Edexcel 9-1 GCSE Biology B3 Summary; Genetics

Asexual and Sexual Reproduction

Asexual reproduction requires only one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes and no mixing of genetic information, therefore no variation. The offspring produced are genetically identical to the parent and to each other. They are clones. Plants can be cloned artificially using cuttings or tissue cultures, animals can be cloned using embryo transplants or fusion cell cloning. Sexual reproduction requires two parents and involves the fusion of gametes, called fertilisation. It allows some of the genetic information from each parent to mix, producing offspring that resemble the parents but are not identical. This allows for variation in the offspring and species overall. Both animals and plants can reproduce using sexual reproduction. Each method has its advantages an disadvantages. For example, asexual reproduction is a lot quicker than sexual reproduction and is very efficient as only one parent is needed, whereas sexual reproduction allows opportunities for variation so the offspring won't have the same weaknesses as the parents and can better adapt to changes in the environment. 

Meiosis

A diploid cell has the full 23 pairs of chromosomes . Each cell in the body is diploid apart from the gametes and red blood cells (which have no nucleus). A haploid cell has 23 individual chromosomes, one of each pair. When gametes fuse in fertilisation, the zygote produced becomes diploid because the 23 chromosomes from each gamete pair up.  Meiosis is a special type of cell division which happens in the reproductive organs. This is what produces gametes for sexual reproduction.  Copies of genetic material are made. Each chromosome has an exact copy of itself. 1st division, chromosome pairs line up in their matched pairs along the centre of the cell Members of each pair split up and move to opposite poles of the cell Cell splits and becomes two, like mitosis so far, still two diploid cells 2nd division, in each cell, pairs of chromosomes split up and go to opposite poles of the cell Both cells split again and become four new haploid cells, each one genetically different from each other and the original parent cell Variation occurs when the inner chromatids from each homologous pair switch genes during divisions. 

Structure of DNA

DNA stands for deoxyribonucleicacid and can be found in the nucleus of a cell in tight coils wrapped around protein, called chromosomes. DNA contains genes which code for proteins. A strand of DNA is described as a double helix structure with a sugar (deoxyribose) phosphate backbone, nitrogen bases and hydrogen bonds holding the strands together by holding the bases together. There are four nitrogen bases and two complementary pairs, adenine and thymine pair up and two hydrogen bonds hold them together, and cytosine and guanine pair up with three hydrogen bonds holding them together.  A nucleotide is one sugar molecule, its attached base, and an attached phosphate group (as shown in the diagram).

Dna Structure (binary/octet-stream)

Protein Manufacture

Proteins are made from different types of amino acids and are made in ribosomes in the cytoplasm of a cell. To make a protein, a copy of our DNA because the genes code for proteins and the copies come out of the nucleus , where the DNA is, into the cytoplasm and to a ribosome.  TRANSCRIPTION Transcription is when a gene is 'unzipped' so that the hydrogen bonds that join the bases are broken. One strand of the double helix is copied with the complementary bases and where there should be a thymine base, a uracil base is put. This strand is the messenger RNA and is a single strand of DNA with uracil instead of thymine.  In the nucleus, RNA polymerase attaches to the non-coding section of DNA and the enzyme separates the two strands RNAp continues to move along the DNA until it gets to the coding section of a gene RNAp adds complementary RNA nucleotides to the template strand RNAp links the RNA nucleotides together to form a strand of mRNA The mRNA travels out of the nucleus through a nuclear pore TRANSLATION Translation is when the mRNA is joined to a ribosome and each 3 bases, called a codon, are translated so that transfer RNAs can bring one of 20 amino acids to that codon. This happens for every codon, and the amino acids form a long chain called a polypeptide. This then folds itself up, according to the amino acids used, to form a protein. In the cytoplasm, a ribosome and mRNA strand attach together A tRNA molecule pairs up with each codon The ribosome joins together the amino acids carried by the tRNA molecules This results in the formation of a polypeptide The types and order of amino acids in the chain cause it to fold into a specific shape, forming a protein  

