Unit 4, topic 1

Question 1

define semiconservative replication

Answer 1

the production of two new DNA double helix molecules, each consisting of one parental strand and one daughter strand.

Question 2

define peptide bonds

Answer 2

the bond forms between adjacent amino acid monomers

Question 3

define start and stop codon and the process they are involved in

Answer 3

start codon is the first codon of an mRNA transcript translated by a ribosome and signals to the ribosome to start translating mRNA.

stop codon is the codon that discontinues the synthesis of the polypeptide chain.

involved in transfer RNA.

Question 4

what is transfer RNA

Answer 4

an RNA molecule that picks up a particular acid from the cytoplasm and then pairs with a specific mRNA codon to deliver the amino acid to growing polypeptide chain.

Question 5

what is the process of translation

Answer 5

the joining of amino acids in a specific order, according to information in mRNA read by ribosome, to form a polypeptide.

Question 6

what is an example of a protein

Answer 6

haemoglobin carries oxygen in blood

Question 7

describe the outcome of alternative splicing of mRNA

Answer 7

When exons from the same mRNA strand are spliced together in different arrangements, varied forms of the mature mRNA are created, which are translated into different versions of the polypeptides. Therefore, alternative splicing causes variations in the expressed proteins.

Question 8

short term vs long term regulation

Answer 8

Short term – ensures that all of the different varieties of specialised cells can carry out their regular everyday functions.

Long term – determines the development of the organism through all the different life stages, including the differentiation of specialised cells.

Question 9

define transcription factor

Answer 9

a regulatory protein whose function is to activate or inhibit transcription of coding DNA by binding to specific non-coding segments near the gene to expressed or repressed.

Question 10

define DNA

Answer 10

a doubled stander helix that is the universal basis of an organisms genetic material; it contains instructions written in chemical code, for the production of proteins by the cell.

Question 11

define gene

Answer 11

region/s of DNA that are made up of nucleotides; the molecular unit of heredity

a section of DNA, a unit of hereditary that transmits information from one generation from one generation to the next, a segment of DNA that codes for polypeptide. Genes can be read and switched on or off independently.

When a genes is switched on – the cellular machinery will read the gene and produce, for EG, a specific protein. The gene is said to be expressed.

Question 12

how is DNA found in eukaryotes

Answer 12

DNA in eukaryotes

DNA is found in nucleus, chloroplasts and mitochondria.

Chromatin – grainy substance, DNA before it condenses.

occurs in unduplicated chromosomes (single, long DNA double helix molecule) coil around histones proteins.

Question 13

how is DNA found in prokaryotes

Answer 13

DNA in prokaryotes

Membrane-bound organelles are not present

DNA forms a single circular chromosome that lies in direct contact with the cytoplasm. Chromosomes are often joined to the plasma membrane at a single point.

DNA can be in a nucleotide (a region within a prokaryotic cell that contains the genetic material).

Plasmids (a small circular piece of DNA, found in bacteria which can replicate independently of the cell’s chromosomes, plasmids carry antibiotic resistance markers) may also be present in the cytoplasm. None essential genes are commonly encoded on these plasmids.

DNA contains very little repetitive and non-coding DNA.

Prokaryotic cells are generally haploid – they only contain one copy of each gene.

Question 14

define homologous chromosome

Answer 14

Homologous chromosomes – a pair of chromosomes that have the same size, shape and genes at the same locations.

Question 15

what is the endosymbiotic theory and what is evidence for it

Answer 15

proposes that eukaryote cells were formed when a bacterial cell was ingested by another primitive prokaryotic cell.

Question 16

what is maternal inheritance

Answer 16

Maternal inheritance

Inheritance of mitochondria from egg cells that have divided after fertilisation to populate the cells of the adult organism. - only from mothers, any trait that is encoded by organelle DNA is contributed by the female.

Question 17

what is supercoiling

Answer 17

In both eukaryotes and prokaryotes, chromosomes needs to fit into a small area, which can be achieved by supercoiling – where a number of architectural proteins act together to fold and condense the DNA.

Question 18

what is DNA composed of and how is it held together

Answer 18

A molecule of DNA is composed of two long strands of subunits called nucleotides, wound around each other to form a double helix. A nucleotide has three distinct chemical components.

A five-carbon sugar (deoxyribose in DNA)

Negatively charged phosphate group

An organic nitrogen base

Weak, base specific Hydrogen bonds hold the double helix together.

Question 19

what is RNA composed of?

Answer 19

Ribonucleic acid (RNA) is a nucleic acid related to DNA, but with three major differences:

Composed of single chain of nucleotides – singe stranded.

The base Thymine is replaced with Uracil (U)

The ribose sugar replaces deoxyribose of DNA.

