The cell membrane is impermeable see analysis on page

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The cell membrane is impermeable. (see analysis on page 86). It has a double-layered structure which regulates the movement of substances into and out of the cell. It regulates processes such as osmosis. Phospholipids help with this, they have hydrophilic heads and hydrophobic tails. Genes: DNA segments that encode the synthesis of a particular protein. They are contained within chromosomes. Each chromosome is a double-stranded molecule of DNA. There are 4 nucleotide basis: Adenine (A) Thymine (T) Guanine (G) Cytosine ©’ A binds with T and G binds with C A human cell has 23 pairs of chromosomes, or 46 chromosomes in all. In a reproductive cell there are only 23 and they’re not paired. A gene is a segment of a DNA strand. The code is the sequence of nucleotide bases. Study figure 3.2. (BINAS transcription) Protein: folded- up polypeptide chain that serves a particular function in the body. A protein’s shape and its ability to change shape and to combine with other proteins are central to its function. They can act as enzymes. Protein molecules that have been synthesized in the cell are wrapped in membranes and marked with addresses to indicate where they are to go, it takes place in the Golgi bodies. Motor molecules move them along many microtubules. A protein may: Be incorporated into the membrane - Channel: opening in a protein that allows the passage of ions - Gate: protein that allows substances to pass through the membrane on some occasions but not others - Pump: protein that actively transports a substance across the membrane Remain within the cell to act as an enzyme Be excreted from the cell by exocytosis
The nucleus of each human somatic cell contains 23 pairs of chromosomes, or 46 in all. One member of each pair comes from the mother, and the other member comes from the father. Pair 1 22 are autosomes and they contain the genes that contribute most to our physical appearance and behaviour. The 23 rd pair are the sex chromosomes, which contribute to our physical and behavioural sexual characteristics. (x and y). Female = xx Male = xy The y chromosome contains the SRY gene that makes the SRY protein, which triggers testes development and hence the male phenotype. Each cell contains two copies of every gene called alleles (one gene pair contains two alleles), if they are identical they are homozygous, if they are different they are heterozygous. The nucleotide sequence most common in a population is called the wild-type allele, whereas a less frequently occurring sequence is called a mutation. In heterozygous pairs: - Only the allele from the mother may be expressed - Only the allele form the father may be expressed - Both alleles may be expressed simultaneously Dominant vs. recessive . Complete dominance / incomplete dominance / codominance. Errors can arise during the reproduction of genes and passing them on to offspring is fallible. The altered alleles are mutations. A mutation may be as small as a change in a single nucleotide base, called a single nucleotide polymorphism (SNP). It results in change in one amino acid in a protein. A mutation can be

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