Genetic Information Flashcards
Describe a DNA molecule
- Double stranded
- Each nucleotide in the strand comprises of a phosphate, deoxyribose sugar and a nitrogenous base
- The bases from one strand pair in a complementary fashion with the bases on the other strand (adenine pairs with thymine, guanine with cytosine)
- Millions of base pairs long
Where is DNA found in prokaryotes?
Prokaryotic (bacterial) cells have a single molecule of DNA formed into a circle. Some cells also have smaller loops called plasmids. The DNA is naked and is not associated with histone proteins
Where is DNA found in eukaryotic cells?
- DNA is found in the nuclei of eukaryotes, in a very long linear molecule which is associated with histone proetins
- DNA is also found in the chloroplasts and mitochondria in eukaryotes in a short circular molecule, not associated with histone proteins
Together what do DNA and histone proteins form?
A chromosome
How is the DNA and histone proteins arranged to form a chromosome?
The DNA molecule is wrapped around the histone proteins to fix it into position, the DNA histone complex is then coiled into solenoids and then further coiled. This means that a very long molecule can be condensed into a single chromosome (lots of genetic information can be stored in the nucleus of each cell
When can a chromosome be seen as a distinct structure?
When the nucleus is about to divide and the DNA molecule is already replicated by semi-conservative replication (this is prophase of the cell cycle)
What is a diploid organism?
An organism that inherits the full set of chromosomes from each parent, as a consequence diploid cells have homologous pairs of chromosomes
What diploid number do humans have?
2n=46
What is it called when one member of the pair is inherited from the mother?
Maternal chromosome
What is it called when one member of the pair is inherited by the father?
Paternal chromosome
What is a homologous pair of chromosomes?
Carry the same genes at the same locus but do not necessarily carry the same alleles of a gene. They are not genetically indentical
What is a gene?
A base sequence of DNA that codes for either:
1 - The amino acid sequence of a polypeptide
2 - A functional RNA (including rRNA and tRNA)
Describe the structure of a gene
- A gene occupies a fixed position called a locus on a particular DNA molecule. One DNA molecule carries many genes
- In eukaryotes most of the DNA molecule does not code for amino acid sequences in the polypeptides (on 2% in humans). There are non-coding multiple repeat sequences between genes
- Not all of the base sequences within genes codes for amino acid sequences.
Which part of the genes code for amino acid sequences?
Exons
Which part of the genes do not code for amino acid sequences and are separated by one or more coding sequences?
Inrons
Define the term genome
The complete set of all the genes in a cell, including the genes in the nucleus, mitochondria and chloroplasts
Define the term proteome
The full range of all the proteins that a cell is able to produce using its genome
Define the term allele
Some genes have two or more alternative forms, each allele has a different sequences of bases and consequently codes for a different polypeptide
Define the term gene pool
All of the different alleles of all the genes found within a population
How many bases in a gene sequence code for a specific amino acid?
Every 3 bases forms a triplet that codes for 1 amino acid
How many possible combinations of triplets are there?
4x4x4 = 64 possible combinations
What is meant by the term degenerate?
Some amino acids can be coded for with different triplets e.g. AGA and AGG both code for ‘ARG’
What does the fist triplet always code for?
Methionine
Why do only 61 triplets code for amino acids out of the 64 possible combinations?
3 of the triplets are stop triplets
Where is a stop triplet always found?
At the end of each gene
What is meant when saying the code is universal?
It is the same in all organisms
What is meant when saying the code is non-overlapping?
Each base is read only once, each base is [art of only one triplet
What two types of RNA are needed in addition to the DNA to synthesise proteins?
mRNA and tRNA
Describe RNA molecules
- Single stranded polynucleotide
- The nucleotides comprise of a phosphate, a ribose sugar and one of 4 nitrogenous bases - adenine, uracil, guanine and cytosine
- Phosphodiester bonds between nucleotides
How many nucleotides long is mRNA?
Up to a few thousand nucleotides long
What is every 3 bases named in mRNA?
A codon
How many nucleotides long is tRNA?
About 75 nucleotides long ,
Describe a tRNA molecule
- The strand folds back on itself and hydrogen bonds form between complementary base pairs
- The overall shape is called clover leaf
- Three bases are exposed, these are called the anticodon
- The anticodon on a tRNA molecule bonds to a complementary codon on a mRNA nmolecule during the process of translation
What is the first step to making proteins?
Transcription
What does transcription do?
It makes a molecule of pre-mRNA, the whole gene is transcribed including the introns as well as the exons
Describe the process of transcription
- One gene unwinds due to hydrogen bonds between complementary bases being broken
- Complementary free RNA nucleotides bind to the exposed bases on the template strand e.g. A-U, T-A, C-G and G-C
- The sugar phosphate backbone between the free RNA nucleotides is joined using RNA polymerase (creates phosphodiester bonds)
- Transcription stops at a stop triplet such as ATC. This marks the end of a gene and causes the disengagement of the RNA polymerase
- A gene is transcribed repeated to make many pre-mRNA at any time
- Most genes are transcribed to make pre-mRNA however a few genes are transcribed to make rRNA and tRNA
What is the second stage of making a protein?
Splicing mRNA
Where does transcription take place?
In the nucleus
Where does splicing take place?
In the nucleus
Describe splicing mRNA
pre-mRNA is made into mRNA
- In eukaryotes nulcei the pre-mRNA is then spliced, the non-coding sections are cut out and the coding sections are edited together to produce mRNA which is a copy of the exons only
- Prokaryotic DNA and mitochondrial and chloroplast DNA does not have introns so the mRNA does not need splicing
What does translation do?
Makes polypeptides
Where does translation place?
Either in the cytoplasm or RER