Chapter 8 - DNA, Genes And Protein Synthesis Flashcards
What is a gene?
• A section of DNA that contains coded information – making polypeptides and functional RNA
What do you polypeptides make?
Proteins
What is a triplet?
Three bases making an amino acid
How many amino acids are needed to make up a protein?
Around 20 amino acids
What is another word for a triplet?
Codon
How many triplets are needed to code an amino acid?
One triplet
What are the features of a genetic code?
- triplets can only read in one direction
- Start of DNA sequence is methionine (MET)
- End of DNA will have a stop codon which isn’t part of the coded DNA
- non-overlapping
- universal
- degenerate code
What is meant by degenerate code?
Most amino acid are coded for by more than one triplets
What is meant by non-overlapping?
Each letter in the sequence is only read once
What is meant by universal?
The same triplet codes for the same amino acid in all organisms
Describe RNA
•molecules that transfer DNA information •made up of -ribose pentose sugar -organic base U/A/C/G -phosphate group
Describe mRNA and its function
- long strand/single helix
- contains genetic code sequence of mRNA nucleotide bases
- easily broken down - only exists when needed
- involved in transcription from DNA
- found in nucleus/cytoplasm
Describe tRNA
•transfer RNA bringing amino acid from cytoplasm to ribosome
•clover-leave structure
-amino acid attachment site
-anti-codon is complementary to the triplets ok the mRNA
-held together by hydrogen bonds between the complementary base pairs
Referring to DNA or tRNA or mRNA:
Double or single polynucleotide chain?
DNA - double
mRNA - single
tRNA - single
Referring to DNA or tRNA or mRNA:
Size order?
DNA - biggest
mRNA - medium
tRNA - smallest
Referring to DNA or tRNA or mRNA:
Structure?
DNA - helix/twist
mRNA - long strand/helix
tRNA - clover leave
Referring to DNA or tRNA or mRNA:
Type of pentose sugar?
DNA - deoxyribose
mRNA - ribose
tRNA - ribose
Referring to DNA or tRNA or mRNA:
Organic bases?
DNA - ATGC
mRNA - AUGC
tRNA - AUGC
Referring to DNA or tRNA or mRNA:
Where is it found?
DNA - Nucleus
mRNA - nucleus and cytoplasm
tRNA - ribosome and cytoplasm
Referring to DNA or tRNA or mRNA:
Chemical stability?
DNA - very stable
mRNA - less stable than DNA and tRNA
tRNA - more stable than mRNA but less then DNA
What is transcription?
The formation of Pre-mRNA from through complimentary base pairing
What is translation?
MRNA and tRNA are used to translate the genetic code into the polypeptide code to form proteins
Describe the process of transcription
1) DNA helicase breaks hydrogen bonds between nucleotides. Unwinding the helix
2) RNA polymerase moves along template strand and makes pre-mRNA using complementary bases
3) DNA strand rejoins to the template strand behind the RNA polymerase
4) RNA reaches stop codon and detaches
What is pre-mRNA splicing?
- sections of DNA are non-coding = introns
- Coding regions of DNA = exons
- The pre-mRNA includes both introns and exons, so the introns are removed as they would interfere with polypeptide synthesis
- once leaving all the exons, this sequence becomes mRNA
What does DNA polymerase do?
Rejoins active nucleotides
What is a codon?
A sequence of three adjacent nucleotide in mRNA that codes for an amino acid
What happens after transcription but before translation?
MRNA it leaves the nucleus through nuclear pores to where it will randomly collide with a ribosome
Describe the process of translation
1) ribosome attaches the mRNA at the initial codon
2) met – tRNA diffuses to ribosome and attached to the initiation codon by complementary base pairing
3) The next amino acid-tRNA attaches to the adjacent mRNA codon. The anticodon on the tRNA binds to the codon on the mRNA
4) Bond formed between the two amino acid (peptide bond). The previous tRNA is cut from the amino acid. Catalysed by ribozyme
5) The ribosome moves along one codon so that a new acid – tRNA can attach. Free tRNA can leave to collect more amino acids.
