4. DNA, Chromosomes, Transcription and Translation Flashcards
Describe how the DNA is organised/arranged in eukaryotic cells
In eukaryotic cells (nucleus of eukaryotes)
DNA is linear (Linear DNA) with two ends
DNA molecules are much longer (compared to DNA molecules in prokaryotic cells)
The DNA molecules are tightly wrapped around proteins called histones (histone proteins) – forming complex structures called chromosomes
Describe how the DNA is organised/arranged in prokaryotic cells
In Prokaryotic cells
DNA is circular with no free ends (Circular DNA)
DNA molecules are relatively short
DNA in prokaryotes is not bound to histones - so Prokaryotic DNA does not exist as a chromosome
How does the DNA in mitochondria and chloroplasts compare to the DNA in prokaryotes
The DNA in mitochondria and chloroplasts are very similar to the DNA we find in prokaryotes
It is relatively short, circular and not attached to histones
What is the dark material in the nucleus
What is it not possible to see at this point
The dark material in the nucleus is the DNA bound to histones
Not possible to see any distinct chromosomes
This is because at this stage, the chromosomes have a relatively open structure
At this point, we refer to the DNA and histones as chromatin
what happen before a cell divides
What happens to these two copies
Before a cell divides all of the chromosomes are copied
These two copies remain attached at a point called the centromere
Describe the structure of chromosomes
Before a cell divides all of the chromosomes are copied
These two copies (chromosomes) remain attached at a point called the centromere
Now the two DNA molecules are called chromatids (left hand and right hand chromatid)
At this stage, we now refer to the whole structure as a chromosome
At this point the chromosomes condense
The DNA and histones form densely packed loops and coils and the chromosomes become visible in the cell
Describe what is meant by homologous chromosomes
a pair of chromosomes, one maternal and one paternal, that carry the same genes (but not necessarily the same alleles of the genes)
two chromosomes in a homologous pair have the same genes
Define locus
The position of a gene on a chromosome is called the locus for that gene
A gene occupies a fixed position, called a locus, on a particular
DNA molecule
how many chromosomes in human cells
In human cells, we find 46 chromosomes
what would the homologous pair of chromosome 9 contain
chromosome 9
paternal and maternal chromosome containing genes
What is a gene
A gene is a section of DNA which encodes (codes for) the amino acid sequence of a polypeptide
CERTAIN REGIONS OF CHROMOSOMES ARE CALLED GENES
A gene is a base sequence of DNA that codes for:
* the amino acid sequence of a polypeptide
* a functional RNA (including ribosomal RNA and tRNAs).
How can genes occur in slightly different versions
Sometimes random mutations take place
This means that genes can occur in slightly different versions
What is an allele
Versions of a gene
how do humans inherit homologous chromosomes
They inherit one of each of the homologous chromosomes in a pair from their father and one from their mother
This means that on a homologous pair of chromosomes, the alleles do not have to be the same
e.g. inheriting the A allele from their father and the B allele from their mother
They inherit one of each of the homologous chromosomes in a pair from their father and one from their mother
What does this mean about the alleles on a homologous pair of chromosomes
This means that on a homologous pair of chromosomes, the alleles do not have to be the same
e.g. inheriting the A allele from their father and the B allele from their mother
Does the majority of the DNA found in chromosomes code for polypeptides?
A lot of the DNA that we find in chromosomes does not code for polypeptides
Describe where non-coding sequences of DNA can be found
Between the genes there are large amount of repeating base sequences. These repeating sequences are non coding
Within functional genes, we find stretches of non-coding DNA. These are called introns
What are introns
non-coding sequences of DNA found within function genes (exons)
what is an exon
Even within a gene only some
sequences, called exons, code for amino acid sequences
sequences of DNA within a gene which code for amino acid sequences
How the sequence of DNA is used to determine the amino acid sequence of protiens
What do genes play a key role in
Genes play a key role in protein synthesis
This is because the nucleotide sequence of a gene, encodes the primary structure of a polypeptide (or protein) (the sequence of amino acids)
What are the two main stages in protein synthesis
The first stage is called transcription and this takes place in the nucleus
During transcription, the base sequence of a gene is copied into the complementary base sequence of a molecule called messenger RNA (mRNA)
The mRNA molecule then moves to the cytoplasm
In the cytoplasm, the information encoded in the nucleotide sequence of the mRNA is used to join a specific order of amino acids, forming the polypeptide
This stage is called translation
Describe the stages of transcription
Part of a gene encoding a specific polypeptide
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In the first stage of transcription, DNA helicase breaks the hydrogen bonds between the two strands
Now complementary RNA nucleotides move into place and form hydrogen bonds with the bases on the exposed nucleotides on one of the DNA strands
At this point, the enzyme RNA polymerase, joins the RNA nucleotides, by forming phosphodiester bonds
The enzymes continue making their way along the DNA until they reach the end of the gene
A strand of messenger RNA has been produced
The base sequence of the mRNA is the same as the base sequence of the top DNA strand (except thymine has been replaced by uracil)
The top DNA stand is called the sense strand
The mRNA is complementary to the other DNA strand
This is called the antisense or template strand
