Highly repetitive sequences, or satellite DNA, which account for 5-45%.
The repetitive sequences are typically between 5 and 300 base pairs long and may be duplicated as many as 100 000 times in a genome.
Repetitive Sequences
Much of the DNA in eukaryotes consists of repetitive base sequences, which are not translated. Highly repetitive sequences, sometimes called satellite DNA, are sequences of between 5 and 300 bases, that may be repeated as many as 10 000 times. These constitute 5-45% of typical eukaryote DNA. Its function is not yet clear.
A surprisingly small proportion of eukaryotic DNA is single copy, or unique genes.
7.1.5 State that eukaryotic genes can contain exons and introns.
→ Many genes in eukaryotes contain introns – sequences of bases that are transcribed, but not translated. Exons are sequences of bases that are transcribed and translated.
→ A typical eukaryote gene consists of a series of exons and introns. After transcription of the whole gene, the introns are removed to form mature mRNA, in a process called post-transcriptional modification. Prokaryotes do not usually have introns in their genes.
7.2 DNA Replication
7.2.1 State that DNA replication occurs in a 5’ → 3’ direction.
The 5’ end of the free DNA nucleotide is added to the 3’ end of the chain of nucleotides that is already synthesized.
7.2.2 Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates.
Leading strand: DNA replication takes place towards the replication fork.
Lagging strand: DNA replication takes place away from the replication fork.
7.2.3 State that DNA replication is initiated at many points in eukaryotic chromosomes.
The rate of replication in fruit flies (Drosophila) is 2600 nucleotides/minute. The largest chromosome of Drosophila is 6.5 x 107 nucleotides. If DNA replication started at both ends of the chromosome, it would take 8.5 days to replicate the chromosome. In fact, it only takes 3- 4 minutes. In order to explain this discrepancy, scientists have determined that replication must start at many points along the same DNA helix at the same time.
This process is needed in eukaryotic cells because they have very large amounts of DNA, which would take much longer to replicate if replication was done from one end of the chromosomes to the other.
7.3 Transcription
7.3.1 State that transcription is carried out in a 5’ → 3’ direction.
Transcription is the enzyme-controlled process of synthesizing RNA from a DNA template. It is carried out in a 5’ to 3’ direction (of the new RNA strand). This means that new RNA nucleotides are added to the 3’ end of the growing RNA strand.
7.3.2 Distinguish between the sense and antisense strands of DNA.
The sense strand (coding strand) has the same base sequence as mRNA with uracil instead of thymine. The antisense (template) strand is transcribed.
7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.
Transcription begins at a specific point on the DNA molecule called the promoter region. Only one of the two strands is used as a template for transcription and this is called the antisense strand. The other DNA strand is called the sense strand. RNA polymerase uses free nucleoside triphosphates (NTPs)
7.3.4 State that eukaryotic RNA needs the removal of introns to form mature mRNA.
Introns are sequences of bases that are transcribed but not translated.
Exons are sequences of bases that are transcribed and translated.
mRNA (includes introns and exons) → mature mRNA (no introns, but has exons)
→ PTM – post-transcription modification
7.4 Translation
7.4.1 Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy.
Each amino acid has a specific tRNA-activating enzyme (the name aminoacyl-tRNA synthetase is not required). The shape of tRNA and CCA at the 3’ end should be included.
- = Hydrogen bonds
All tRNA molecules have:
Sections that become double stranded by base pairing, creating loops
A triplet of bases called the anticodon, in a loop of seven bases
Two other loops
The base sequence CCA at the 3’ terminal, which forms a site for attaching an amino acid
These features allow all tRNA molecules to bind to three sites on the ribosome
7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites.
The large subunit has 3 binding sites for tRNA and the small subunit had the binding site for mRNA.
7.4.3 State that translation consists of initiation, elongation (including translocation) and termination.
Translation occurs in three stages:
Initiation
Elongation (including translocation)
Termination
Initiation
mRNA binds to the small sub-unit of the ribosome
ribosome slides along mRNA to start codon
anticodon of tRNA pair with codon on mRNA
complementary base pairing between codon and anti-codon
anticodon of tRNA with methionine pairs with start codon/AUG is start codon
second tRNA pairs with next codon
Elongation
Peptide bond forms between amino acids
Ribosome moves along the mRNA by one codon
Movement in 5’ to 3’ direction
tRNA that has lost its amino acid detaches
Another tRNA pairs with the next codon/moves to A site
tRNA activating enzymes
Link amino acids to specific tRNA
3. Termination
7.4.4 State that translation occurs in a 5’ → 3’ direction.
During translation, the ribosome moves along the mRNA towards the 3’ end. The start codon is nearer to the 5’ end.
There are 3 stop codons.
7.4.5 Draw and label a diagram showing the structure of a peptide bond between two amino acids.
7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons and stop codons.
Codon: three consecutive bases in DNA (or RNA), which specify an amino acid.
Anticodon: three consecutive bases in tRNA, complementary to a codon on RNA.
7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes (in RER) synthesize proteins primarily for secretion or for lysosomes.
7.5 Proteins
7.5.1 Explain the four levels of protein structure, indicating the significance of each level.
The bonds in a tertiary structure form between R groups.
by
coffeeandtv (student)