6) A polypeptide chain is formed and peels away from the ribosome, folding up into a protein. This process carries on until a stop codon is reached, the ribosome detaches leaving the finished protein
Describe a chromosome
- only visible during cell division
- one molecule of DNA
- humans = 46
- daughter cell contain exact copied = genetically identical
Describe a chromatid
- when When chromosomes are visible, there appears to threads joined at a centromere
- each replicated thread is chromatid
- They come as a pair but when separated during mitosis they will end up in different daughter
What is allele ?
An alternative form of a gene
What is a histones
Proteins which together DNA, make up the chromosomes of eukaryotic cell
What is a cell’s genome?
The complete set of genes in a cell
What is a cell’s proteome?
The full range of proteins a cell is able to produce
What are homologous pairs?
Pairs of matching chromosomes, which are the same size and have the same genes
What is the diploid number?
The total number of chromosomes in an organism
How do alleles differ?
They contain different base sequences so code for different proteins, which create different versions of the same polypeptide.
Why is is DNA degenerate?
Most amino acids are coded for by more than one triplet
Why is the genetic code described as universal?
The same triplet codes for the same amino acid in all living organisms
How do you get the mRNA sequence from a DNA sequence?
You use the complimentary base pairs, but always switch T for U
What does T stand for?
Thymine
What does A stand for?
Adenine
What does G stand for?
Guanine
What does U stand for?
Uracil
What does C stand for?
Cytosine
What is a codon?
A triplet of bases that code for one amino acid
How do you work out tRNA from mRNA?
The complementary pairs (tRNA contains uracil, not thymine)
How do you work out the sequence of amino acids from a section of mRNA?
Break it down into codons and use information in the table provided
How are di-nucleotides formed?
Two nucleotides join together in a condensation reaction between the sugar on one nucleotide and the phosphate on another
What bond is formed in polynucleotides?
Phosphodiester
What are the purine bases?
Guanine and adenine
What are the pyrimidine bases?
Thymine, uracil and cytosine
What is RNA made from?
A single polynucleotide strand in which each nucleotide is made from a ribose sugar, a base (A, C, G, U) and a phosphate group
Function of mRNA
After it is formed through transcription, it exits the nucleus through nuclear pores, where it travels to the ribosome. It acts as a template for protein synthesis
Structure of tRNA
A single-stranded molecule which is folded into a clover leaf shape, held in place by strong hydrogen bonds between complementary bases. At one end, there is an anticodon, which is complementary to the triplet on the mRNA, and at the other there is an amino acid bonding site
Function of tRNA
It carries amino acids used to make proteins to the ribosome
What is transcription?
The formation of pre-mRNA from DNA through complementary base pairing
What is translation?
mRNA and tRNA are used to convert the genetic code into the polypeptide chains needed to form proteins
Process of transcription
DNA helicase breaks the hydrogen bonds between nucleotides
This causes the double helix to unravel
One of the strands acts as a template strand. Using this, RNA polymerase attaches complementary nucleotides from the cytoplasm to the exposed strand
As the polymerase moves through the molecule and assembles the pre-mRNA, the double helix rejoins behind it and hydrogen bonds reform
When the polymerase reaches a stop codon, it stops and detaches
Why doesn’t pre-mRNA need to be spliced in prokaryotes?
There are no introns in prokaryotes’ DNA
How are introns removed from eukaryotic DNA?
Splicing
What happens to pre-mRNA to make mRNA?
It is spliced - introns are removed and exons are joined together
What happens to a polypeptide chain after it has been made?
It is coiled to form the secondary structure
The secondary structure is coiled to form the tertiary structure
Different polypeptide chains are linked to form the quaternary structure
Process of translation
A ribosome becomes attached to the mRNA
A tRNA molecule, carrying an amino acid, with a complementary anticodon to the codon on the mRNA sequence attaches itself to the mRNA by complementary base pairing
A second tRNA molecule attaches to the mRNA in the same way
The two amino acids attached to the tRNA molecules join together with a peptide bond
The first tRNA molecule moves away, leaving the amino acid behind
A third tRNA molecule attaches to the mRNA
By the same process, its amino acid joins to the two amino acids already there
The second tRNA molecule moves away
This continues until the stop codon is reached. The polypeptide chain moves away from the ribosome; translation is complete
What differs between amino acids?
The ‘R’ group
Do tRNA molecules have hydrogen bonds?
Yes
Which molecules are made from a single polypeptide strand?
mRNA and tRNA
