Topic 4A - DNA, RNA And Protein Synthesis Flashcards
Describe nuclear eukaryotic DNA, and other DNA.
It is linear and exists as chromosomes - thread-like structures, each made up of one long molecule of DNA. Chromosomes are found in the nucleus. The DNA molecule is really long, so to fit in the nucleus, it is wound around proteins called histones. They also help to support the DNA. The DNA and protein is then coiled up very tightly to make a compact chromosome. The mitochondria and chloroplasts in eukaryotic cells also have their own DNA. This is similar to prokaryotic DNA because it’s circular and shorter than DNA molecules in the nucleus. It’s not associated with histone proteins.
Describe prokaryotic DNA
Prokaryotes also carry DNA as chromosomes, but the DNA molecules are shorter and circular. It is not wound around histones and it condenses to fit in the cell by supercoiling.
What are genes? Give details about what they do.
A gene is a sequence of DNA bases that codes for either a polypeptide or functional RNA. The sequence of amino acids in a polypeptide forms the primary structure of a protein. Different polypeptides have a different number and order of amino acids. It’s the order of bases in a gene that determines the order of amino acids in a particular polypeptide. Each amino acid is coded by a sequence of three bases in a gene called a triplet. To make a polypeptide, DNA is first copied into messenger RNA. This is the first stage of protein synthesis. Genes that don’t code for a polypeptide code for functional RNA instead. Functional RNA is RNA molecules other than mRNA, which perform special tasks during protein synthesis, e.g tRNA and ribosomal RNA, which forms parts of ribosomes. A cell’s genome is the complete set of genes in the cell. A cell’s proteome is the full range of proteins that the cell is able to produce.
What are the DNA in eukaryotic cells that don’t code for polypeptides?
Some genes don’t code for polypeptides, and code for functional RNA. In eukaryotic DNA, genes that do code for polypeptides contain sections that don’t code for amino acids. These sections of DNA are called introns. There can be several introns within a gene. All the bits that do code for amino acids are called exons. Introns are removed during protein synthesis - so they don’t affect the amino acid order. Their purpose isn’t known for sure, and prokaryotic DNA doesn’t have introns. Eukaryotic DNA also contains regions of multiple repeats outside of genes. These are DNA sequences that repeat over and over. These areas don’t code for amino acids either, so they’re called non-coding repeats.
What are alleles? Describe homologous pairs of chromosomes.
A gene can exist in more than one form - alleles. The order of bases in each allele is slightly different, so they code for slightly different versions of the same polypeptide. For example, blood type is O, A and B. In a eukaryotic cell nucleus, DNA is stored as chromosomes. Humans have 23 pairs of chromosomes. Pairs of matching chromosomes are called homologous pairs. In a homologous pair, both chromosomes are the same size and have the same genes, although they could have different alleles. Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosome in a homologous pair.
What is mRNA? What is tRNA?
mRNA is made during transcription. It carries the genetic code from the DNA to the ribosomes, where it’s used to make a protein during translation. mRNA is a single polynucleotide strand. In mRNA, groups of three adjacent bases are usually called codons (or triplets or base triplets). tRNA is involved in translation. It carries the amino acids that are used to make proteins to the ribosomes. tRNA is a single polynucleotide strand that’s folded into a clover shape. Hydrogen bonds between specific base pairs hold the molecule in this shape. Every tRNA molecule has a specific sequence of three bases at one end called an anticodon. They also have an amino acid binding site at the other end.
Describe transcription.
During transcription, an mRNA copy of a gene is made from DNA. In eukaryotic cells, transcription takes place in the nucleus (prokaryotes don’t have a nucleus, so transcription takes place in the cytoplasm). Transcription starts when RNA polymerase attaches to the DNA double-helix at the beginning of a gene. In eukaryotes, the hydrogen bonds between the two DNA strands in the gene are broken by a DNA helicase attached to the RNA polymerase. This separates the strands and the DNA molecule uncoils at that point, exposing some of the bases. One of the strands is then used as a template to make an mRNA copy. The RNA polymerase lines up free RNA nucleotides alongside the exposed bases on the template strand. The free bases are attracted to the exposed bases. Specifics complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (except the base T is replaced by U in RNA). Once the RNA nucleotides have paired up with their specific bases on the DNA strand, they’re joined together by RNA polymerase, forming an mRNA molecule. The RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand. The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double-helix. (In prokaryotes, the DNA strands are separated by RNA polymerase, not DNA helicase). When RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detached from the DNA. In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place.
How is transcription different in eukaryotes and prokaryotes?
In eukaryotes, the introns and exons are both copied into mRNA during transcription. mRNA strands containing introns and exons are called pre-mRNA. A process called splicing then occurs - introns are removed and the exons joined together - forming mRNA strands. This takes place in the nucleus. The mRNA then leaves the nucleus for translation. In prokaryotes, mRNA is produced directly from the DNA without splicing because there are no introns.
What happens in translation?
In both eukaryotes and prokaryotes, translation occurs at the ribosomes in the cytoplasm. During translation, amino acids are joined together to make a polypeptide chain, following the sequence of codons carried by the mRNA. The mRNA attached itself to a ribosome and tRNA molecules carry amino acids to it. ATP provides the energy needed for the bond between the amino acid and the tRNA molecule to form. A tRNA molecule (carrying an amino acid), with an anticodon that’s complementary to the first codon on the mRNA, attaches itself to the mRNA by specific base pairing. A second tRNA molecule attaches itself to the next codon on the mRNA in the same way. The two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule moves away, leaving its amino acid behind. A third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away. This process continues, producing a chain of linked amino acids until there’s a stop signal on the mRNA molecule. The polypeptide chain moves away from the ribosome and translation is complete.
What is genetic code? Describe it.
The genetic code is the sequence of base triplets (codons) in mRNA which code for specific amino acids. In the genetic code, each base triplet is read in sequence, separate from the triplet before it and after it. Base triplets don’t share their bases - the code is non-overlapping. The genetic code is also degenerate - there are more possible combinations of triplets than there are amino acids (20 amino acids but 64 possible triplets). This means that some amino acids are coded for by more than one base triplet. Some triplets are used to tell the cell when to start and stop production of the protein - these are start and stop signals. They’re found at the beginning and end of the mRNA. The genetic code is also universal - the same specific base triplets code for the same amino acids in all living things.