Question 20

what is the process of DNA replication

Answer 20

DNA replication process

DNA helicase unzips the long, helical molecule of double-stranded DNA by breaking the weak hydrogen bonds between the nucleotides and thus exposing the nucleotide bases.

This separation of the parental DNA strands happens along a small section at a time. The hydrogen bonds that hold the two strands of DNA molecule are weak and the enzyme is easily able to separate them.

The junction between the unwound single strands of DNA and the intact double helix is called the replication fork. The replication fork moves along the DNA so that there is a continuous unwinding of the parental strands.

Stockpiles of free nucleotides attach to the exposed bases according to the base-pairing rule with the help of the enzyme DNA polymerase, which then joins the nucleotides to form a new complementary strand.

Both strands act as templates for the production of new DNA strands, with nucleotides being linked together in what is termed a 5’ to 3’ direction.

The outcome of DNA replication is two double helix DNA molecules, each consisting of one original parental strand.

Thus one of the two strands on conserved or retained, from one generation to the next, while the other strand is new – this is semiconservative replication.

Question 21

distinguish between asexual and sexual reproduction and provide an example of each

Answer 21

Asexual reproduction – a form of reproduction in which offspring are produced from a single parent. As there is only one parent – offspring typically shows great resemblance the parent.

EG: binary fission in bacteria.

Sexual reproduction – a form of reproduction in which offspring are produced from two parents. Difference in characteristics between one generation and the next is greater than asexual reproduction.

EG: meiosis

Question 22

what is a gamete

Answer 22

Gamete – a cell produced in sexual reproduction, which combines at fertilisation in humans, the gametes are ova and sperm cells; in flowering plants, pollen grains contain male gametes and ova contain female gamete.

Question 23

what does haploid and diploid mean

Answer 23

Diploid – 2n.

Haploid – 1n.

Question 24

what happens in prophase 1

Answer 24

Prophase 1:

chromatin condense and chromosomes become visible.

A spindle begins to form, originating from the centrioles, if present, and attached to the centromere of each chromosome.

Homologous chromosomes come to lie side by side, in a process called synapsis. The pairs of homologous chromosomes, one maternal and paternal coil around each other to form a bivalent.

At the end of prophase the nuclear membrane breaks down.

Question 25

what happens in metaphase 1

Answer 25

The homologous chromosomes move together to line up across the equator of the spindle, still attached at the chiasmata.

Question 26

what happens in anaphase 1

Answer 26

Anaphase 1:

The maternal and paternal chromosomes of homologous pairs are pulled towards opposite poles of the cell by the spindle fibres. Sister chromatids remain attached at their centromeres, moving towards the same pole.

The separation of disjunction of each pair of homologous chromosomes occurs independently of the other chromosome pairs

Question 27

what happens in telophase 1

Answer 27

Telophase 1:

Haploid set of chromosomes is seen at each pole, but each chromosomes still has two sister chromatids.

The spindle breaks down, the cell starts to separate across its middle, and nuclear envelopes from around the two new nuclei.

Question 28

what happens in cytokinesis 1

Answer 28

Cytokinesis:

The division of the cytoplasm, completes the first stage in meiosis.

At the end of meiosis 1, a brief interphase usually occurs. DNA does not duplicate during the this interphase.

Question 29

what happens in prophase 2

Answer 29

Prophase 2:

New spindle fibres form

Question 30

what happens in metaphase 2

Answer 30

Metaphase 2:

Chromosomes move to equator of the cell.

Question 31

what happens in anaphase 2

Answer 31

Anaphase 2:

Sister chromatids separate and move apart from each other to opposite poles of the cell.

Once at opposite ends, the chromosomes become the chromosomes of daughter cells and enter telophase 2.

Question 32

what happens in telophase 2

Answer 32

Telophase 2:

The spindle apparatus disappears the chromosomes de-condense to their thread-like form and new nuclear envelopes form.

Question 33

what happens in cytokinesis 2

Answer 33

Cytokinesis:

With meiosis completed, cytoplasmic division follows, with four haploid cells being formed from the original single diploid parent cell. In humans, females produced an ovum containing 22 autosomes and one X chromosome, and males produce sperm containing 22 autosomes and either an X or Y chromosome.

Question 34

describe the process of crossing over and recombination and demonstrate how they contribute to genetic variation

Answer 34

An important event involving homologous chromosomes during meiosis is crossing over.

During synapsis in prophase 1, when the pairs of homologous chromosomes coil around other to form a bivalent, the non-sister chromatids become attached at points called chiasmata.

Here they may exchange segments of genetic material. This recombination scrambles pieces of maternal and paternal genes and rearranges the combinations of alleles available on each homologous chromosome.

