DNA , genes and protein Flashcards
what is a gene
a gene is a section of DNA that contains the coded info for making polypeptides and functional RNA
what is the coded info in the form of
the coded info is in the form of a specific sequence of bases along the DNA
why are genes important
polypeptides make up proteins and so genes determine the proteins of an organism
Enzymes are proteins. As enzymes control chemical reactions they are responsible for an organisms development and activities
In other words, genes along with environmental factors determine the nature and development of all organisms
what does the gene code for
a gene is a section of DNA located at a particular position, called a locus ,on a DNA molecule
The gene is a base sequence of DNA that codes for:
- the amino acid of polypeptide
- or a functional RNA, including ribosomal RNA and transfers RNA
how many bases code for one amino acids
scientists suggested that there must be a minimum of three bases that coded for each amino acid
as the code has three bases for each amino acid, each one is called a triplet
why did scientists suggest that three bases coded for one amino acid
there is only 20 different amino acids regularly occur in proteins
each amino acid must have its own code of bases on the DNA and there is only four bases (A, T,C,G) are presented in DNA
if each base coded for a different amino acid, only four different amino acids could be coded for
Using a pair of bases, 16 (4 to the power of 2) different codes are possible, which is still inadequate
three bases produces 64 (4 to the power of 3) different codes is possible. This is more than enough to satisfy the requirement of 20 amino acids
why are there some amino acids coded for more than one triplet
as there are 64 possible triplet and only 20 amino acids, it follows that some amino acids are code for by more than one triplet
what are the features of the genetic code
Further experiments have revealed the following features of the genetic code:
- A few amino acids are coded for by only a single triplet
- the remaining amino acids are coded for between two and six triplets each
- the code is known as a “degenerate code” because most amino acids are coded for by more than one triplet
- a triplet is always read in one particular direction along the DNA strand
- the start of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for amino acid methionine.
If this first methione molecule does not form part of the final polypeptide, it is later removed
6.Three triplets do not code for any amino acid. These are called “stop codes” and mark the end of the polypeptide chain
They act in much the same way as a “full stop” at the end of a sentence
- the code is non - overlapping, in other words each base in the sequence is read only once.
This six bases numbered 123456 are read as 123 456 rather than triplets 123, 234, 345, 456
This code is universal with a few minor exceptions each triplet codes for the same amino acid in all organisms - this is indirect evidence for evolution
how much of the DNA in eukaryotes code for polypeptide
much of the DNA in eukaryotes does not code for polypeptides
e.g. between genes there are non - coding sequences made up of multiple repeat of base sequences
Even within genes, only certain sequences code for amino acids
what are coding sequences called
these coding sequences are called exons
Within the gene these exons are separated by further non - coding sequences called introns
what are non coding sequences called
introns
what do some genes code for
ribosomal RNA and transfer RNA
what are the difference in in the DNA of prokaryotic and eukaryotic cells
in prokaryotic cells, the DNA molecules are shorter, form a circle and are not associated with protein molecules
- prokaryotic cells therefore do not have chromosomes
in eukaryotic cells, the DNA molecules are longer , form a line (are linear) rather than a circle and occur in association with proteins called histones to form structures called chromosomes
what do the DNA of mitochondria and chloroplasts of eukaryotic cells have in common
the mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotic cells
- it is short circular and not associated with proteins
when are chromosomes visible
chromosomes are only visible and distinct structure when a cell is dividing
for the rest of the time they are widely dispersed throughout the nucleus
what is the chromosomes made up of
when they first become visible at the start of cell division chromosomes appear as two threads, joined at a point
Each thread is called a chromatid because DNA has already replicated to give two identical DNA molecules
