Biology Unit 2 As Model Answers

A nucleotide is the monomer (1); a nucleic acid is the polymer (1)

- DNA is double-stranded; RNA is single-stranded (1) - DNA contains thymine; RNA contains uracil (1) - DNA contains the sugar deoxyribose; RNA contains ribose (1) - Both contain guanine, cytosine and adenine (1) - Both contain a 5-carbon sugar (1) - Both contain a phosphate group (1)

• DNA helix unzips where it is to be copied (sense strand)
• Hydrogen bonds between strands are broken
• Free mRNA nucleotides base pair with complementary exposed bases on the DNA
• RNA polymerase joins the individual nucleotides together
• Introns are spliced out of the mRNA
• mRNA is exported out of the nucleus

• mRNA travels through the cytoplasm; ribosome attaches to start codon (AUG)
• ribosome moves along the mRNA 3 bases at a time and reads the codons
• tRNA molecules bring amino acids to the ribosome
• the anticodon of the tRNA binds with complementary codon of the mRNA
• amino acids join together by peptide bonds
• tRNA is recycled

It is non-overlapping (specific start an stop codons) (1); it is degenerate (some amino acids are coded for by more than one codon) (1)

- double helix unzips - hydrogen bonds break - free DNA nucleotides complementary base pair with exposed bases on both strands - DNA polymerase joins nucleotides together on the new strands (catalyses formation of ester bonds)

Two new molecules are formed which are exact copies of the original (1); one of the chains in each new molecule was present in the original molecule (1)

DNA controls the production of enzymes (1); enzymes control metabolic pathways (1); metabolic pathways influence the phenotype of an organism (1)

• prophase – chromosomes coil (shorter and fatter); nuclear envelope disappears; spindle (protein fibres) forms in the cell
• metaphase – chromosomes line up on equator of spindle; spindle fibres attach to chromosomes at the centromere
• anaphase – spindle fibres contract and pull copies of chromosome towards opposite ends of the spindle
• telophase – new nuclear envelope forms; chromosomes become long and thin; cytoplasm divides to form two new cells
• interphase – daughter cell grows more cytoplasm; DNA replication; organelle replication

Cut end tip off growing root (1); add aceto-orcein stain (1); squash (1)

Root tip end has fast growing cells (1); stain to make chromosomes visible (1); squash to get a thin tissue layer letting enough light through for the optical microscope (1)

G1 (cells prepare for DNA replication); S (DNA replication); G2 (short gap before mitosis); M (mitosis)

- meiosis produces gametes; mitosis produces other body cells - meiosis produces cells with 23 chromosomes (haploid - n); mitosis produces cells with 46 chromosomes (diploid - 2n) - meiosis produces cells that are genetically different; mitosis produces cells that are genetically identical - both go through prophase, metaphase, anaphase and telophase

- in first division, homologous chromosomes pair up - equivalent portions of chromatids may cross over - homologous pairs separate during meiosis 1, with one chromosome from each pair going into the two daughter cells - this happens randomly and is called independent segregation - chromatids move apart during anaphase of the second meiotic division - after the second division, four haploid daughter cells are formed

Recombination of sister chromatids after crossing over during prophase of meiosis 1 (1) produces new combinations of alleles (1). Independent segregation of homologous chromosomes during anaphase of meiosis 1 (1) produces gametes with randomly assorted chromosome combinations (1).