The Genetic Code

The genetic code tells us which codon translates to which amino acid. Some amino acids have several codons, so if one of the bases is incorrect due to a mutation (*), then the correct amino acid may still be coded for, MAYBE. Some amino acids have only one possible arrangement of bases, so if one is incorrect the entire protein will be wrong, which could cause serious side effects, depending on the protein and what went wrong. 3 out of the 20 amino acids stop the process of translation and are at the end of coding sections of genes. If there is a mutation in the non-coding section of the gene where the RNA polymerase binds, then the enzyme may not be able to join to the gene, meaning no protein is made.  * A mutation is a change in the bases of a gene. It can be caused when the DNA is not copied correctly in cell division. Environmental factors can also cause mutations. These factors are called mutagens. Some mutations can change an organism's phenotype, which is a scientific term for the characteristics that are shown when a gene is expressed.  

Alleles and Inheritance

Different versions of a gene are called alleles (a-leels). Heterozygous means that one has two different alleles, homozygous means that one has two of the same allele. A dominant allele is an allele whose phenotype will always be expresses, regardless of the other allele's phenotype. A recessive allele means that both alleles must be the same in order for its phenotype to the expressed, if the other allele is a dominant one, then its phenotype will be shown rather than the recessive allele phenotype. For example, in matters of gender, the allele for a male child is dominant as only one of its allele is necessary for a male. For a female, two 'X' alleles are needed, one from the female gamete, one from the male gamete. The female gamete can ONLY provide an 'X' allele, whereas the male gamete can provide either the 'X' allele, meaning a female, or the 'Y allele, meaning a male.  The allele for brown eyes is a dominant allele, so only one is needed to have brown eyes. The allele for blue eyes is recessive, so two are needed to have blue eyes. If two blue eyed people had a child, then that child would certainly have blue eyes (unless there was a mutation). If two brown eyed people had a child, then that child could be either brown or blue eyed, depending on each parent's own alleles.  The allele for cystic fibrosis is a recessive allele, so both parents would need to provide the c.f allele for the child to the afflicted, they are either carriers or have c.f themselves. C.f affects many organs in the body, mainly the lungs and the pancreas, causing them to be clogged with a thick sticky mucus. Those with c.f are also often infertile and have blocked breathing and digestive systems. The mucus produced is a good breeding ground for bacteria, causing bad lung infections. Treatment for cystic fibrosis includes physiotherapy, antibiotics, and enzymes. Punnet squares can represent the parents' respective alleles and therefore display the likelihood of the offspring inheriting each combination of alleles. Family pedigrees can show if someone in the family is a carrier of a genetic disorder and who they have inherited it from.

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Caption: : Example of a family pedigree for cystic fibrosis

Multiple and Missing Alleles

Everyone's blood group is one of the four main groups, A, B, AB, or O. Your blood type is determined by marker molecules on the outside of your red blood cells. There are 3 types of markers, A, B, and O. The gene responsible for blood has three alleles written as I^A, I^B, and I^O. I^O is recessive, whereas I^A and i^B are co-dominant, meaning that if the genotype is AB, both phenotypes will be expressed.  The human Y sex chromosome is missing some of the genes found in the human X sex chromosome. If the allele for one of these X chromosomes causes a genetic disorder, then a male will develop it, whether it's recessive or dominant. A female, however, will only develop it if it is dominant or both inherited alleles are recessive. 

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Caption: : Example of a Punnet square for blood type. There is a 25% chance of the offspring having any of the four main blood types.

Mutations and Variations

Mutations can happen when there is a mistake in copying DNA during cell division. For example, one base in a DNA sequence could be replaced by a different one. A base could also just completely be deleted, or a random base could just be added. These are known as substitution, deletion, and addition, respectively. Any of these could happen naturally but are more likely to occur due to the DNA being damaged by radiation or chemicals.  Characteristics may vary between individuals of the same species. Some variation is caused by differences in alleles (genetic variation). Some variation is caused by how the environment affects the individual (environmental variation). Continuous variation can take any value in a data set, for example the length of hair. If you can measure it with a scale (e.g. in cm), it's continuous data. Discontinuous variation is where the characteristics can take only one of a limited set of values in a data set, for example the number of legs.  Normal distribution is a curve definition. The theoretical curve shows how often an experiment will produce a particular result. It is bell-shaped and symmetrical, showing that trials will usually give a result near the average.