A2 Biology Unit 5- Genetics


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- A polynucleotide, made up of lots of nucleotides joined together

- Each nucleotide is made from a pentose sugar (5 carbon atoms), a phosphate group and  nitrogenous base

- Sugar in DNA nucleotides is a deoxyribose sugar

- Each nucleotide has the same sugar and phosphate, the base on each nucleotide can vary though

- There are four possible bases, adenine (A), thymine (T), cytosine (C), and guanine (G)

- DNA nucleotides join to form polynucleotide strands (join between phosphate and sugar making a sugar- phosphate backbone)

- Two DNA polynucleotide strands join by hydrogen bonding between bases

- Complementary base pairing- A and T (2 H bonds), G and C (3 bonds)

- Two strands wind up to form a DNA double helix

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Genes are sections of DNA

- Found on chromosomes, they code for polypeptides (proteins) (including enzymes)

- Proteins are mde from amino acids, different proteins have a different number and order of amino acids.

- The nucleotide bases determine the order of amino acids in a protein

- Each amino acid is coded for by a sequence of 3 bases - the triplet code/ codon

- Different sequences of bases code for different amino acids- the genetic code

- Sequence of bases is the template used in protein synthesis

To know for later:

DNA molecules are found in the nucleus of the cell, but ribosomes for protein synthesis are found in the cytoplasm. DNA is too large to move out of the nucleus so is copied into RNA by transcription.

- The RNA leaves the nucleus and joins with a ribosome in the cytoplasm where it can be used to synthesise a protein. Process is called translation

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- Ribonucleic acid is made up of nucleotides that contain one of four bases

- The nucleotides also form a polypeptide strand, however RNA differs in 3 ways:

1. Sugar in RNA nucleotides is a ribose sugar not deoxyribose

2. Nucleotides form a single polynucleotide strand not a double

3. Uracil (U) replaces thyamine (T) as a base. Uracil always pairs with Adenine

Two types of RNA = messenger RNA (mRNA) and transfer RNA (tRNA)

Shape: DNA= double strand twisted in a double helix held by H bonds. mRNA= single stranded. tRNA = single stranded, folded in clover shape, held by H bonds.

Sugar: DNA= deoxyribose. mRNA= ribose. tRNA= ribose


Features. DNA= 3 adjacent bases, triplet. mRNA= 3 adjacent bases,codon. tRNA= specific sequence of three bases called anitcodon, amino acid binding site. 

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Messenger RNA:

- Single polynucleotide strand

- 3 adjacent bases called codons (sometimes base triplets)

- made in the nucleus during transcription

- Carries genetic code from DNA in the nucleus to the cytoplasm, where its used to make a protein during translation.

Transfer RNA:

- Single polypeptide strand folded into a clover shape. Hydrogen bonds between specific base pairs hold the molecule in shape

- tRNA molecule has a specific sequence of 3 bases called an anticodon at one end, and an amino acid binding site at the other.

-  tRNA is found in the cytoplasm where it is involved in translation

- Carries the amino acids that are used to make proteins to the ribosomes

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- During transcription an mRNA copy of a gene is made in the nucleus

1. Starts when RNA polymerase (an enzyme) attaches to the DNA double helix at the beginning of a gene

2. Hydrogen bonds between the two DNA strands in the gene break, separating the strands, and the DNA molecule uncoils at that point.

3. One of the strands is then used to make an mRA copy.

4. RNA polymerase lines up free RNA nucleotides alongside the template strand. Specific 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)

5. Once the RNA nucleotides have paired up with their specific bases on the DNA strand they're joined together, forming an mRNA molecule.  

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6. RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand

7. The H bonds between the uncoiled strands of the DNA reform once the RNA polymerase has passed by and the strands coil back into a double helix

8. When the RNA polymerase reaches a particulr sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA

9. The 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. 

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mRNA is modified

- Genes in eukaryotic DNA contain sections that don't code for amino acids

- These sections are called introns. All the bits that do code are called exons.

- During transcription both introns and exons are copied into mRNA. This is called pre-mRNA

- Introns are removed from pre-mRNA by a process called splicing. Introns are removed and exons join forming mRNA strands. This takes place in the nucleus.

- Exons can be joined together in different orders to form different mRNA strands

- This means more than one amino acid sequence and so more than one protein can be produced from one gene

- After splicing, the mRNA leaves the nucleus for the next stage of protein synthesis (translation). 

Transcription = 1st stage and translation = 2nd stage protein synthesis

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- Occurs at the ribosomes in the cytoplasm. During translation, amino acids are joined together to make a polypeptide chain (protein) following the sequence of codons (triplets) carried by the mRNA

mRNA attaches to a ribosome.tRNA molecules carry amino acids to ribosome. A tRNA molecule, with an anticodon that's complementary to the first codon on the mRNA, attaches itself to the mRNA by specific base pairing. First codon that's transcribed is called a start codon

 A 2nd tRNA molecule attaches to the next codon on 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

Process continues, producing a chain of linked amino acids (a polypeptide chain) until there's a stop codon on the mRNA molecule. These tell the ribosome when to stop translation. Polypeptide chain moves away from the ribosome, translation is complete.

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Genetic Code

-Genetic code is the sequence of base triplets in mRNA and DNA that 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. The code is non-overlapping (triplets don't share bases)

- Genetic code is degenerate- more possible combinations of triplets than there are amino acids. (20 amino acids but 64 possible triplets). This means that the same amino acids are coded for by more than one base triplet. e.g. tyrosine can be UAU or UAC. 

- Some triplets are used to tell the cell when to start and stop the production of a protein. These are called start and stop codons, and are found at the beginning and end of the mRNA

- Genetic code is universal- the same specific base triplets code for the same amino acids in al living things. 

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Transcription factors

- All cells in an organism carry the same genes (DNA), but structure and function of cells vary. This is because not all the genes in a cell are expressed (transcribed and used to make a protein).

-Because different genes are expressed, different proteins are made and those proteins modify the cell - they determine the cell structure and control cell processes.

-Transcription of genes is controlled by protein molecules called transcription factors:

1. Transcription factors move from the cytoplasm to the nucleus

2. In the nucleus they bind to specific DNA sites near the start of their target genes- the genes they control the expression of

3. They control the expression by controlling the rate of transcription

4. Some transcription factors called activators, increase the rate of transcription e.g. they help RNA polymerase bind to the start of the target gene and activate transcription

5. Other factors, called repressors, decrease the rate of transcription e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription

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- Expression of genes can also be affected by other molecules in the cell e.g. oestrogen

- Oestrogen is a hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex.

- The complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene

- The complex can either act as an activator, e.g. helping RNA polymerase, or as a repressor, e.g. blocking RNA polymerase

- Whether the complex acts as a repressor or activator depends on the type of cell and target gene

- So the level of oestrogen in a particular cell affects the rate of transcription of target genes

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Gene expression is also affected by a type of RNA called small interfering RNA:

1. siRNA molecules are short, double-stranded RNA molecules that can interfere with the expression of specific genes

2. Their bases are complimentary to specific sections of  target gene and the mRNA that's formed from it 

3. siRNA can interfere with both transcription and translation of genes

4. siRNA affects translation through a mechanism called RNA interference:

* In the cytoplasm, siRNA and associated proteins bind to the target mRNA

*The proteins cut up the mRNA into sections so it can no longer be translated

* So the siRNA prevents the expression of the specific gene as its protein can no longer be made during translation

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:) thanx

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