Revision for Unit 4

What does DNA stand for?

Deoxyribonucleic acid

Where is DNA generally found?

in the nucleus of cells of all living things

what is DNA

is a complex molecule arranged in very long strands that contains genetic information.

what does DNA do?

it makes proteins used for body and cell division, replication and repair; and managing chemical reactions (enzymes)

how many DNA molecules in a human?

46 DNA molecules (23 pairs), each molecule is one chromosome

what is the structure of the DNA molecule?

very long strands like a ladder with deoxyribonucleic sugar, phosphate and rungs of base pairs

what are base pairs joined together by?

hydrogen bonds

Guanine bonds with…

cytosine

Thymine bonds with…

Adenine

how many hydrogen bonds between C and G?

3 hydrogen bonds

how many hydrogen bonds between T and A?

two hydrogen bonds

DNA strands are twisted into a…

helix, a ‘double helix’

what is a nucleotide composed of?

phosphate, deoxyribose sugar and a base

What does the DNA look like in a non dividing cell?

the double helix is wound around histones, this forms a coil structure forming a tangled web of chromatin in the nucleus.

Wha does the DNA look like in a dividing cell?

coils of DNA become super coiled and forms the chromosome structure.

when does DNA replication occur?

during interphase of cell division, the chromatids replicate ready for division. chains of base pairs separate, and new base pairs join to each half

What are genes?

Genes are long sequences of DNA base triplets that come for amino acids to be assembled into long strands. These strands fold and bind and form specifically structured proteins

each human gene codes for how many proteins?

1 to 3

what are the functions of proteins?

they are the structural material for cells, they control chemical reactions (enzymes are proteins), they control some aspects of membrane transport (as protein channels/ or preceptors)

genes can be…… depending on the cells purpose

switched on or off

what are proteins?

they are complex structures made of chains of amino acids

proteins are assembled by…

ribosomes, tRNA which carries amino acids and corresponding triplet codon, mRNA carrying instructions

Other than the nucleus, where else can DNA be found in a cell?

in the mitochondria

what does mitochondria DNA look like?

they are small circular molecules rather than large strands.

what does mitochondrial DNA do?

it codes for enzymes used in cellular respiration

where is mitochondrial DNA inherited from

the mother

What is a Karyotype?

is a picture of all the chromosomes in the cell

How many genes in the human genome?

around 30 000 genes in the human genome

what is the genome?

it is the total number of genes in the 23 chromosome pairs

does each chromosome pair contain the same genes?

yup

where do each chromosome pairs come from?

each hand of the pair contains genes for the same things but different variants, one from mum and one from dad

when does crossing over occur?

during meiosis 1

what is crossing over?

homologous pairs exchange some alleles, this results in far more gamete combinations than if crossing over didn’t occur

what would happen if crossing over didn’t occur?

without crossing over, there would only be two possible gametes for alleles

where does crossing over occur?

in multiple places (loci) in each chromosome

when does random assortment happen?

metaphase 1 (meiosis 1)

what happens during random assortment?

homologous pairs seperate, one ends up in each cell at the end of meiosis 1. Containing 23 chromosome pairs.

what does random assortment allow?

random assortment allows lots of possible different combinations depending on which of each pair ends up in cell.

what happens in the first meiotic division?

homologous pairs engage in “crossing over”. they exchange a little of each others DNA. Homologous pairs separate, so one of each pair moves to each pole. At the beginning of prophase 2, a haploid karyotype of single chromosomes is present.

what happens in the second meiotic division?

chromosomes line up on spindle, they are pulled apart during anaphase. the end result is each cell with a unique haploid karyotype of unduplicated chromatids

what is variation?

differences between organisms within a species

human variation is from…

a combination of genetics and the environment

what are the genetic sources of variation?

random assortment of chromosomes during meiosis, crossing over and recombination during meiosis, non- disjunction during meiosis, random fertilisation and mutations

what is non- disjunction during meiosis?

this is where some chromosome pairs do not seperate properly during meiosis. This results in karyotype with an abnormal chromosome number, after fertilisation.

when does non- disjunction occur?

can occur in meiosis 1 or 2

what does non- disjunction usually result in (birth)

generally results in severe birth defects, or miscarriage

non- disjunction is generally not an…

advantageous source of genetic variation

what are some examples of non- disjunction?

Trisomy 21 (Down syndrome), Trisomy 13 (Patau syndrome) and Turners syndrome (Momosomy 23 (Just one X chromosome))

what is random fertilisation?

it refers to the fact that an egg can be fertilised by any of of many different sperm, meaning more variation in offspring.

what are mutations?

they are random permanent changes in DNA

can some variations be advantages, why or why not?

some of the population is likely to be able to survive during adversity. For example, people who store fat are more easily able to survive a famine. And people heterozygous with sickle cell disease don’t get malaria as badly.