The resultant recombination chromosomes produce new combinations of genes, which increases genetic diversity in the offspring.

Question 35

describe the process of oogenesis

Answer 35

Begins in ovaries in females during embryonic development before a women is born. There, primary oocyte completes meiosis 1 in each month to form a secondary oocyte and a structure called a polar body. Cytokenesis is unequal, with almost all of cytoplasm going into the secondary oocyte. The polar body degenerates. The second is meiotic division, which produces a haploid ovum (egg), and a second polar body, occurs only a sperm fertilises the egg.

Polar body – a very small cell produced during oogenesis, containing a nucleus, but very little cytoplasm.

Question 36

describe the process of spermiogenesis

Answer 36

Spermiogenesis:

Stem cells in the testes undergo mitotic division, each time producing a new stem cell that continues to divide, and a diploid primary spermatocyte. The latter divides in meiosis 1 to form 2 secondary spermatocytes, which in turn divide in meiosis 2 to form spermatids, which are haploid and develop into four sperm cells. This process occurs throughout a males lifetime and is capable of producing at least 3 million sperm every day.

Question 37

contrast spermiogenesis and oogenesis

Answer 37

Differences between oogenesis and spermiogenesis:

In oogenesis, cytokinesis in both meiosis 1 and 2 is unequal and produces only a single egg, and polar bodies that degenerate. By contrast, meiosis in spermiogenesis produces four sperm.

At birth, ovary contains all the cells that will ever develop into eggs. Once puberty is reached, sperm are produced throughout a man’s lifetime.

Sperm are produced continuously, whereas oogenesis has long breaks between stages of division. This is because primary oocytes begin meiosis in the female embryo, but some eggs will not complete meiosis 1 until the woman reached menopause usually between the ages of 45 and 50 years.

Question 38

demonstrate how the process of independent assortment alters the variations in offspring genotypes

Answer 38

Independent assortment

During metaphase 1, the chromosomes line up in homologous pairs across the equator of the cell – each pair consists of one maternal and one paternal chromosomes and the way they line up is independent of the way the other pairs orient themselves. When the chromosomes pairs separate and move to opposite poles in anaphase 1, the original maternal and paternal chromosomes are distributed randomly to the gametes instead of as a predefined set from either parent. This process of called independent assortment.

Ensures that each resultant haploid cell contains a mixture of genes from the organism’s mother and father. As the homologous pairs carry different genetic information, independent assortment increases the number of different combinations of genes carried by the gametes.

The number of different combinations of chromosomes can be calculated as n^2, where n is the haploid number of the organism.

Question 39

demonstrate how random fertilisation alters the variation of an offspring’s genotype

Answer 39

Random fertilisation

Increases the possible combinations of alleles inherited because any egg that contains just one more than 8 million possible chromosomes combinations, can be fertilised by sperm, which also contains just one of more than 8 million combinations. Therefore meiosis and fertilisation shuffle alleles into different combinations in each individual from one generation to the next. This greatly increases the potential for variation in the genotypes of the offspring.

Question 40

define genome

Answer 40

all the genetic material in the chromosomes of an organism, including its genes and DNA sequences

Question 41

define exon

Answer 41

genes contain exons, which are codons that code for DNA

Question 42

define non-coding DNA

Answer 42

section of the gene that does not code for DNA but

includes a variety of transcribed proteins such as functional RNA (i.e tRNA), centromeres, telomeres and introns

Question 43

explain the central dogma of molecular biology

Answer 43

Is used to describe the one-way sequence of information transfer where:

DNA acts as template for its own replication, and also as a template for the production mRNA.

The newly formed mRNA is modified to become mature mRNA, before mature mRNA moves out the cytoplasm to act as a template for the compilation of amino acids into a polypeptide by a ribosome.

There are examples to show that this one-way sequence of info is not true

Retroviruses (a group of viruses) uses the enzyme reverse transcriptase to produce DNA from the RNA genome. (EG: HIV)

Genome results in polypeptide production, chemical modification of DNA can affect the expression of genes; genes work together in networks.

Question 44

define mRNA

Answer 44

mRNA (messenger RNA) - a ribonucleic acid molecule formed in the nucleus during gene transcription; has nitrogenous base sequence complementary to DNA template segment; it travels to cytoplasm where ribosomes attach.

Question 45

define ribosome

Answer 45

Ribosome – organelle where polypeptide synthesis occurs in all cells, locks onto mRNA molecule and moves along it to translate its code and link amino acids formed in nucleolus.

Question 46

what is the process of transcription

Answer 46

DNA in the region of a gene unwinds, and then unzips, exposing the nucleotides bases of both DNA strands. However only the template strand is used to direct the synthesis of mRNA. The other strand of DNA is called the non-template strand, or complementary strand.