Once the mRNA has been synthesised, the RNA polymerase detaches from the DNA and the DNA goes back to its normal double helix structure
At this stage, the mRNA now moves out of the nucleus through a nuclear pore
Once in the cytoplasm, the mRNA can take part in translation
most mRNA molecules are hundreds of nucleotides long
Describe how splicing leads to messenger RNA
a large amount of DNA is non-coding
non-coding DNA is found both between genes and also within genes
structure of a gene: exon intron exon intron exon
non-coding within a gene are called introns
coding regions within a gene are called exons
In humans, many genes contain a large number of introns
During transcription, both exons and introns are copied into RNA
That means that the RNA contains non-coding regions
This is called pre-mRNA
Once the pre-mRNA is formed, the introns are then removed and the ends of the exons connected
This process is called splicing
Splicing converts the pre-mRNA into functional mRNA
Gene
exon intron exon intron
pre-mRNA
exon intron exon intron
| splicing
\/
mRNA
exon exon
transcription
\/
Explain why splicing does not take place in prokaryotes
Splicing does not take place in prokaryotes
This is because introns are uncommon in prokaryotic cells
What do some genes encode for rather than amino acid sequence of a polypeptide
Some genes do not encode for polypeptides
Instead, these genes encode/codes for functional RNA molecules
Examples of functional RNA molecules include transfer RNA and ribosomal RNA
how many pairs of chromosomes do humans have
23 pairs of chromosomes
Describe what is meant by the genome
All of the genes in a cell is called its genome
In eukaryotes, this includes both the genes on the chromosomes and the genes in mitochondria and in chloroplasts
Describe what is meant by the proteome
This is because the nucleotide sequence of a gene, encodes the primary structure of a polypeptide (or protein) (the sequence of amino acids)
All of the proteins produced by the genome of an organism is called the proteome
Only a fraction of those proteins will be produced in any particular cell type
Cells can produce a different range of proteins depending on what the cell is doing (its function)
How are mRNA nucleotides read
mRNA nucleotides are read as a series of triplets (three bases)
These triplets are called the genetic code (UUU UUA -> Phe table etc.)
These are the shorthand versions of each of the twenty amino acids that we find in proteins
These are these are the mRNA triplets which encode those amino acids
The mRNA triplets are the genetic code
key features of the genetic code
Most amino acids have more than one triplet (multiple triplets code for the same amino acid)
E.g. Leucine has 6 triplets
Because of this, it is said that the genetic code is called a degenerate code
The triplet code is non-overlapping
This means that no base is read more than once
e.g. AUG GCA CUG - triplet 1, triplet 2, triplet 3
if code was overlapping AUG would be triplet 1, UGG would be triplet 2 etc.
The genetic code is universal
The same triplets encode the same amino acids in the vast majority of organisms on planet Earth
What is each triplet in the mRNA called
A codon
Triplet - 3 bases
Describe the structure of tRNA
tRNA has two important parts
At the top of the tRNA molecule, we have a binding site for an amino acid
At the bottom of the tRNA molecule, we have a triplet of bases called the anticodon
tRNA for the amino acid methionine
The tRNA anticodon is complementary to the mRNA codon for that amino acid
e.g.
on the mRNA, the first codon is AUG. This is the start codon but its also the codon for methionine
The anticodon, on the tRNA carrying methionine (amino acid) is complementary to the mRNA codon for methionine
There is a complementary tRNA for the codons encoding every amino acid
Many amino acids are encoded by several triplets
Each one of these will have a corresponding transfer RNA
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Describe the stages of translation
In the genetic code, we can see that one of the triplets determines where to start translating the mRNA molecule. This is called the start triplet
The start triplet also encodes the amino acid methionine
Three triplets determine where translation stops
These are called stop triplets
In translation the nucleotide sequence of the mRNA is used to determine the amino acid sequence of a polypeptide
Each triplet in the mRNA molecule is called a codon
In order for the mRNA to be read, another type of RNA is involved
This is called transfer RNA (tRNA)
Once the mRNA moves from the nucleus to the cytoplasm, the small subunit of a ribosome binds with the mRNA at the start codon
Ribosomes contain a number of different proteins
They also contain a type of RNA called ribosomal RNA
Now a tRNA molecule with an anticodon complementary to the start codon attaches
This is held in place by hydrogen bonds between the complementary base pairs on the mRNA and tRNA
Now a second tRNA molecule moves into place
The anticodon on this tRNA is complementary to the second codon on the mRNA
This codon encodes for a specific amino acid. This amino acid is brought in by the tRNA
Now a peptide bond is formed between the two amino acids
This is catalysed by the enzyme peptidyl transferase - which is a part of the ribosomal RNA molecule
The formation of the peptide bond requires energy provided by ATP
Now the ribosome moves to the next codon and forms a peptide bond to the next amino acid
The first tRNA molecule is released
tRNA molecules that have been released are later attached to their amino acids by enzymes in the cytoplasm
The ribosome continues moving down the mRNA, forming the polypeptide
When the ribosome comes to a stop codon, it detaches and the polypeptide chain is released
Once the first ribosome has started moving along, another ribosome can attach to the start codon and start translating. We might see a whole line of ribosomes making their way along the mRNA
This means that a large number of polypeptide molecules can be produced rapidly
Once the polypeptide has correctly folded, it can then carry out its function in the cell
