4A DNA, RNA and Protein Synthesis Flashcards
What is Nondisjunction?
Nondisjunction is where chromosomes fail to segregate during meiosis
What can Nondisjunction be due to?
A failure of homologous chromosomes to segregate in meiosis 1 or a failure of sister chromatid to segregate in meiosis 2
What happens if a gamete with the incorrect no. chromosomes is fertilised?
The resulting embryo may not be viable and might die or else an individual may be born with a genetic disease
What are the two ways which meiosis is a source of variation?
- Independent Assortment
2. Crossing Over
What is a homologous chromosome?
Homologous chromosomes are a pair chromosomes, one maternal and one paternal, that have the same gene loci
What is an allele?
An allele is a form of a gene
What is a gene locus?
A gene locus is a fixed position on a chromosome that is occupied by a gene
When does crossing over occur?
Phase 1 of meiosis
Why does crossing over happen?
Crossing over occurs as piece of chromatid from one chromosome touches a piece of chromatid from the homologous partner.
What happens during crossing over?
The chromatids of each pair become twisted around each other (crossing over). This creates tensions and causes portions of the chromatids to break off.
What is recombination?
After crossing over, the broken portions might then rejon with the chromatids of its homologous partner
What does recombination result in?
The exchange of genetic material between homologous chromosomes
–> Each homologous chromosome has a different combo of alleles compared to what they had originally
Why do chromosomes stay the same length in crossing over?
The genetic material is exchanged between homologous chromosomes and at the same gene loci, no genetic material is lost
In meiosis, how many chromosomes from each homologous pair enters a gamete?
One
Which of the two chromosomes from each pair enters a gamete is dependant on what?
Dependent on how the homologous pairs line up at the equator of the cell ( THAT MAKES BLOODY SENSE DONT IT)
Shut up Dan
In metaphase 1, how do chromosomes align along the equator of the cell?
Chromosomes randomly align along the equator of the cell (independent assortment)
What is independent assortment?
Chromosomes randomly align along the equator of the cell in metaphase 1
What does the random alignment of chromosomes actually refer to?
The way in which one homologous pair lines up at the equator in meiosis is independent of how any other pair aligns
How does the random alignment of chromosomes affect the chromosomes in daughter cells?
The combo of chromosomes of maternal & paternal origin that go into the daughter cell at meiosis 1 is a matter of chance
(independent segregation)
What do you need to know to calculate the number of possible combinations in the gametes?
The haploid number –> 2^n where n is the haploid no.
What is cross fertilisation?
Where the female and male gametes fuse
What can cause further variation other than meiosis?
Cross fertilisation
What mixes up allelic combination?
Independent assortment and crossing over during meiosis and the random nature of fertilisation
What is genetic novelty?
New variations of alleles or genes produced
What can produce genetic novelty?
Mutations
Is genetic novelty produced during meiosis?
No - it mixes up allelic combination but does not produce genetic novelty
What are the two possible types of variation?
- Genetic
- Environmental
How does crossing over create genetic variation?
It creates genetic variation between daughter cells in meiosis by creating new allelic combinations
How does independent assortment add to genetic variation?
By shuffling the combination of maternal and paternal chromosomes in the daughter cells
How does cross fertilisation add to genetic variation?
Adds to genetic variation as which male gamete fertilises which female is entirely random
What effect do mutations have on variation?
Mutations create genetic novelty, this can also add to genetic variation
How is DNA passed from one generation to the next?
By gametes
Sperm cells join to egg cells which form what?
Zygote - which develops into a new organism
Normal body cells have how many chromosomes?
The diploid number (2n) - means each chromosome has one from mum, one from dad
Gametes have how many chromosomes?
Haploid number (n) of chromosomes - only one copy of each chromosome
What happens in fertilisation?
Haploid sperm cell fuses with a haploid egg - makes an egg with the diploid no. chromosomes
How do sperm and egg cells increase variation?
- Fertilisation is random
- Random fertilisation produces zygotes with different combos of chromosomes to both parents
- Any sperm can fertilise any egg
How are gametes formed?
Via meiosis
What sort of DNA is found in eukaryotic cells?
Linear DNA that exist as chromosomes which are found in the nucleus
Why does DNA have to be wound up?
DNA is a very long molecule therefore it has to be wound up to fit into the nucleus
How is DNA wound up?
The DNA is wound around proteins called histones - these histones also help to support the DNA
Once the DNA is wound around the histone, what happens?
The DNA (and protein) is then coiled up very tightly to make a compact chromosome
Which cell components in eukaryotic cells have their own DNA?
The mitochondria and chloroplasts
What is the DNA in mitochondria and chloroplasts like?
It is pretty similar to prokaryotic DNA as it’s circular and shorter than DNA molecules in the nucleus
What is DNA like in prokaryotic cells?
The DNA is also carried as chromosomes
DNA molecules are shorter & circular in prokaryotes
How does the DNA in prokaryotic cells coil up?
DNA isn’t wound around histones - it condenses to fit in the cell by supercoiling
What is a gene?
A sequence of DNA bases that codes for either a polypeptide or functional RNA
What forms the primary structure in DNA?
The sequence of amino acids in a polypeptide forms the primary structure of a protein
How do polypeptides vary based on their structure?
Different polypeptides have a different no. and order of amino acids
What determines the order of amino acids in a polypeptide?
The order of the bases in a gene determine the order of amino acids
How is an amino acid coded for?
Each amino acid is coded for by a sequence of 3 bases in a gene called a triplet
How is a polypeptide made in protein synthesis?
DNA is first copied into mRNA (this is the first stage of protein synthesis)
If a gene doesn’t code for a polypeptide, what does it code for?
Functional RNA
What is functional RNA?
