evidence of evolution?
The code is universal (with a few minor exceptions), as each triplet codes for the same amino acid in all organisms.
Therefore it suggest that all organisms have an ancestor where this trait began.
When does code overlap?
Never.
Each base pair is only read once when and so the DNA 123456:
- is read as 123 and 456
- NOT 123, 234, 345, 456.
What is an exon and intron
Exons: coding sequences of DNA.
Introns: non-coding sequences of DNA.
What are the differnces between prokaryotic and eukaryotic DNA?
Prokaryotic: DNA is much shorter, form a circle, and are not associated with protein molecules (histones).
Eukaryotic: DNA is much longer, form a line (linear). and occur in assoation with proteins called histones to form a chromosome. The mitochondria and chloroplasts also contain DNA which is like that of the prokaryotic DNA.
What do histones do?
They fix the DNA into position.
How many molecules of DNA make up a chrmosome?
One.
It is very long and is coiled up many times.
How do chromosomes vary between species?
chromosomes differ in number (in typical indiviuals in a species, ignoring mutations such as down syndrome)
E.g humans have 46 chromosomes whilst dogs have 78.
(this number is usually even in adult cells)
What are homologous pairs?
Chomosomes of that have the same gene loci (but can be different alleles), one maternal, the other paternal.
What is diploid?
2 of each chromosome.
2n
What is haploid?
1 of each chrmosome.
n.
What is an allele?
One of a number of alternative forms of a gene.
What is a mutation?
Any change in the quantity or base sequence of a gene.
May or may not produce a new allele of that gene and result in a different sequence of amno acids being coded for.
Why might a mutation have serious consequneces for an organism?
The diffence in sequnce of amino acid as a result of a new allele will lead to the production of a differnt amino acid, hence a differnt protein.
A different proten may not function properly or not at all
If the protein produced in an enzyme, it may have a different shape. The new shape may not fit the enzyme’s substrate.
As a result the enzyme may not function, having serious consequences on an organism.
Where does protein systhesis take place?
The cytoplasm.
How does genetic code get to the site of protein synthesis? Why is this possible?
Via mRNA.
mRNA is small enough to leave the nucleus through nuclear pores and enter the cytoplasm.
Define codon.
A sequence of three basees on mRNA that code for a single amino acid.
Define genome.
The comlete set if genes in a cell, including those in mitochondria and/or chloroplasts.
Define proteome.
The full range of proteins produced by the genome.
(sometimes called complete proteome, where proteome refers to the proteins produced by a given type of cell under a certain set of conditions.)
What is the structure of RNA?
- A polymer made up of repeating mononucleotide sub-units. A nucleotide of RNA consists of:
- the pentose sugar ribose.
- one of the organic bases (A, U, G, C).
- a phosphate group.
What RNA is important in protein synthesis?
- mRNA
- tRNA
Describe the stucture of mRNA and tRNA.
mRNA:
- thousonds of mononucleaotides in a long single helix.
- its base sequence is determined by the sequence of bases on DNA
- great variety in different mRNA
tRNA:
- small molecule, made up of around 80 nucleotides.
- single stranded folded into a cover-leaf shape.
- one end extendeds beyond the other where the amino acid can easily attach.
- at the opposite end there are three other organic bases called an anticodon.
- There are as many different varieties as there is coding triplets.
- Each has an anticodon specific to one amino acid.
What is the complimentary base paring between bases?
Guanin to Cytosine
Adenine to Uracil (RNA) or Thymine (DNA).
What are the overall steps of protein sythesis?
- DNA provides the instructions in the form of a long sequence of bases.
- A complimentary section of part of this sequence is made in the form of pre-mRNA. (Transcription)
- Pre-mRNA is spliced into mRNA. (Splicing)
- The mRNA is used as a template to which complimentary tRNA molecules attach and the amino acids they carry link to form a polypeptide. (Translation)
Describe the process of transcription.
- An enzyme acts on a spesific region of the DNA causing the two strands to separate and expose the nucleotide base in that region
- The nucleotide bases on one of the template strand pair with complimentary nucleotides (A to U, T to A, G to C, C to G) from the pool which is present in the nucleus.
- RNA polymerase moves along the strand and joins the nucleotides together to form pre-mRNA adding them one at a time.
- DNA strands rejoin behind it, so few base pairs are exposed at one time (around 12).
- When RNA polymerase reaces a particular sequence of bases on the DNA that is recoginsed as a ‘stop’ triplet code, it detaches.
What is the difference between transcription in prokayotes to eykaryotes?
The result of transcription in prokaryotes is mRNA, but in eukaryotes is pre-mRNA.
Therefore splicing is only required in eukaryotes.
What happends during splicing?
Introns are removed from pre-mRNA and the exons are joined together, resulting in mRNA.
Why does splicing need to take place?
The introns would prevent the sythesis of polypeptides.`
When will mRNA diffuse out of the nucleous?
Never, as it is to large.
Instead it leaves via nuclear pores
What happends to the mRNA once it leaves the nucleaus?
It is attracted to the ribosomes to which it attaches to.
How many different tRNA are there?
Around 60.
Describe the process of translation.
- A ribosome becomes attached to the starting codon at one end of the mRNA molecule.
- tRNA with the complimentary anticodon moves into the ribosome and pairs up with the codon on the mRNA.
- This tRNA carries a specific amino acid.
- Anther tRNA with the complimentary anticodon pairs with the next codon on the mRNA.
- This tRNA carries another specific amino acid.
- The two amino acids on the tRNAs are joind by a peptide bond using an enzyme and ATP, which is hydrolysed to provide energy for the condensation reaction.
- The ribosome moves along to the third codon on the mRNA, again linking the amino acids but from the 2nd and 3rd tRNA.