A particular nucleotide sequence at the beginning of the unzipped section of DNA signals the start of a gene to be transcribed. Complementary RNA nucleotides are progressively joined together by RNA polymerase enzyme moving along the length of the DNA.

A base sequence at the end of the gene serves as the stop signal and the mRNA is released as a single strand. The DNA ‘zips up’ again and twists itself back into a double helix once the mRNA molecule has peeled off.

Question 47

what is transcription in protein synthesis

Answer 47

Transcription – the formation of an mRNA molecule against the template strand of DNA molecule in the nucleus by complementary nucleotide base pairing.

Question 48

describe how adjustments are made to a newly formed mRNA molecule

Answer 48

Adjustments to the newly formed mRNA molecule

The mRNA strand at this stage is called pre-mRNA, before it leaves the nucleus, it is modified by certain chemical addictions to its ends that ensure the stability of mRNA molecule as it moves out of the nucleus. Also some segments of the mRNA molecule, called introns, are removed. Both introns and exons are transcribed into pre-mRNA but introns are removed and exons joined back together in a process called splicing, before mature mRNA leaves the nucleus.

Introns – are regions of base sequences that do not code for polypeptide production and interspersed with regions of DNA called exons.

Exons - coding regions and do contain the information for polypeptide formation.

Question 49

how are different versions of proteins produced

Answer 49

Different versions of produced proteins

When exons from the same pre-mRNA are spliced together differently, alternative forms of the mature mRNA are created and are then translated into different versions of the polypeptides and thus, different final proteins.

Introns direct this alternative splicing.

In this way, particular tissue types in multicellular organisms contain uniquely different versions of proteins that have been encoded by the same gene. Alternative splicing such as occurs most of the time.

Question 50

define the process of translation

Answer 50

When mature mRNA moves from the nucleus into the cytoplasm, a ribosome attaches to it, and moves along until it comes to an AUG start codon. This start codon is the signal to begin the process of assembling amino acids into the polypeptide chain specific to the particular nucleotide sequence in the mRNA. - this process is translation.

Question 51

describe the process of translation in protein synthesis

Answer 51

Translation of mRNA into polypeptides using transfer RNA (tRNA)

Transfer RNA transfers or carries amino acids to ribosomes. TRNA molecules are folded back in themselves to form a compact 3-D structure.

Transfer RNA gas a 3 base nucleotide sequence called the anticodon on at end, which bonds to the complementary codon on the mRNA strand.

Each of the amino acids can bond with 1-4 tRNA molecules called amino acid binding sites. Its ability to bond with a particular tRNA is associated with which anticodon is present in that tRNA molecule.

When bonded, the amino acid at the binding site is in a position to be joined to the growing chain of amino acids by a peptide bond.

The ribosome continues moving along the mRNA strand, facilitating the tRNA delivery of an amino acid and its bonding to the growing chain until it comes to a stop codon, for which there is no corresponding tRNA molecule. At this point, the synthesis of the specific polypeptide molecule is complete.

Each tRNA molecule peels off from the mRNA and long with other tRNA molecule in the cytoplasm can subsequently pick up specific amino acids again and again when required. The mRNA strand is released from the ribosome and broken down by the cell, enabling the RNA nucleotides to be reused over and over again.

In this way the amino acids are linked in an order corresponding to the sequence of nucleotide base codon in the mRNA. As the mRNA base sequence is determined by the sequence of base condoms in the original DNA, it follows that the base sequence in the DNA determines the order in which amino acids line up.

A protein molecule is later formed from one or more polypeptide chains joined together to form a specific and often very intricate, 3D structure such as a haemoglobin.

Question 52

what are the stop and start codons

Answer 52

Note* - three codons UAA, UAG, UGA do not actually code for an amino acid, instead they act as a stop codon.

the codon, AUG is a start codon.

Question 53

what is the purpose of gene expression

Answer 53

to synthesis a functional gene produce (protein or functional RNA).

for example - Enzymes are vital functional proteins because they speed up every chemical reaction in every cell. Without enzymes, reactions would be slow as to hardly proceed at all, this would be incompatible with the maintenance of life itself.

Genes that code for a required time-specific or cell type specific proteins in a cell may be inactive, while genes coding for non-required proteins are inactive. This ensures the cell does not waste energy and resources producing unwanted proteins, as well as ensuring the cell does not produce proteins whose functions may interfere with the cell properly performing its specialised role.

Even when genes are expressed there are controls over how fast specific genes are transcribed and translated.

Whether genes are expressed seems to depend on the type of the cell, its stage of development and conditions within and around the cell.