what is the length of each DNA found in the cell
around 2m in every human cell
The DNA is coiled and folded to form a chromosome
how does DNA coil into chromosomes
DNA molecule
DNA molecule combined with histones
DNA - histones complex is coiled
Coils fold to form loops
Loops coil and pack together to form the chromosome
we already know that DNA is a double helix, this double helix wounds around histone to fix it in position
This DNA - histones complex is then coiled
The coil in turn iss then looped and further coiled before being packed into the chromosome
In this way a lot of DNA is condensed into a single chromosome
This chromosome is made from a single molecule of DNA although, this is very long
how many genes does a single DNA has
a single molecule of DNA had many genes along its length
each gene occupies a specific position ( locus)
how many chromosomes do humans have in each cell
humans have 46 chromosomes
the number of chromosomes vary from species to species e.g.a dog has 76 chromosomes
what are sexually produced organisms e.g. humans a result of
they are the result of a fusion of a sperm and egg each of which contributes one complete set of chromosomes to the offspring
Therefore, one of each pair is derived from the chromosomes provided by the mother in the egg (maternal chromosomes) and the other is derived from the chromosomes provided by the father in the sperm (paternal chromosomes)
what is a homologous pair
These are known as homologous pair is always two chromosomes that carry the same genes but not necessarily the same alleles of the gene
the total number of homologous pairs is referred to as the diploid number which is 46 in humans
what is an allele
an allele is one of a number of alternative forms of a gene
we have seen that genes are sections of DNA that contain coded info in the form of specific sequences of bases
Each gene exists in two, occasionally more, different forms
Each of these forms is called an allele
how does an individual obtain an allele
each individual inherits ann allele from each of its parents
These two alleles may be the same or they may be different
how are alleles different from one and other
when they are different, each allele has a different base sequence, therefore a different amino acid sequence so produces a different polypeptide
what happens when there is a change in base sequences
any changes in the base sequence of a gene produce a new allele of that gene (mutation) and result in a different sequence of amino acids being coded
why is the change in a base sequence a bad thing
This different amino acid sequence will lead to the production of a different polypeptide and hence a different protein
Sometimes this different protein may not function properly or may have a different shape
The new shape may not fit the enzyme’s active site
As a result, the enzyme may not function and this can have serious consequences for the organisms
what is meiosis
meiosis is a type of cell division that produced four daughter cells within half the number of chromosomes as the daughter parent cell
why is meiosis important
in sexual reproduction the two gametes (egg and sperm) fuse to give rise to new offspring
if each gamete had a full set of chromosomes (diploid number) then thee cell that they produce will be double this
e.g. in humans, the diploid number of chromosomes is 46, which means that this cell would have 92 chromosomes
This doubling of the number of chromosomes would continue at each generation
It follows that, in order to maintain a constant number of chromosomes in the adult of a species, the number of chromosomes must be halved at some stage of the life cycle
how do cells half the number of chromosomes
halving of chromosomes occurs as a result of meiosis
In most animals meiosis occurs in the formation of gametes
what is the process of meiosis
- Before meiosis starts the DNA unravels and replicates so there are two copies of each chromosome called chromatids
- the DNA condenses to form double - armed chromosomes, each made from two sister chromatids
The sister chromatids are joined in thhe middle by a centromere - Meiosis 1 (first division) - the chromosomes arrange themselves into homologous pairs
- these homologous pairs are then separated, halving the chromosome number
- Meiosis 2 the pairs of sister chromatids (second division) that make up each chromosome are separated (the centomere is divided)
- Four haploid cell that are genetically different from each other produced
what do every diploid cell have in common
every diploid cell of an organism has two complete sets of chromosomes
one set provided by parent.