RNA molecules other than mRNA, which perform special tasks during protein synthesis
e.g. tRNA and rRNA, which form part of ribosomes
What is a cell’s Genome?
The complete set of genes in the cell
What is a cell’s Proteome?
The full range of proteins that the cell is able to produce
In eukaryotic DNA, genes that code for polypeptides contain what?
Sections of code that don’t code for amino acids
What are introns?
Sections of gene that do not code for amino acids
There can be several introns within a gene
What are exons?
The bits of gene that do code for amino acids
When are introns removed?
During protein synthesis
Why are introns removed?
So they don’t affect the amino acid order
What is the purpose of introns?
Their purpose isn’t known for sure - prokaryotic DNA however doesn’t have introns
What do eukaryotic cells contain outside of genes?
Eukaryotic DNA also contains regions of multiple repeats outside of genes
What are multiple repeats in eukaryotic cells?
These are DNA sequences that repeat over and over
i.e CCTTCCTTCCTT
What is the use of multiple repeats in eukaryotic cells?
These areas don’t code for amino acids (like introns) so they’re called non-coding repeats
Genes can exist in different forms, what is this called?
Alleles
What is the order of bases like in different alleles and what does this mean?
The order of bases in each allele is slightly different
This means they code for slightly different versions of the same polypeptide
Give an example of an allele?
The gene that determines blood type exists as one of three alleles
One determines type O, another type A and the other type B
What are a homologous pair of chromosomes like?
Both chromosomes are the same size & have the same genes, although they could have different alleles
What is the locus on a chromosome?
Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosome in a homologous pair
What are the two stages of protein synthesis?
Transcription and translation - they both involve RNA
What is RNA like generally?
A single polynucleotide strand and it contains uracil instead of thymine (uracil always pairs with adenine)
What are the 3 types of RNA?
- mRNA
- tRNA
- rRNA
When is mRNA made?
During transcription
What does mRNA do?
It carries the genetic code from the DNA to the ribosomes
At the ribosomes it is used to make a protein during translation
What is the structure of mRNA?
A single polynucleotide strand
In mRNA groups of three adjacent bases are usually called codons
What is tRNA involved in?
It is involved in translation
What is the role of tRNA?
It carries the amino acids that are used to make proteins to the ribosomes
What is the structure of tRNA?
tRNA is a single polynucleotide strand that’s folded into a clover shape
How is tRNA held in the clover shape?
Hydrogen bonds between specific base pairs hold the molecule in this shape
What does every tRNA molecule have?
They all have a specific sequence of three bases at one end called an anticodon
Also have an amino acid binding site at the other end
What is the first stage of protein synthesis?
Transcription
What happens during transcription?
An mRNA copy of a gene is made from DNA
Where does transcription take place in eukaryotes?
The nucleus
Where does transcription take place in prokaryotes?
They don’t have a nucleus, so it takes place in the cytoplasm
What are the stages of transcription?
- RNA polymerase attaches to DNA at beginning of a gene
- Hydrogen bonds between strands break - DNA uncoils, bases exposed
- One of strands is used as template to make mRNA copy
- Complementary base pairing happens
- RNA polymerase joins DNA strands - forms mRNA molecule
- RNA polymerase moves along the DNA, separating the strands & assembling the mRNA strand
- DNA reforms once RNA polymerase has passed & strands coil into double helix
- RNA polymerase detaches at a stop signal
- mRNA moves out of nucleus & attaches to a ribosome
Does transcription make the same products in prokaryotes and eukaryotes?
No, the products are different
How is mRNA produced differently in prokaryotes to eukaryotes?
Prokaryotes - mRNA produced directly from DNA, no splicing (unspliced DNA)
Eukaryotes - introns & exons both copied in transcription splicing then occurs, this takes place in the nucleus then mRNA leaves the nucleus (spliced DNA)
What is the process of producing mRNA in eukaryotes?
- Introns & exons both copied into mRNA during transcription
- mRNA containing introns & exons are called pre-mRNA
- Splicing then occurs (introns removed, exons join) forms mRNA strands
- mRNA leaves the nucleus for next stage of protein synthesis
What is the second stage of protein synthesis?
Translation
Where does translation occur in eukaryotes and prokaryotes?
It occurs at the ribosomes in the cytoplasm
What happens during translation?
Amino acids are joined together to make a polypeptide chain (protein), following the sequence of codons (triplets) carried by mRNA
What can protein synthesis also be called?
Polypeptide synthesis
What happens in translation?
- mRNA attaches to ribosome
- tRNA with complementary anticodon attachest to mRNA by specific base pairing
- 2nd tRNA molecule attaches to next codon on mRNA
- Two amino acids attached to tRNA joined by peptide bond - first tRNA moves away, leaves amino acid
- 3rd tRNA binds to next codon on mRNA - its amino acids bind to first 2 & 2nd tRNA moves away
- Process cont. - produces chian of linked amino acids (polypetide chain) until stop signal on mRNA
- Polypeptide chian moves away from ribosom & translation is complete
What is the gentic code like?
- Non-overlapping
- Degenerate
- Universal
How is genetic code non-overlapping?
In the genetic code, each base triplet is read in a sequence from the triplet before it & after it
Base triplets don’t share their bases - making them non-overlapping
How is genetic code degenerate?
More combos of triplets than there are amino acids (20 amino acids –> 64 possible triplets)
Means some aminos acids are coded for by more than one base triplet
e.g. tyrosine cna be coded for by UAU or UAC
What are start/stop codons?
Triplets that are used to tell whent to start & stop production of the protien
Found at the beggening or end of the mRNA e.g. UAG is a stop signal
How is the genetic code universal?
The same specific base triplets code for the same amino acids in all living things
e.g. UAU codes for tyrosine in all organisms
