DNA (Further) Flashcards
How is DNA stored in eukaryotic cells
Each chromosome (found in nucleus) is made up of 1 long DNA molecule and its associated proteins. As the DNA molecule is so long it is wound around proteins called histones. The DNA and proteins are then coiled very tightly, making a compact chromosome.
What else do histone proteins do (other than get wound up in DNA)
Help to support the DNA.
What do mitochondria and chloroplast in eukaryotic cells also have
Their own DNA. Their DNA is similar to prokaryotic DNA because its circular and shorter than DNA molecules in nucleus.
Is mitochondria and chloroplast DNA associated with histone proteins?
No
How is DNA stored in prokaryotic cells?
Prokaryotes also carry DNA as chromosomes - but they’re shorter and circular. It’s not wound around histones - it condenses to fit in the cell by supercoiling.
What is a gene and where are they contained
Contained within DNA, a gene is a sequence of DNA bases that codes for either a polypeptide or functional RNA.
What is different about each polypeptide chain
The number and order of its amino acids
What determines the order of amino acids in a particular polypeptide
The order of the bases in a gene
What is each amino acid coded for by
A sequence of 3 bases in a gene called a triplet or codon
What is the first stage of protein synthesis
DNA is copied into messenger RNA (mRNA)
What do genes that don’t code for a polypeptide code for
Functional RNA instead
What are functional RNA
RNA molecules other than mRNA, which perform special tasks during protein synthesis e.g. tRNA and ribosomal RNA (rRNA), which forms part of ribosomes.
What is the complete set of genes in a cell known as
The cells genome
What is the full range of proteins that a cell is able to produce called
It’s proteome
In eukaryotes what does a lot of the nuclear DNA do
Doesn’t code for polypeptides
What do even genes that do code for polypeptides contain
Sections that don’t code for amino acids
What are the sections of DNA that don’t code for amino acids in genes that code for polypeptides called
Introns
How many introns can be in a gene and what is there purpose
Can be several introns and there purpose isn’t known for sure
What happens to introns in eukaryotes during protein synthesis
They are removed so they don’t affect the amino acid order.
Does prokaryotic DNA have introns
No
What are the bits of a gene that do code for amino acids called
exons
What does eukaryotic DNA contain other than genes
Regions of multiple repeats, these are DNA sequences that repeat over and over. E.g. CCTTCCTT. The areas don’t code for amino acids, so they’re called non-coding multiple repeats
What is an allele
One of many forms of a gene
What is different about each allele
The order of bases, so they code for different versions of the same polypeptide
How many homologous pairs does each human cell have
23
What is different / the same in each homologous pair
Same size and genes, but can have different alleles
Where are alleles coding for the same characteristic in a homologous pair found
At the same fixed position (locus) on each chromosome in a homologous pair
How many amino acids are there
20
what is each amino acid coded by
a triplet code / codon, which consists of 3 nucleotide bases
Define Degenerate
One amino acid can be coded for by more than one codon
Define Universal
The nucleotides which make up the genetic code are the same in all organisms
Define non - overlapping
3 bases will code for one amino acid. Codons are read sequentially, therefore, after 3 bases of one codon have been read, the next three will be read to code for a different amino acid
What are stop codons
3 non - coding stop codons - no amino acid produced - stops production of more amino acids
What % of human DNA is non coding
98%, we are unsure of it’s purpose
what % of human DNA codes for proteins
2%
What is the difference between DNA and RNA, based on how easily hydrolysed they are
DNA is not so easily hydrolysed, whereas RNA is easily hydrolysed
What is the difference between DNA and RNA, based on how much information they can store
DNA can store lots of information, RNA can store some information
Why is DNA not used as a transporter
It is too valuable
Why is RNA used as a transporter
Smaller than DNA, so can leave nucleus via nuclear pores and its exposed single strand means amino acids can assemble easily
What is transcription
First stage in gene expression - producing an mRNA code from a sequence of DNA nucleotides
Where does transcription occur in eukaryotes
In the nucleus
Where does transcription occur in prokaryotes
In the cytoplasm
What occurs in the first stage of transcription
RNA polymerase attaches to DNA at the beginning of a gene. In eukaryotes, hydrogen bonds between bases are broken by DNA helicase (DNA helicase is attached to DNA polymerase). This separates strands, uncoiling the DNA, exposing some bases to be copied to produce mRNA.
What does transcription produce
An mRNA copy of a gene made from DNA
What happens in the second stage of transcription
Free RNA bases line up alongside exposed DNA bases. the free bases are attracted to the complementary exposed bases, once paired with their complementary base the free RNA is joined together by RNA polymerase (forms mRNA), the mRNA strand then becomes a complementary copy of the exposed DNA (template)
What happens in the 3rd stage of transcription
RNA polymerase moves down exposed DNA, reforming its hydrogen bonds and assembling mRNA, the uncoiled strand of DNA then coils back into a double helix shape once RNA polymerase has passed
What happens during the 4th stage of transcription
When RNA polymerase reaches a DNA stop codon, it detaches from DNA and stops making mRNA., finishing with mature mRNA. In eukaryotes, mRNA leaves the nucleus via a nuclear pore and attaches to a ribosome in the cytoplasm. Afterwards translation takes place
What is the first step of translation
mRNA attaches to a ribosome
What happens after mRNA attaches to a ribosome
Charged tRNA, with complementary anticodon base sequences to the first codon on the mRNA, binds to the mRNA at the P site
What happens after charged tRNA binds to the mRNA
Hydrogen bonds form between the codon on the mRNA and the anticodon on the tRNA
What happens after hydrogen bonds form between tRNA and mRNA
A second ‘loaded’ tRNA binds to the next codon on mRNA at the A site. Two tRNA molecules bind at once
What happens after a second ‘loaded’ tRNA binds to the mRNA
Two ‘loaded’ amino acids are in close proximity. A peptide bond joins the amino acids.
What happens after a peptide bond joins the amino acids
The ribosome moves along, replacing the A site with the tRNA that attaches first, with a complementary anticodon to the mRNA codon
what happens after the A site is replaced with the first complementary tRNA to attach
Once the amino acids of the original tRNA and the second loaded tRNA have joined, the original tRNA (now at the E site) will become unloaded (leaves amino acid) and disassociate from the ribosome.
What happens after the original tRNA disassociates from the ribosome
The amino acid chain continues to grow. The next ‘loaded’ tRNA molecule binds, until a stop codon is reached.
What is meant by a loaded tRNA
is attached to an amino acid
What is meant by an unloaded tRNA
has left its amino acid (become disassociated from amino acid), so is not attached to one
What does tRNA look like
A clover leaf (has 5 arms, 3 of which have a loop at the end, one which has no loop (amino acid attachment site) and a short arm that pokes out diagonally between two arms called the ‘variable arm’
What is the A site
Aminoacyl site - The A site is where the incoming tRNA carrying an amino acid binds during translation.
What is the P site
Peptidyl site - The P site holds the tRNA that is currently attached to the growing polypeptide chain.
What is the E site
Exit site - The E site is where the empty tRNA (after having donated its amino acid) exits the ribosome.