During meiosis, homologous pairs of chromosomes separate , so that only one chromosome from each pair enters a daughter cell
This is known as the haploid number of chromosomes which, in humans, is 23
When two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored
what else does meiosis do
in addition to halving the number of chromosomes, meiosis also produced genetic variation among the offspring, which may lead to adaptations that improve survived chances
how does meiosis brings about genetic variation
meiosis brings about this genetic variation in the following two ways:
- independent segregation of homologous chromosomes
- new combinations of maternal and paternal alleles by crossing over
QUICK REMINDER: what is a homologous chromosomes
a pair of chromosomes, one maternal and one paternal, that have the gene loci
what is independent segregation/ assortment
during meisosis 1, each chromosome line up alongside its homologous partner
In humans e.g. this means that there will be 23 homologous pairs of chromosomes lying side by side
When these homologous pairs arrange themselves in this line they do so at random
One of each will pass to each daughter cell
Which one of the other pairs, depends on how the pairs are lined up in the parent cell
Since the pairs are lined up at random, the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is also a matter of chance (2 to the power of 23 which means that there is 8 million different combination)
how does genetic combinations produce genetic variety
each member of a homologous pair of chromosomes has exactly the same genes and therefore determines the same characteristics
e.g., tongue rolling and blood group
However, the alleles of these genes may differ (e.g., they may code for rollers or non-rollers, or blood group A or B
The independent assortment/ segregation, of these chromosomes, therefore produced new genetic combination
how does independent assortment work
in this example, we are looking at two homologous pairs
STAGE 1: one of the pair of chromosomes includes the gene for tongue rolling and carried one allele for roller and one for non roller
The other chromosome includes the gene for blood group and carries the allele for blood group A and allele for blood group B
There are two possible arrangements (P and Q) of the two chromosomes at the tart of meiosis
Both arrangements are equally probable but each produces a different outcome in terms of characteristics that may be passed on via the gametes
STAGE 2:
at the end of meiosis 1, the homologous chromosomes have segregated into two separate cells
STAGE 3:
at the end of meiosis 2, the chromosomes have segregated into chromatids providing four gametes for each arrangement
The actual gametes are different, depending on the original arrangement (P or Q) of the chromosomes at stage 1
why are the cells produced at the end of meiosis genetically different
the gametes produced at the end of meiosis is genetically different as a result of the different combinations of the maternal and paternal chromosomes/ alleles they contain
what do haploids ( gametes) do during fertillisation
the haploid gamete produced by meiosis fuse to restore the diploid state
Each gamete has a different make - up and their random fusion therefore produces variety in the offspring
how does genetic recombination by crossing over work
during meiosis 1, each chromosome lines up alongside is homologous partner
The following events then take place:
- the chromotids of each pair become twisted around one another
- during this twisting process tensions are created and portions of the chromotids 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 exchanged
- in this way new genetic combinations of maternal and paternal alleles are produced
the chromatids cross over one another many times and so the process is known as crossing over.
The broken off portions of chromatid recombine with another chromatid, so this process is called recombination
what happens if recombination did not take place
if there is no recombination by crossing over only two different types of cell are produced
However, if recombination does occur, four different types are produced
Crossing over therefore increases genetic variety even further
what equation do we need to use to determine the number of possible combinations of chromosomes for each daughter cell after meiosis
the formula is:
2 to the power of n
n being the number of pairs of homologous chromosomes
e.g. an organism with 4 homologous pairs of chromosomes can produce 2(2 to the power of 4) or 16 possible combinations of chromosomes of maternal and paternal origin i its daughter cells as a result of meiosis
what equation do we use to work out the number of homologous pairs after fertillisation
we use the equation (2n)2 to the power of 2
where n is the number of pairs of homologous chromosomes
e.g.
