DNA, genes and protein synthesis- chapter 8 Flashcards
what is a gene
-a section of DNA that contains coded information for making polypeptides and functional RNA
-the coded information is in the form of a specific sequences of bases
-polypeptides makes proteins and enzymes which then control chemical reactions
what is a locus
when a gene (section of DNA) is located at particular positions on a DNA molecules
a gene is a base sequence that codes for…
-The amino acid sequence of a polypeptide
- Functional RNA, including ribosomal RNA and transfer RNAs
what is the genetic code
and what is scientists reasoning for this suggestion
there must be a minimum of three bases (triplet) that coded for each amino acids
- Only 20 different amino acids regularly occur in proteins.
- Each amino acid must have its own code of bases on the DNA.
- Only four different bases are present in DNA.
- If each base coded for a different amino acid that would only be 4.
- Using a pair of bases, 16 codes are possible.
- Three bases produce 64 different codes which is more than enough for the 20 amino acids.
why is the genetic code known as degenerate
as most amino acids are coded for by more than one triplet
what does it mean that the genetic code is non-overlapping
each base is only read once
what are further reasonings for the genetic code
- a few amino acids are coded for by only a single triplet.
- Remaining are coded for by between two and six triplets each.
The code is known as degenerate code. - A triplet is always read in one particular direction.
- The start of a DNA sequence that codes for a polypeptide is always the same triplet (methionine)
- Three triplets do not code for any amino acid, these are stop codes.
- The code is non-overlapping
- Code is universal
- much of the DNA does not code for polypeptides
what is the coding sequence called
what are introns
extrons
within the gene exons are separated further non-coding sequences
when are chromosomes visible
only when the cell is dividing
what is each thread of a chromosome called
a chromatid
what is the DNA in chromosomes held by
histones
what are homologous pairs
Sexually produced organisms are the result of fusion of a sperm and an egg cell. So one of each pair is derived from the chromosomes of the mother and the other by the father. These are known as homologous pairs.
what are alleles
they are one of a number of alternative forms of a gene as each gene exists in 2, occasionally more, different forms
what does the change in base sequence of a gene produce and result in
produces= a new allele of that gene (mutation)
results= in a different sequence of amino acids being coded for, this leads to the production of a different polypeptide hence a different protein
what is messenger RNA
transfers DNA code from the nucleus to the cytoplasm as it is small enough to leave the nucleus through the nuclear pores and enter the cytoplasm
what is a codon
sequence of three bases on mRNA that codes for a single amino acid
what is a genome
the complete set of genes in a cell
what is a proteome
full range of proteins produced by the genome
what is an RNA structure
Polymer made up of repeating mononucleotide sub-units.
Forms a single strange in which each nucleotide is made up of:
- Pentose sugar ribose
- One of the organic bases.
- Phosphate group
explain messenger RNA (mRNA)
-Long strand (thousands of mononucleotides) arranged in a single helix
- Base sequence of RNA is determined by the sequence of bases on DNA in a process called transcription.
- Once formed it leaves the nucleus via the pore in the nuclear envelope and enters the cytoplasm where is associates with the ribosomes where it acts as a template for protein synthesis
explain transfer RNA (tRNA)
- Relatively small molecule that is made up of around 80 nucleotides
- Single stranded and folded into a clover leaf shape.
- One end extends more than the other leaving an area where amino acids can easily attach.
- At the opposite end is a sequence of three other organic bases known as the anticodon.
- Each tRNA molecules is specific to one amino acid and has an anticodon that is specific
what is the basic process of transcription and translation
- A complementary section of part of this sequence is made in the form of a molecules called pre-mRNA- a process called transcription
- The pre-mRNA is spliced to form mRNA.
- The mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide- a process called translation.
explain the process of transcription
- an enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases.
- The nucleotide bases on one of the two DNA strands (Template strand), pair with their complementary nucleotides. The enzyme RNA polymerase then moves along the strand and joins the nucleotides together to form a pre mRNA molecule.
- As the RNA polymerase adds the nucleotides one at a time to build a strand of pre mRNA, the DNA strands rejoin behind it (only about 12 base pairs on the DNA are exposed at any one time)
- When RNA polymerase reaches the stop codon it detaches.
explain splicing of mRNA
- In eukaryotes transcription results in the production of pre-mRNA which is then spliced to form mRNA.
- DNA strand is made of exons and introns, the intervening introns would prevent the synthesis of a polypeptide, the introns are removed and the functional exons are joined together. Splicing
- Once spliced the mRNA leaves via a nuclear pore.
- The mRNA then attached to a ribosome ready for translation.
explain translation- polypeptide synthesis
- A ribosome becomes attached to the starting codon (AUG) at one end of the mRNA.
- The tRNA molecule with the complementary anticodon sequence moves to the ribosomes and pairs up with the codon on the mRNA. The tRNA carries the specific amino acid.
- A tRNA with a complementary anticodon pairs with the next codon on the mRNA which carries another amino acid.
- The ribosomes moves along the mRNA, bringing together two tRNA molecules are any one time.
- The amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide energy.
- This happens on the 3rd codon, as this happens the first tRNA is released and is free to collect another amino acid.
- The process continues with up to 15 amino acids being added each second until the ribosome reaches the stop codon.
