4.1 & 4.2 Genes and the genetic code Flashcards
What is a gene?
A section of DNA that contains the coded information for making polypeptides and functional RNA
Where is a gene located?
A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule
What do genes code for?
- The gene is a base sequence of DNA that codes for:
- the amino acid sequence of a polypeptide
- or a functional RNA, including ribosomal RNA and transfer RNAs
What is the genetic code?
- The order of bases on DNA. Consists of codons (triplets of bases that code for a particular amino acid)
- Scientists suggested that there must be a minimum of three bases that coded for each amino acid because…
> Only 20 different amino acids regularly occur in proteins
> Each amino acid must have its own code of bases on the DNA
> Only 4 different bases are present in DNA
> If each base coded for a different amino acid, only 4 different amino acids could be coded for - Using a pair of bases, 16 different codes are possible, which is still inadequate
- 3 bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids
What 4 bases are present in DNA?
- Adenine
- Guanine
- Cytosine
- Thymine
What are the features of the genetic code?
- A few amino acids are coded for by only a single triplet
- The remaining amino acids are coded for by between 2 and 6 triplets each
- 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 the amino acid methionine. If this first methionine molecule does not form part of the final polypeptide, it is later removed
- 3 triplets do not code for any amino acid. These are called ‘stop codes’ and mark the end of a polypeptide chain
- The code is non-overlapping
- The code is universal
- Degenerate
What does non-overlapping mean?
Each base in the sequence is only read once. Thus 6 bases numbered 123456 are read as triplets 123 and 456
What does universal mean? (genetic code)
With a few minor exceptions each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution
Explain what exons and introns are.
- Much of DNA in eukaryotes does not code for polypeptides
- For example, between genes there are non-coding sequences made up of multiple repeats of base sequences
- Even within genes, only certain sequences code for amino acids
- These coding sequences are called EXONS
- Within the gene these exons are separated by further non-coding sequences called INTRONS
- Some genes code for ribosomal RNA and transfer RNAs
Describe DNA in prokaryotic cells.
- In prokaryotic cells such as bacteria, The DNA molecules are shorter, form a circle and are not associated with protein molecules
- Prokaryotic cells therefore do not have chromosomes
Describe DNA in eukaryotic cells.
- 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 chromosomes
- The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotic cells, is short, circular and not associated with proteins
What is the structure of a chromosome?
- Chromosomes are only visible as distinct structures when a cell is dividing
- For the rest of the time they are widely dispersed throughout the nucleus
- When they first become visible at the start of cell division chromosomes appear as 2 threads, joined at a single point.
- Each thread is called a chromatid because DNA has already replicated to give 2 identical DNA molecules
- The DNA in chromosomes is held by histones
- The considerable length of DNA found in each cell is highly coiled and folded
What is the structure of a DNA-histone complex?
- DNA helix is wound around histones to fix it in position
- The DNA-histone complex is then coiled
- The coil is looped and further coiled before being packed into the chromosome
- In this way a lot of DNA is condensed into a single chromosome
How much DNA does a chromosome contain?
- The chromosome contains just a single molecule of DNA, although this is very long
- This single DNA molecule has many genes along its length
- Each gene occupies a specific position along the DNA molecule
What are homologous chromosomes?
- Sexually produced organisms, such as humans, are the result of the fusion of a sperm and an 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 and the other is derived from the chromosomes provided by the father in the sperm (maternal and paternal)
- These are known as homologous pairs and the total number is referred to as the diploid number
What is a homologous pair?
Is always two chromosomes that carry the same genes but not necessarily the same alleles of the genes
What is an allele?
- An allele is one of a number of alternative forms of a gene
- Each gene exists in two, occasionally more, different forms
- Each of these forms is called an allele
- Each individual inherits one allele from each parent
- These two alleles may be the same or they may be different
What happens when the alleles are different?
- Each allele has a different base sequence, therefore a different amino acid sequence, so produces a different polypeptide
- Any changes in the base sequence of a gene produces a new allele of that gene and results in a different amino acids being codes for
- This will lead to the production of a different polypeptide, and hence a different protein
- Sometimes this different protein may not function properly or may not function at all
What happens if the protein in the production of a different polypeptide doesn’t function?
When the protein produced is an enzyme, it may have a different shape
- The new shape may not fit the enzyme’s substrate
- As a result the enzyme may not function and this can have serious consequences for the organism
How is the coded information on the DNA in the nucleus transferred to the cytoplasm where it is translated into proteins?
Sections of the DNA code are transcribed onto a single-stranded molecule called ribonucleic acid
What is 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 on a length of DNA in the process of transcription
- 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 synthesis.
- Its structure is suited to this function because it possesses information in the form of codons
- The sequence of codons determines the amino acid sequence of a specific polypeptide that will be made
What is a codon?
The term codon refers to the sequence of three bases on the mRNA that codes for a single amino acid
What is a genome?
The complete set of genes in a cell, including those in mitochondria and/or chloroplasts
What is a proteome?
The full range of proteins produced by the genome. Sometimes called the complete proteome, where the term proteome refers to the proteins produced by a given type of cell under a certain set of conditions
What is RNA?
- Ribonucleic acid is a polymer made up of repeating mononucleotide sub-units
- It forms a single strand
What is each nucleotide made up of in RNA?
- The pentose sugar ribose
- One of the organic bases adenine, guanine, cytosine and uracil
- a phosphate group