calculate the number of possible chromosome combinations produced from the fertiliation of two gametes from separate individuals whose dipliod number is 12 (assume no crossing over)
as you are looking at the possible chromosomes in the offspring (after fertilisation), you must use the formula:
(2 to the power of n) to the power of 2
first you need to half the diploid number to find the homologous chromosomes
substituting the numbers into the the formula we get 4096
where does the synthesis of proteins take place
in the cytoplasm of cells
how does the coded info on the DNA of the nucleus get transferred to the cytoplasm where it is translated into proteins
sections of DNA code are transcribed onto a single - stranded molecule called ribonucleic acid (RNA)
what are the two types of RNA important in protein sythesis
mRNA
tRNA
what does mRNA do
mRNA transfers the DNA code from the nucleus to the cytoplasm acts as a type of messenger and is hence given the name messenger RNA or mRNA for short
mRNA is small enough to leave the nucleus through the nuclear pores and to enter the cytoplasm, where the coded information that it contains is used to determine the sequence of amino acids in the proteins which are sythesised there
what is a codon
the term codon refers to the sequence of three bases on mRNA that codes for a single amino acid
what is a genome
a genome is the complete set of genes in a cell including mitochondria and/ or chloroplasts
what is a proteome
it is the full range of proteins provided by the genome
This is sometimes called the complete proteome, in which case the term proteome refers to the proteins produced by a given type of cell under a certain set of conditions
what is the structure of ribonucleic acid
ribonucleic acid is a polymer made up of repeating mononucleic sub - units
It forms a single strand in which each nucleotide is made up of:
1.the pentose sugar ribose
- one of the organic bases adenine (A), gunaine (G), cytosine (C) and uracil (U)
- a phosphate group
what is the structure of messenger RNA (mRNA)
- mRNA is a long strand that is arranged in a single helix
- The base sequence of mRNA is determined by the sequence of bases of on a length of DNA in a process called transcription
- great variety of different types of mRNA
what happens once mRNA is formed
once formed, mRNA leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm, where it associates with the ribosomes
There, it acts as a template for protein sythesis
how does mRNA suited for its function
its structure is suited to this function because it possesses info in the from of codons ( three bases that are complementary to a triplet, that codes for an amino acid, in DNA)
The sequence of codons determines the amino acid sequence of a specific polypeptide that will be made
what is the structure of tRNA
- transfer RNA (tRNA) is relatively small molecule that is made up of around 80 nucleotide
- It is a single stranded chain folded into a clover leaf shape, with one of the chain extending beyond the other - this is the part of the tRNA molecule to which an amino acid can easily attach
- there are many types of tRNA, each of which binds to a specific amino acid
- at the opposite end of tRNA molecule is a sequence of three other organic bases known as the anticodon
why is there many tRNA molecules
given that the genetic code is degenerate there must be as many tRNA molecules as there are coding triplets
because each tRNA is specific to one one amino acid
what are the organic bases that pair up in RNA
In RNA, the base thymine is always replaced by similar base called uracil
RNA can join with both DNA and RNA
the complementary bass that RNA forms are:
- guanine with cytosine
- adenine with uracil (in RNA) or thymine (in DNA)
describe the differences between DNA molecule, mRNA and tRNA
DNA
1. double polynucleotide chain
- largest molecule of the three
- double helix molecule
- pentose sugar is deoxyribose
- found mostly in the nucleus
- quantity is constant for all cells of a species
- chemically very stable
mRNA
1. single polynucleotide chain
- molecule is smaller than DNA but larger than tRNA
- single - helix (except in a few viruses)
- pentose sugar is ribose
- quantity varies from cell to cell with level of metabolic activity
- less stable than DNA or tRNA as individual molecules are usually broken down in cells within a few days
tRNA
1. single polynucleotide chain
- smallest molecule of the three
- clover- shaped molecule
- pentose sugar is ribose
- manufactured inn the nucleus but found throughout the cell
- quantity varies from cell to cell and with level of metabolic activity
how do organisms make proteins
the biochemical machinery in the cytoplasm of each cell has the capacity to make every protein just from the 20 naturally occurring amino acids
what does the manufacturing of proteins depend on
exactly which proteins organisms manufacture depends upon the instructions that are provided with at any given time, by the DNA in the cell’s nucleus
what is the basic process of protein sythesis
The basic process is as follows:
- DNA provides the instructions in the form of a long sequence of bases
- a complementary section of a part section of part of this sequence is made in the form of molecule called pre - mRNA a process called transcription
- the pre -mRNA is spliced to form mRNA
- the mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry out are linked to form a polypeptide a process called translation