Question 54

what is non-coding DNA used for

Answer 54

a considerable amount of non-coding DNA is made up of repetitive sequences of ribosomal RNA (rRNA) and transfer RNA (tRNA) both of which are necessary for protein synthesis.

Centromeres and telomeres are segments of non-coding DNA

Centromeres – hold duplicate chromosomes (sister chromatids) together, and are the side of attachment for spindle fibres to move chromosomes in cell division.

Telomeres – are extensions of DNA at the ends of chromosomes that act to prolong the life of chromosomes. At each replication of a DNA molecule, some of the repeating segments making up the telomeres are lost, but the coding sections are preserved.

Question 55

define gene regulation

Answer 55

Gene regulation – refers to the process within cells that enable a gene to be expressed just in particular cells and at certain specific times and rates. These capabilities are present in all known life on earth.

Question 56

define housekeeping genes

Answer 56

Housekeeping genes – a gene that encodes a polypeptide as part of a proteins (often an enzyme) required to maintain basic cellular processes and hence must be expressed continually.

EG: genes that express proteins for glycolysis or for biosynthesis of essential macromolecules are housekeeping genes.

Question 57

what are the two types of gene regulation

Answer 57

There are two different types of gene regulation:

Short term – ensures that all of the different varieties of specialised cells can carry out their regular everyday functions.

Long term – determines the development of the organism through all the different life stages, including the differentiation of specialised cells.

Question 58

describe chemical modification at the transcription stage

Answer 58

Chemical modification of chromatin at the transcription stage

DNA is packaged with histone proteins to form the DNA-protein complex (chromatin).

The most fundamental unit of chromatin is the nucleosome – in which the double helix is wound twice around 8 histone proteins that are arranged in a ball shape. Nucleosome form the scaffold for the packaging of DNA molecules.

Each of the histones has a tail that extends out from the nucleosome. When DNA is packaged and coiled into the chromatin, the genes within the DNA are not available for expression because they are wrapped and coiled tightly into a very condensed structure. RNA polymerase cannot access the DNA in the chromatin to begin transcription of the mRNA molecule from the DNA template strand. Therefore, such genes are ‘switched off.’

However, chromatin can be chemically remodelled to allow segments of DNA containing genes to become unwrapped and exposed so that transcription can occur. To allow segments of DNA containing genes to become unwrapped and exposed, so that transcription can occur. The addition of acetyl chemical groups, ‘histone acetylation’ to specific amino acids in the histone protein tails results in loosening of the association of the histones with DNA, so that transcription of the exposed DNA can occur.

The addition of methyl groups, ‘histone methylation’ results in gene inactivation, due to attraction between methylated histones and DNA that block transcription.

Question 59

give an example of how chemical modification of DNA at the transcription stage

Answer 59

Chemical modification of DNA at transcription stage

Methylation and acetylation can also occur directly to chromosol DNA (rather than to the histone protein tails) to either disable or stimulate the transcription of DNA to mRNA.

The addition of phosphate groups and small proteins also acts to regulate transcription in this way.

EG: DNA methylation is used to deactivate one of the two X chromosomes in the body cells of female mammals. Cats fur colour (orange and black) is carried on the X chromosome. During embryonic development the colour of the cats fur will depend on the X chromosome that is active.

Question 60

describe how gene expression can be influenced by products of other genes at transcription

Answer 60

Regulation of gene expression by products of other genes at transcription

Transcription can be influenced by the genome itself.

Regulatory proteins – are products of genes that regulate the expression of genes other than their own. Specific regulatory proteins that bind to the DNA are called transcription facts. Most of these activate gene expression but some repress it.

Activating proteins – bind to non-coding DNA genes and enable it to unwind from histone proteins. Genes near this unwound DNA can then be transcribed. Activators also assist the binding of RNA polymerase to coding DNA segments to begin their transcription to mRNA. They may bind to several different specialised segments of the DNA, and even to introns. Some activating transcription factors can turn on multiple genes at the same time.

Repressing proteins – may bind to particular region on the DNA and block RNA polymerase from attaching for transcription.

Another role of regulatory proteins is to interact with other proteins such as signalling molecules like hormones, that relay info about the physiological or developmental state of the organism.

Question 61

what is an example of an environmental influences at gene expression

Answer 61

Environmental influences of gene expression

Example:

Fur colour of Himalayan rabbit which as a white body that black ears, nose, feet, and tail. In heat, the rabbit fur is white but if a cold pack is placed on its fur for a long time, black fur will grow. This is because black fur only grows on the body where it is cool enough.

