topic 4A: DNA, RNA and protein synthesis Flashcards
prokaryotic DNA
-in prokaryotic cells, the DNA molecules are:
-short
-circular
-not associated with histone proteins
eukaryotic DNA
-eukaryotic DNA is found in the nucleus of eukaryotic cells
-DNA moleculs are:
-very long
-linear
-associated with histone proteins
-the DNA molecule and its associated histone proteins forms a chromosme
Compare and contrast DNA in eukaryotic cells with DNA in prokaryotic cells
SIMILARITIES
-NUCLEOTIDE structure is identical - deoxyribose attached to phosphate and a base
-Adjacent nucleotides joined by PHOSPHODIESTER BONDS, complementary bases joined by HYDROGEN BONDS
-DNA in MITOCHONDRIA / CHLOROPLASTS have similar structure to DNA in prokaryotes
SHORT, CIRCULAR, NOT associated with PROTEINS
DIFFERENCES
-Eukaryotic DNA is LONGER
-Eukaryotic DNA is LINEAR, prokaryotic DNA is CIRCULAR
-Eukaryotic DNA is associated with HISTONE PROTEINS, prokaryotic DNA is not
-Eukaryotic DNA contain INTRONS, prokaryotic DNA does NOT
DNA in mitochondria and chlorplasts
-the mitochondria and chloroplasts of eukaryotic cells also contain DNA which, lie the DNA of prokaryotes, is short, circular and not associated with proteins
What is a gene?
-a gene is a sequence of DNA (nucleotide) BASES that codes for:
-the AMINO ACID SEQUENCE of a POLYPEPTIDE
-or a FUNCTIONAL RNA (eg. ribosomal RNA or tRNA)
What is a chromosome?
-LONG, LINEAR DNA and its associated HISTONE PROTEINS
-In the NUCLEUS of EUKARYOTIC cells
What is a locus?
FIXED POSITION a GENE occupies on a particular DNA molecule
Describe the nature of the genetic code
TRIPLE CODE: A sequence of 3 DNA BASES, called a triplet, codes for a SPECIFIC AMINO ACID
UNIVERSAL: The SAME BASE TRIPLETS code for the SAME AMINO ACIDS in ALL organisms
NON OVERLAPPING: Each base is part of only one triplet so each triplet is read as a DISCRETE unit
DEGENERATE: An amino acid can be coded for by MORE THAN ONE BASE TRIPLET
What are ‘non-coding base sequences’ and where are they found?
Non-coding base sequence - DNA that does not code for amino acid sequences / polypeptides:
1. Between genes - eg. non-coding multiple repeats
2. Within genes - introns
In eukaryotes, much of the nuclear DNA does not code for polypeptides
What are introns and exons?
EXON: Base sequence of a gene coding for amino acid sequences (in a polypeptide)
INTRON: Base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells
Define genome
GENOME: the complete set of genes in a cell (including those in mitochondria and /or chloroplasts)
Define proteome
PROTEOME: The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)
Describe the two stages of protein synthesis
- TRANSCRIPTION: Production of messenger RNA (mRNA) from DNA, in the nucleus
- TRANSLATION: Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes
structure of mRNA (messenger)
-during TRANSCRIPTION
-single polynucleotide strand
mRNA is linear / straight
-mRNA doesn’t have HYDROGEN BONDS between complementary based pairs
-mRNA is a LONGER, VARIABLE length (more nucleotides)
-mRNA has CODONS (a group of three adjacent bases, called triplets or base triplets)
-mRNA doesnt have a AMINO ACID BINDING SITE
structure of tRNA (transfer)
-involved in TRANSLATION
-single polynucleotide strand
-tRNA is folded into a ‘CLOVER LEAF SHAPE’
-tRNA has hydrogen bonds between specific complementary paired bases
-tRNA is a SHORTER, FIXED LENGTH
-tRNA molecule has a specific sequence of 3 bases at one end called an ANTICODON
-tRNA has an AMINO ACID BINDING SITE at the other end
Compare and contrast the structure of tRNA and mRNA
SIMILARITIES
-both single polynucleotide strand
DIFFERENCES
-tRNA is folded into a ‘clover leaf shape’, whereas mRNA is linear / straight
-tRNA has hydrogen bonds between paired bases, mRNA doesn’t
-tRNA is a shorter, fixed length, whereas mRNA is a longer, variable length (more nucleotides)
-tRNA has an anticodon, mRNA has codons
-tRNA has an amino acid binding site, mRNA doesn’t
Describe how mRNA is formed by transcription in eukaryotic cells
- HYDROGEN BONDS between DNA bases break
- Only ONE DNA strand acts as a TEMPLATE
- Free RNA NUCLEOTIDES align next to their COMPLEMENTARY BASES on the template strand
○ In RNA, URACIL is used in place of thymine (pairing with adenine in DNA) - RNA POLYMERASE joins ADJACENT RNA nucleotides
- This forms PHOSPHODIESTER BONDS via CONDENSATION reactions
- Pre-mRNA is formed and this is SPLICED to REMOVE INTRONS, forming (mature) mRNA
Describe how production of messenger RNA (mRNA) in a eukaryotic cell is
different from the production of mRNA in a prokaryotic cell
-Pre-mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
-Because genes in prokaryotic cells DONT CONTAIN INTRONS so NO SPLICING in prokaryotic cells
Describe how translation leads to the production of a polypeptide
- mRNA attaches to a RIBOSOME and the ribosome moves to a START CODON
- tRNA brings a SPECIFIC AMINO ACID
- tRNA anticodon binds to COMPLEMENTARY mRNA CODON
- Ribosome moves along to NEXT CODON and another tRNA binds so 2 AMINO ACIDS can be joined by a CONDENSATION REACTION forming a PEPTIDE BOND
○ Using energy from HYDROLYSIS OF ATP - tRNA RELEASED after amino acid joined polypeptide
- RIBOSOMES MOVES ALONG mRNA to form the polypeptide, until a STOP CODON is reached
Describe the role of ATP in translation
ATP
-Hydrolysis of ATP to ADP + Pi RELEASES ENERGY
-so AMINO ACIDS join to tRNAs and PEPTIDE BONDS form between amino acids
Describe the role of tRNA in translation
tRNA
-Attaches to / transports a SPECIFIC AMINO ACID, in relation to its anticodon
-tRNA ANTICODON COMPLEMENTARY BASE PAIRS TO mRNA CODON, forming hydrogen bonds
-2 tRNAs bring AMINO ACIDS together so PEPTIDE BOND can form
Describe the role of ribosomes in translation
Ribosomes
-mRNA binds to ribosome, with space for 2 codons
-Allows tRNA with ANTICODONS to bind
-Catalyses formation of PEPTIDE BOND between amino acids (held by tRNA molecules)
-MOVES ALONG (mRNA to the next codon) / translocation
