EDEXCEL UNIT 2

GENES AND GENETICS 

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GENE EXPRESSION CONTROL

1. By gene expression we mean

   1. the transcription of a gene into mRNA and its subsequent

   2. translation into protein.

2. Gene expression is primarily controlled at the level of transcription, largely as a result of binding of proteins to specific sites on DNA.

3. The structural genes contain the code for the proteins products that are to be produced.

4. Regulation of protein production is largely achieved by modulating access of RNA polymerase to the structural gene being transcribed.

5. The promoter gene doesn't encode anything; it is simply a DNA sequence that is initial binding site for RNA polymerase.

6. Enhancers can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or thousands of nucleotides away. When a DNA -BENDING PROTEIN BINDS to the DNA, the shape of the DNA changes, which allows interactions between the activators and transcription factors to occur.

7. The operator gene is also non-coding; it is just a DNA sequence that is the binding site for the repressor.

8. The regulator gene codes for synthesis of a repressor molecule that binds to the operator and blocks RNA polymerase from transcribing the structural genes.

 

MARKING SCHME 

1. ref to correct stimulus e.g. chemical ;

2. some genes {switched off / switched on / eq}

3. mRNA from {switched on / eq} genes ;

4. mRNA translated / eq ;

5. idea of {protein synthesised / different proteins produced} ;

6. which (permanently) modify cell (to become specialised) /description of a modification / eq ;

 

 

LAC OPERON

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RNA splicing 

 

RNA splicing is a process that removes the non-coding sequences of genes (introns) from pre-mRNA and joins the protein-coding sequences (exons) together in order to enable translation of mRNA into a protein.

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1. reference to transcription ;

2. reference to {post-transcriptional modification / splicing} of (pre-) mRNA ;

3. reference to spliceosomes ;

4. reference to removal of introns ;

5. idea that exons are arranged in different combinations ;

6. idea that the (m)RNAs are different ;

7. reference to translation ;

8. idea of different {primary structure /sequence of amino acids} ;

9. idea that this results in different bonds ;

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Phenotype à Genotype + Environment

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1. skin cancer is the result of an interaction between genotype and the environment.

1. ultraviolet light is an environmental { factor / effect } / eq ;

2. idea that ultraviolet light { causes mutations / is a mutagen } ;

3. idea that DNA { replication / repair / eq } is affected ;

4. reference to { formation of an oncogene / tumour suppressor genes being affected } ;

5. idea that control of cell cycle is lost ;

 

TWIN STUDY

1. non-identical twins are genetically different /eq ;

2. identical twins {are genetically the same / have the same genotype / same genes / same alleles ;

3. so difference (in height / mass / phenotype) is due to {environment / named environmental factor / eq} ;

4. greater difference in traits for non-identical twins / eq ;

5. idea of difference due to genetic effects e.g. genes have a bigger effect ;

 

 

 

CONTINUOUS AND DISCONTINUOUS

VARIATIONS 

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Continuous variation is where the different types of variations are distributed on a continuum

while discontinuous variation is where the different types of variations are placed into discrete, individual categories.

Examples of continuous variation include things like a person's height and weight.

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EPIGENETIC

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The study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself

 

METHYLATION OF DNA

Methylation is adding methyl groups (CH3), a type of epigenetic marker, to DNA. The methyl group is added between the bases guanine and cytosine on DNA and makes it harder for RNA polymerase to bind , so the gene is transcribed less and therefore translated less, effectively turning it off.

 

ACETYLATION/METHYLATION  OF HISTONES

In order for a gene to be transcribed the DNA must be uncondensed so the transcription enzymes can bind to it , the addition or removal of acetyl groups - a type of epigenetic marker - affects how easily the DNA can unknot and be transcribed.

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NON CODING RNA

A non-coding RNA (ncRNA) is a functional RNA molecule that is transcribed from DNA but not translated into proteins. 

 

EPIGENETIC CHANGES ACROSS GENERATIONS

Epigenetic changes can be passed onto offspring.

Most epigenetic markers are removed during meiosis, but some remain , meaning environmental changes in grandparent’s lives can affect their offspring’s expression of genes.

Reprogramming is the process of removing the epigenome and occurs at various points in a species lifetime, however 1% of genes evade the reprogramming, meaning some epigenetic markers remain.

 

 

 

 

DIHYBRID INHERITANCE

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MULTIPLE ALLELES, CODOMINANT LINKED GENE & POLYGENIC TRAITS

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MULTIPLE ALLELES - Thus, THREE OR MORE KINDS OF GENES OCCUPYING THE SAME LOCUS in individual chromosome are referred to as MULTIPLE ALLELES.

 

In codominance, however, neither allele is recessive and the phenotypes of both alleles are expressed.

 

Linked genes are genes that are likely to be inherited together because they are physically close to one another on the same chromosome.

 

POLYGENIC TRAITS

1. idea of more than one gene for a single characteristic ;

2. at { many / different } loci -./ eq ;

3. cumulative effect on the phenotype / interaction of many genes ;

(plural form of locus is loci)