Question 62

what are influences on phenotypic expression of genes that can be passed to offspring

Answer 62

influences on phenotypic expression of genes that can be passed to offspring

Environmental factors such as arsenic, heavy metals and organic pollutants affect gene expression. As well as this, it is also thought that lifestyle, stress and even types of food consumed can influence gene expression and can lead to changes that can be passed to the following generation.

These epigenetic forms of chemical gene regulation can be passed on through gametes to offspring and are an alternative form of inheritance.

Epigenetics mechanisms – chemical modifications to gene function that are not due to a change in the DNA sequences.

DNA methylation
Histone modification
Non-coding RNA
Post-translational modifications

EG: agouti gene in a range of animals causes yellowish banding or striped on the hair is switched off by methylation.

Question 63

describe how proteins bonding to DNA can regulate gene expression at translation.

Answer 63

Regulation of gene expression at translation

After a gene is transcribed and processed into mRNA, the mRNA can be prevented from being translated into a polypeptide, so expression is switched off in that way. It can be achieved in a couple different ways:

MRNA binding proteins can attach to the mRNA and block ribosomes from being able to translate it. These proteins bind to specific non-coding RNA sequences near to the coding region, that can bind to DNA and block transcription.

Mirco RNA (miRNA) molecules are transcribed from non-coding DNA and processed into short segments of around 20 nucleotides in length – with these, which are complementary to mRNA, base pair with the mRNA sequences and form double stranded RNA which prevents translation.

Question 64

what are homeobox genes

Answer 64

Homeobox genes

A gene of a group that code for proteins that regulate body formation and patterning in the developing embryo.

This distinctive localised gene expression is accomplishes through specific proteins produced in the egg before fertilisation.

These proteins (called maternal effect proteins) diffuse across the egg, forming concentration gradients that convey positional information in the egg.

EG: zone of highest concentration of one of these proteins, called Bicoid protein will become the embryo’s anterior end and visa versa.

Other such proteins create concentration gradients that mark positions within the egg.

These protein concentration gradients are preserved during the early rounds of cell division in the zygote, after fertilisation, so that positional information is retained by later generations of daughter cells in the developing embryo. These maternal effect proteins interact with non-coding DNA regions immediately adjacent to the homeobox genes that initiate development of the front halves of the animal but repress homeobox genes that are required for development of animal’s rear body parts. Therefore, each homeobox gene is activated by specific combinations of positional proteins. The pattern expression of the homeobox genes ultimately reflects the concentrations of the positional proteins within the developing embryo as it undergoes cell division.

Once expressed - the product of each homeobox genes is a protein that binds DNA to activate gene expression. These proteins activate expression of collections of many other genes within the genome. The group of genes are expressed are all those required for the formation of the intended organ or body part at that position within the developing embryo. - therefore they have a wide raging effect.

Question 65

what is the role of the Y chromosome in sex determination

Answer 65

Sex determination: the role of the Y chromosome

At a certain stage of embryonic development, either male or female sexual characteristics are produced.

In mammals , the sex chromosomes present in every cell determines gender.

The Y chromosomes (for males) must have genes that set the organism on the path to developing into a male. The single most influential gene in this process is the SRY gene (sex-determining region of the Y chromosome) located on the small arm of the Y chromosome.

The gene encodes for a protein that primary launches the genetic program for testes development. As testes develop, they produce hormones such as testosterone that steer the embryo towards the differentiation of male features including the formation of the penis and other components of the male reproductive system.

If the individual lacks a Y chromosome and is unaffected by the SRY gene, they begin the course towards developing into a female. Proper female development relies on the presence of both X chromosomes to direct the formation of fully functional ovaries.

Question 66

describe how a mutation arises from a point mutation

Answer 66

Point mutations

Point mutations – is a mutation that affects a single base-pair position within a gene.

Point mutations are caused by the substitution of a nucleotide.

Single nucleotide polymorphisms (SNPs) - a nucleotide difference that occurs at one given position in the gene.

If a point mutation occurs in a gene coding for a polypeptide chain:

There can be no effect – as a substituted nucleotide in one triplet base sequence may still code for the same amino acid.

There can be an effect – if the substituted nucleotide in the triplet base sequence codes for a different amino acid, resulting in a different protein.

Question 67

what are DNA repair mechanisms

Answer 67

DNA repair mechanisms

During the cell cycle, after DNA has been replicated and before cell division occurs DNA is proofread and any errors that are detected are repaired.

Repair usually relies on one the DNA strands being intact which serves as a template for proofreading and restoration of the damaged complementary strand. However, if the mutation is not repaired or it is improperly repairs, the mutation becomes part of the DNA sequence and persists through subsequent cell divisions.

DNA repair mechanisms are usually highly effective so mutations are rare.