protein sythesis - Bakery analogy
the process of protein sythesis can be likened to a bakery:
- basic equipment and ovens (the organelle)
- the oven makes a variety of cakes (protein)
- cake made from a few amino acids (20 amino acids)
- the cake depends on the recipe (genetic code)
by choosing different recipes at different times, rather than making everything all the time, the baker can make seasonal demand and adapt to change while avoiding any waste
DNA replication - publication analogy
DNA replication can be likened to the replication of many copies of a recipe book (genome)
- making a photocopy of a recipe to use in the bakery is therefore transcription
- making the cake using the photocopied recipe, is translation
- if the book (the genome) is not removed from the library, many copies of the recipe can be made and therefore many cakes can be produced in many places at the same time or over many years
how does transcription work
transcription is the process of making pre - mRNA using part of the DNA as a template
The process goes as follows:
- an enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region
2.the nucleotide bases on one of the two DNA strands, known as the template strand, pair with their complementary nucleotide from the pool which is present in the nucleus
The enzyme RNA polymerase then moves along the strand and joins the nucleotide together to form a pre - mRNA molecule
- In this way an exposed an exposed guanine base on the DNA bind to the cytosine base of a free nucleotide
Similarly,cytosine links to the guanine and thymine joins to adenine
The exception is adenine which links to uracil rather than thymine - as the RNA polymerase adds the nucleotide one at a time to build a strand of pre - mRNA, the DNA strands rejoin behind it. As a result only about 12 base pairs on the DNA are exposed at any one time
- when the RNA polymerase reaches a particular seuqence of bases on the DNA that it recognises as a “stop” triplet code, it detached and the production of pre - mRNA is then complete
why does the pre-mRNA have to be splice
The DNA of a gene in eukaryotic cells is made up of sections called exons that code for proteins and sections called introns that do not
These introns would prevent the sythesis of a polypeptide
Therefore, the base sequence that corresponds to the introns are removed and the functional exons are joined together during a process called splicing
why do prokaryotic cells not need splicing
in prokaryotic cells, transcription results directly in the production of mRNA from DNA
This is because prokaryotic cells do not have introns so splicing is not needed
why must mRNA move out of the nucleus via the nuclear pores
the mRNA molecules are too large to diffuse out of the nucleus and so, once they have been spliced, they leave via a nuclear pore
what does mRNA do outside the nucleus
outside the nucleus, the mRNA is attracted to the ribosomes to which it becomes attached, ready for the next stage of the process:
translation
what is gene mutation
any RANDOM change to the quantity or base sequence of the DNA of an organism is known as a gene mutation
when does mutation occur
happens during interphase as DNA replication happens here
when can mutation be inherited
mutations occurring during the formation of gametes may be inherited often producing sudden and distinct differences between individuals
how does a change in the base sequence of DNA lead to a change in the amino sequence in a polypeptide
a sequence of triplets on DNA is transcribed into mRNA and is then translated into a sequence of amino acids that make up a polypeptide
It follows that any changes to one or more bases in DNA triplets could result in a change in the amino acid sequence of the polypeptide
what are the two types of gene mutation
base substitution and base deletion
what is base substitution
it is a type of gene mutation where a nucleotide in a DNA molecules is replaced by another nucleotide that has a different base
give an example of base substitution
consider the DNA triplet GTC that codes for the amino acid glutamine
If the final base cytosine is replaced by guanine it goes from GTC to GTG
GTG is one of the triplets that codes for amino acid histdine and this then replaces the original amino acid glutamine
Therefore, the polypeptide produced will differ in a single amino acid
how significant is the change produced when bases are substituted
the significance of this difference will depend upon the precise role of the origninal amino acid
e.g.
if the amino acid is important in forming bonds that determine the tertiary structure ( disulfide bonds, ionic bonds and hydrogen bonds) , then the replacement amino acid may not form the same bonds
This results in the protein potentially having a different shape and therefore not function properly
why is a change in a proteins shape bad
if the protein is an enzyme for example, its active site may no longer be able to fit the substrate
This means that it will no longer be able to catalyse a reaction
a change in shape could also lead to a faulty cell receptor
does base substitution always result in a change to a proteins shape/ change in the amino acid sequence
the effect of the mutation may not be significant
this is because of the degenerate nature of the genetic code in which most amino acids have more than one codon
e.g. if the this base, in our example previously, had been replaced with thymine then GTC would become GTT.