- As this happens, the first tRNA is released from its amino acid, and is free to collect another amino acid from the amino acid pool in the cell.
- This process continues, with up to 15 amino acids being added per second, until the polypeptide is built up, and it reaches a stop codon.
- The ribosome separates from the mRNA
- Up to 50 ribosomes can pass immediatley behind the first ro many poypeptides can be assembled simultaneously.
What might happen after translation?
- The polypeptide is coiled or folded, producing a secondary structure.
- The secondary structure is folded, producing a tertiary structure.
- Different polypeptide chains, along with non-protein groups, are linked to form a quaternary structure.
When can gene mutation arise?
Sponataiouly during DNA replication.
Describe the types of mutation.
- Substituion of bases:
A gene mutation where a nucleotide in DNA is replaced by another nucleotide that has a different base. - Deletion of bases:
A gene mutation where a nucleotide is lost from the normal DNA sequnece.
How severe is gene mutation by substitution?
Depends on the precise nucleotide substituted.
- The nucletoide may be an intron and so have no effect on the protein produced.
- The nucleotide may code of the same protein (as a result of degerneate code) and so also not have an effect.
- The nucleotide may code for a protein that vastly effects the tertiary structure of the protein, hugly effecting the organisms (e.g if its an enzyme).
How severe is gene mutation by deletion?
Can have extreme consiquences, as it casues all nucleotides after it to move one to the left when being read. Not only effecting that one triplet but also every triplet after it.
What is a chromosome mutation?
A change in the structure or number of whole chromosomes.
Describe the form chromosome mutations can take?
- Changes in whole sets of chromosomes:
The organism may have three or more chromosomes instead of two (polyploidy) - Changes in the number of individual chromosomes:
Occurs when individual homologous pairs fail to separate during meiosis (non-disjunction), resulting in organisms having one more or one fewer chromosome.
What can non-dijuction lead to after fertilisation?
On fertiliasation with a regular gamete, the resultant offspring have more or fewer chromosomes than normal in all their body cells.
(e.g. Down Syndrome is an additional chromosome 21.)
What is the difference between mitosis and meiosis.
- Mitosis: produces two daughte cells with the same number of chromosomes as the parent cell and as each other.
- Meiosis: produces four daughter cells, each with half the number of chromosomes as the parent cells
What happens during sexual reproduction?
Two gametes fuse to give rise to a new offsprig.
What is an important property of a gamete?
They are haploid.
in most cases
What would happen if a gamete was diploid?
The number of chromosomes would be 92 imeadatly after sexual reproduction (42 from each gamete).
This doubling would increase in each generation exponentially.
This is avoided by gametes being haploid.
What happens overall during meiosis?
Homologuos pairs of chromosomes seperate, so that only one chromosome from each pair enters the daughter cell.
What happends durineg each meiotic division?
Meiosis 1:
- Homologous chromosomes pair up and their chromatids wrap around each other.
- Here crossing over may occur.
- Homologous pairs separate, with one chromosome from each pair going into one of the two daughter cells.
Meiosis 2:
- Chromatids move apart.
- 4 daughter cells have usually been formed.
Other than halving the number of chromosomes, what is achieved as a result of meiosis?
Genetic variation among offspring, which may lead to adaptations that improve survival chances.
How does meiosis cause genetic variation?
- Independaent segregation of homologous chromosomes.
- New combinations of maternal and paternal allels by crossing over.
What is meant by independent segregation?
- During meiosis 1, homologous pairs arange themselves in lines randomly.
- One of each pair will pass to each daughter cell.
- Which one of the pair goes into one daughter cell, and which one goes into the other, depends on how the pairs line up.
- Since they line up, the combination of chromosomes of maternal and paternal that go into the daughter cells (at meiosis 1) is also matter of chance.
Why does variety occur as a result of independent assortment.
- Each chromosome in a homolous pair has the same gene and so determine the same characteistic.
- But the alleles of the genes may differ.
- And so the independent assortment of these chromosomes produce new genetic combinations.
Homologous chromosomes coding for toungue rolling lines up bellow the homologous chromosomes that code for blood group.
There are two arrangements P and Q.
Arrangement P:
- has non-roller and blood type A on one side
- and roller and blood type B on the other side.
Arrangent Q:
- has roller and blood type A on one side
- and non-roller and blood type B on the other.
What are the possible gametes for each arrangement?
Arrangement P:
- 2 roller/B gametes
- 2 non-roller/A gametes
Arrangement Q:
- 2 non-roller/B gametes
- 2 roller/A gametes
Therefore genetic variation can occur as a result of independent segregation.
How and why do haploid gamates fuse?
- How: randomly at fertilisation.
- Why: to restore the diploid state after meiosis.
What happens during genetic recombination by crossing over?
- The chromatids of each pair twist around one another.
- During this process tension builds up and portions of the chromatids break off.
- These broken portions might then rejoin with the chromatids of its homologous partner.
- Usually it is the equivalent portions of homologous chromosomes that are exchanged.
- In this way new genetic combinations of material and paternal alleles are produced.
What happens as a result of meiosis if recombination by crossing over happens as opposed to it not happening?
If it happens: You get 4 different genetic compositions.
If it doesn’t happen: You get 2 different genetic compositions (2 times each).
How do you calculate the number of possible combinations of chromosomes in any daughter cell as a result of meiosis?
How many possible combinations of chromosomes can you have in a organism with 4 homologous pairs, as a result of meiosis?
2ⁿ, where n = the number of homologous pairs.
An oganism with 4 homologous pairs can produce 2⁴ possible different combinations of chromosomes of maternal and paternal origin in its daughter cells as a result of meiosis.
2⁴ = 16 possible different combinations of homologous pairs.