Question 68

describe mutations causing a frameshift

Answer 68

Mutations causing a frameshift

An insertion mutation occurs when one or more nucleotides are added at a site within the original gene sequence.

A deletion mutation is the loss of one or more nucleotides from a locus within the gene.

Collectively these can be referred to as ‘indels’, an abbreviation for ‘insertion’ and ‘deletion’.

The effect of the indel is a frameshift mutation – in which the starting point for the reading of the triplet codon sequence for the amino acids is shifted away from the original position.

As a result all codons are moved ‘downstream’ of the mutation are affected.

If there was an indel of three or multiple of three nucleotide, the original sequence of codons beyond the indel would be restored and there would not be a frameshift for translation of the remainder of the nucleotide sequence.

In the polypeptide produced with a frameshift mutation, the amino acid sequence beyond the location of the mutation bears no resemblance to those in the polypeptide that would have originally been produced.

Question 69

what is the damage mutagens can cause DNA

Answer 69

Damage to DNA by mutagens

Mutations arise spontaneously and are in no sense ‘directed’ by the environment.

Environmental influences can increase the risk of rate of mutation but they cannot induce a particular mutation of occur. This kind of genetic change is random.

Chemical mutagens:

Acridine orange – bases in DNA are added or removed.

Nitric acid – adenine in DNA is deaminated so it behaves like gaunine.

Physical mutations:

Ionising radiation (UV radiation) - affects the chemical structure of nitrogenous bases fusing adjacent thymine or cytosine bases in the DNA sequence.

X-rays cause the deletion of adenine and guanine bases, creating gaps in the double helix.

This disrupts the complementary base pairing during DNA replication causing mutations.

Nuclear radiation – is one physical mutagen that actually causes double-strand breaks in chromosomes. These kinds of anomalies result in large segments of the chromosome being rearranged. A cell can delay cell division to give it time to repair breaks and mutations before it divides, but an accumulation of double stranded breaks upon intense exposure to physical mutagens is lethal. Apoptosis of the cell in this situation is a mechanism to guard against cancer formation.

Question 70

define aneuploidy

Answer 70

Aneuploidy – a genome variant having unconventional chromosomal number due to loss or addition of one or a small number of chromosomes.

Question 71

describe how non-disjunction occurs

Answer 71

Occasionally the two members of the chromosome pair go into the same cell during anaphase 1 instead of separating. This is non-disjunction. It results in the formation of two types of gametes in equal proportions – one has two copies of a particular chromosome and the other type has none.

Non-disjunction can also occur in meiosis 2 when sister chromatids fail to segregate into separate cells.

Trisomy – when a normal gamete gives a zygote with three such chromosomes (3n) instead or a normal diploid zygote.

Monosomy – fusion of a gamete without a particular chromosome with a normal gamete with only one chromosome results in an individual with only one of this particular type of chromosome (1n) in each cell.

Question 72

what is an example of non-disjunction of autosomal chromosomes

Answer 72

Nondisjunction of autosomal chromosomes in meiosis

EG: down syndrome (trisomy 21) is caused by the presence of an extra chromosome 21 in every cell thus giving three copies of this chromosome.

Question 73

what is an example of non-disjunction of sex chromosomes

Answer 73

Non-disjunction of sex chromosomes in meiosis

EG: Klinefelter syndrome may result from either the fusion of a Y sperm with an XX egg or the fusion of a XY sperm with an X egg. Although XXY individuals are phenotypically men, they have very small genitals and are infertile, and develop breasts but testosterone therapy during puberty can help alleviate breasts.

EG: Turner syndrome is due to the absence of one of the sex chromosomes. Foetuses with 22 normal pairs of autosomes and a single Y chromosome never survive birth. However children can be born with 22 normal pairs of autosomes and a single X chromosome. The phenotypic effects are minor but the person is infertile, normally shorter, webbed neck,

Question 74

define neutral mutation

Answer 74

Neutral mutation – if the protein product is unchanged & does not affect survival. EG: ABCA1 gene codes for the protein involved in cholesterol transport, a substitution in a codon will not affect functioning of protein.

Question 75

how can mutations be passed on to the offspring

Answer 75

The effect of mutation depends on whether it has occurred in non-reproductive (somatic) which are produced by mitosis or in reproductive (germ-line) cells which are produced by meiosis.

Mutations in somatic cells: occurs only in that cell and its daughter cells – cancer.

Mutations in germ-line cells: can be passed onto next generation. Often mutations can cause the offspring to spontaneously be aborted but if carried through to birth, these mutations usually result in congenital disorders with varying severity. Occasionally the mutation will enhance the function of the encoded protein which may enhance chance of survival.