As both of the DNA triplets code for glutamine, there is no change in the polypeptide produced and so the mutation has no effect
what happens during the deletion of bases
a gene mutation by deletion is when a nucleotide is loss from the normal DNA sequence
what are the consequences of gene deletion
the loss of a single nucleotide from the thousand in a typical gene may seem a minor change but the consequences can be considerable
usually the amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function properly
why do amino acids sequence become entirely different from the original when a nucleotide has been removed (deleted)
as the sequence of bases in DNA is read in units of three bases ( a triplet)
one deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base
what is chromosome mutation
changes in the structure or number of whole chromosomes are called chromosomes mutations
what are the two ways that chromosome mutations can occur
chromosome mutations can arise SPONTANEOUSLY and take two forms:
- changes in whole set of chromosomes
- changes in the number of individual chromosomes
how do changes to a whole chromosome set lead to mutations
changes in whole chromosome sets occurs when organisms have three or more sets of chromosomes rather than two
This is called POLYPLOIDY and occurs mostly in plants
how do changes to the number of individual chromosomes lead to mutation
sometimes individual homologous pairs of chromosomes fail to separate during meiosis
This is known as NON - DISJONCTION and usually results in a gamete having either one or one fewer chromosome
On fertillistaion with a gamete that has the normal complement of chromosomes, the resultant offspring have more or fewer chromosomes than normal in their body cells
e.g. humans with down syndrome have an additional chromosome 21
what happens after the triplet codes of DNA is transcribed into a sequence of codons (genetic code) on messenger RNA (mRNA)
the next stage is too translate the codons on the mRNA into a sequence of amino acids that make up a polypeptide
how many tRNA are there
there are about 60 different tRNAs
what do each particular tRNA have in
a particular tRNA has a specific anticodon and attaches to a specific amino acid
each amino acid therefore has one or more anticodon bases
synthesising a polypeptide - process
once mRNA passes out of the nuclear pore, it determines the synthesis of a polypeptide
PROCESS
1. a ribosome becomes attached to the starting codon (AUG) at one end of the mRNA molecule
- the tRNA molecule with the complementary anticodon sequence (UAC) moves to the ribosome and pairs up with the codon on the mRNA
This tRNA carries a specific amino acid (methione) - a tRNA molecule with a complementary anticodon (UGC) pairs with the next codon on the mRNA (ACG).This tRNA molecule carries anothr amino acid (threonine)
- the ribosome moves along the mRNA, bringing together wto tRNA molecules at any one time, each pairing up with the corresponding two codons on the mRNA
- the two amino acids on the tRNA molecules are joined by a peptide bond using an enzyme and ATP which is hydrolyed to provide the required energy
- the ribosome moves on to the third codon (GAU) in the sequence on the mRNA, thereby linking the amino acids on the second and third tRNA molecules
- 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
- process continues
how many ribosomes can pass behind the first one
up to 50 ribosomes can pass immediately behind the first, so that many identical polypeptide can be assembled simultaneously
when does the synthesis of proteins end
the synthesis of a polypeptide contains until a ribosome reaches a stop codon
At this point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain is complete
how do genes determine the sequence of
the DNA sequence of triplets that make up a gene determine the sequence of codons on mRNA
the sequence of codons on mRNA determine the order in which the tRNA molecules line up
they in turn, determine the sequence of amino acids in the polypeptide
In this way genes precisely determine which proteins a cell manufactures
-Add many of these proteins are enzymes, genes effectively control the activities of cells
how do you assemble a protein
sometimes a single polypeptide chain is a functional protein often, a number of polypeptides are linked together to give functional proteins:
- the polypeptide is coiled or folded, producing its secondary structure
- the secondary structure is folded, producing the tertiary structure
- different polypeptide chains, along with any non - protein groups are linked to form the quaternary structure