Question 76

what is polygenic inheritance

Answer 76

Polygenic inheritance – when one characteristic is controlled by two or more genes. Polygenic inheritance shows continuous variation (variation of traits caused by two or more genes the range of different phenotypes is wide with small, smooth gradations between differences).

EG: height is determined by many genes.

Discontinuous variation – occurs when only one gene is involves and results in a small number of phenotypes as in all the patterns of inheritance.

Question 77

explain gene cloning as a form of recombinant DNA technology

Answer 77

Recombinant DNA technology

Gene cloning – the process of using plasmids and bacteria to make numerous identical copies of a gene.

Cloned gene can be studied or used to genetically engineer animals.

Using bacterial plasmids as vectors to transport the gene of interest from an unrelated organism into bacterial cells is a common method of producing recombinant DNA.

Steps in making recombinant DNA:

Plasmids are extracted from bacteria by rupturing the cell membrane and cell walls. Similarly, the DNA of interest is isolated from the donor organism.

The restriction enzyme is used to cut the plasmid DNA and DNA of the gene of interest producing sticky ends.

The plasmid vectors and the gene of interest are mixed together and their ends pair.

DNA ligase joins the two segments to form recombinant plasmids.

The recombinant plasmids are added to bacterial culture, where they are taken up by a process called transformation. When the bacteria reproduce by dividing, the plasmid is also replicated. This generates numerous copies of the recombinant DNA.

Question 78

explain PCR

Answer 78

Amplifying DNA: PCR

PCR – a cyclical reaction in which DNA polymerase is used to copy a DNA template making copies of the same piece of DNA.

A thermal cycler is the machine used to carry out PCR which rapidly raises and lowers the temperature of the reaction. Taq polymerase is used (as the type of DNA polymerase) because it is resistant to high temperatures.

Denaturation – temp is increased (95 degrees), breaking the hydrogen bonds between nitrogen bases and thus causing two strands to separate.

Annealing – temp is decreased (50-60 degrees), allowing primers to anneal (join) to complementary sequences according the base paring rule. The reduced temp allows base paring and the formation of hydrogen bonds to occur.

Extension – the temp is increased (72 degrees), the optimum temperature for the DNA polymerase used in PCR. Starting from the primers new DNA strands are synthesised using DNA polymerase using free-floating nucleotides. At the end of this process – 2 new copies of the double stranded DNA are created.

Primers – a single-stranded DNA molecule that acts as the start of the amplification process (short, around 20 nucleotides) and are complementary to nucleotide sequences at either end of the DNA section that is to be copied. DNA polymerase can only extend a DNA strand from an existing nucleotide, it cannot create new complementary strand without primers to extend from.

Question 79

explain gel electrophoresis

Answer 79

Separating fragments of DNA by gel electrophoresis

Gel electrophoresis is used to separate fragments of DNA or other macromolecules such as RNA and proteins according to their size and charge.

DNA samples are loaded into well-like indentations within agarose gel. The gel is placed in a tray filled with a buffer solution, and positive and negative electrodes are attached at each end of the gel.

When the electric current runs the fragments move towards to the positive end (due to their negative charge) moving through the porous gel. Smaller fragments experience less resistance migrate through the cross-linked gel matrix faster and further than the large fragments.

Ethidium bromide or another florescent DNA –binding dye is added the fluoresces under UV light, showing a pattern of bands that are photographed.

To determine the size of particular fragments molecular size markers (pieces of DNA that have a known number of base pairs) are used to compare the size of the samples against a known base pair size.

Question 80

explain DNA sequencing

Answer 80

DNA sequencing:

Can identify the exact nucleotide sequences of sections of DNA – helps understand that many genetic diseases result from simple mutations, such as single base substitutions or deletions. DNA

The four nucleotides are labelled with four different coloured fluorescent dyes. As electrophoresis proceeds a laser cans across the bottom of the gel, detecting the different dyes and consequently the base sequence. A computer then analyses the info from different gel to read the base sequence.

Can be used to identify the type of cancer, risk of cancer – female carrier of one or more mutations in breast cancer susceptibility gene 1 are at high risk of developing ovarian cancer. The proteins encoded but these genes play a role in repairing double strand breaks – blood tests can analyse nucleotides of this gene, compared with an international database and assessed.

Question 81

explain DNA profiling

Answer 81

DNA profiling:

A process used to compare base sequences of two or more individuals to determine how similar they are. Used to explore relatedness between people and if biological samples can match a crime scene.

Short tandem repeats are sections of non-coding DNA that are repeated many times. DNA profiling identifies people according to the differences in the length of their DNA repeats. One person normally has two alleles for each STR, one from each homologous chromosome. Each person has their own number of STR’s – this forms the basis of identification.