Chapter 2: Processes for the Continuity of Life

Question 1

mitosis and meiosis similarities

Answer 1

types of cell division
begin with diploid cell
include the substages prophase, metaphase, anaphase and telophase
dna replication occurs during interphase before mitosis or meiosis begins

Question 2

mitosis and meiosis differences

Answer 2

mitosis has identical daughter cells, one division, produces somatic cells

meiosis has genetically unique daughter cells, two divisions, produces sex cells

Question 3

what is meiosis

Answer 3

meiosis is a type off cell divison where 1 parent cell divides twice to produce 4 non-identical daughter cells.

Question 4

briefly outline the steps of meiosis I

Answer 4

parent cell -> meiosis I -> DNA replicated -> chromatin shortens and thickens to form chromosomes -> chromosomes line up in homologous pairs (random segregation) -> crossing over occurs (homologous chromosomes swap sections) -> spindle fibres attach, shorten, and homologous chromosomes break move apart

Question 5

briefly outline the steps of meiosis II

Answer 5

parent cell -> meiosis I -> meiosis II -> chromosomes line up individually -> spindle fibres attach to centromeres -> spindle fibres shorten, centromeres break apart and chromatids move apart -> cytoplasm separates, cell and nuclear membranes form, chromosomes uncoil -> four non-identical daughter cells.

Question 6

how many daughter cells produced after one round of meiosis?

Answer 6

One round of meiosis in one parent cell produces four daughter cells.

Question 7

what are Homologous chromosomes?

Answer 7

Homologous chromosomes are matching pairs of chromosomes: so chromosomes that carry the same genes, and so are about the same length. Although they carry the same genes, they may have different alleles of each gene.

Question 8

what must happen before a cell undergos meiosis

Answer 8

replication of its DNA and organelles

Question 9

define genetic variation

Answer 9

genetic variation is the variation in the genome of individuals of the same species.

Question 10

define independant assortment

Answer 10

pairs of homologous chromosomes arrange independently to each other.

Question 11

what is the result of indeneptent assortment

Answer 11

as a result, when the chromatids segregate (split), the set of chromosomes in the daughter cells is random
different combinations = genetic variation

Question 12

define crossing over

Answer 12

when two homologous chromosomes swap sections to produce new gene combinations

Question 13

why does crossing over contribute to genetic variation

Answer 13

crossing over contributes to genetic variation because it changes the genetic composition of the chromosomes.

Question 14

why does fertilisation contribute to genetic varaition

Answer 14

becasue it allwos for different gametes (containing different alleles) to combine. there are lots of different individuals within a species, which all have tons of different gametes (due to meiosis). any of these gametes can combine during fertilisation to form different zygotes.

Question 15

Describe the changes that occur to DNA during mitosis.

Answer 15

During mitosis, DNA undergoes a number of changes:

It condenses from chromatin into tightly wound chromosomes for seperation
It aligns along the equator of the cell for division and attaches to spindle fibres
The sister chromatids are separated and pulled to opposite sides of the cell by the spindle fibres, dividing the replicated DNA in half

Question 16

Explain how the segregation of chromosomes during meiosis leads to a wide variety of gametes being produced.

Answer 16

In meiosis, there is a reduction division. Chromosome pairs line up together and one of each pair goes into the daughter cells. For each pair, this process is independent – hence a large number of combinations is possible.

Question 17

Scientists have tried to achieve a viable embryo by fusing two ova (eggs) from the same female.

Explain whether the offspring produced using this process would be a clone of the female whose two ova were used. Use your knowledge of gamete formation and sexual reproduction to support your answer.

Answer 17

The offspring would not be a clone of the female because the offspring and the female would not be genetically identical.

The offspring was produced using two ova. The nuclei of the two ova are different to each other and to the nuclei in the female’s body cells. The ova were produced by meiosis of a body cell of the female.

During meiosis the processes of independent assortment of chromosomes and crossing over occur. In crossing over, segments of DNA swap positions on homologous chromosomes so new gene combinations are possible. Independent assortment results in different sorting of chromatids into ova. This results in genetically unique ova.

If these two genetically unique ova fuse there will be a new unique combination of genetic information in the offspring that is not identical to that of the female who produced the ova.

Question 18

Describe how crossing over and independent assortment create genetic variation.

Answer 18

Crossing over is the exchange of alleles between homologous chromosomes during the first stage of meiosis. This produces gametes with unique combinations of alleles.

Independent assortment describes how pairs of homologous chromosomes arrange and split independently of each other during the first stage of meiosis. This gives random combinations of maternal and paternal chromosomes in the gametes.

Therefore, both crossing over and independent assortment contribute to genetic variation.

Question 19

define binary fission

Answer 19

in binary fission, a parent cell divides once to produce two identical daughter cells. DNA replication and segregation occur at the same time. binary fission is a form of asexual reproduction, dividing without mixing of genetic material.

Question 20

how many steps involved in binary fission

Answer 20

Only one step is involved in the process of binary fission! This is because the DNA replicates and segregates at the same time, while the cytoplasm is in the process of dividing.

Question 21

name and describe the process by which a bacteria cell reproduces

Answer 21

a bacterial cell divides through binary fission where:
- the cell’s genetic material is replicated
- as genetic material is replicating, each copy moves towards opposite ends of the cell
- as it’s moving, the cell splits in two identical daughter cells through cytokinesis.

Question 22

Identify the type of cellular reproduction used in eukaryotic and prokaryotic cells.

Answer 22

Eukaryotes: mitosis
Prokaryotes: binary fission

Question 23

Bacteria proliferate exponentially, via asexual reproduction.

Outline the process behind this.

Answer 23

Binary fission is the process by which bacteria reproduce. DNA replication and segregation occur at the same time, such that it’s a single-step process. The bacterial cell then divides, producing two identical bacterial cells. Binary fission only occurs in prokaryotes; and since all prokaryotes are unicellular, it acts as their way of performing asexual reproduction.

Question 24

With reference to binary fission, contrast sexual and asexual reproduction.

Answer 24

Sexual reproduction involves the production of genetically unique offspring from two parents. Conversely, asexual reproduction involves the production of offspring that are genetically identical to the parent and each other. An example of asexual reproduction is binary fission. This is the process by which bacteria reproduce, as one bacterial cell replicates its DNA and divides into two identical cells.

Question 25

Define a protein

Answer 25

a protein is a block of amino acids that are joined via peptide bonds

Question 26

Functions of proteins

Answer 26

structural proteins = maintain cell shape
enzymes = act as biological catalysts
can act has hormones
can be antibodies for immunity

Question 27

Contrast enzymes and structural proteins, including an example of each.

Answer 27

Enzymes are proteins that act as biological catalysts, facilitating a reaction with substrate molecules. An example of an enzyme is sucrase, which breaks down sucrose into fructose and glucose. Structural proteins provide cells and tissues with its structural properties. An example of this is collagen, which provides skin and bone with its flexibility. Unlike enzymes, structural proteins do not have metabolic function.

Question 28

Describe the synthesis of an enzyme

Answer 28

Transcription
Initially, the double-stranded DNA unzips and a promoter molecule binds to the start of the gene of the enzyme. RNA polymerase binds to the promoter, allowing transcription of the gene. RNA polymerase uses free nucleotides in the nucleus to synthesise a complementary template of the gene known as mRNA. However, it is single-stranded and the thymine nucleotide is replaced by uracil. In eukaryotes, the mRNA is then modified.

Translation
The mRNA then leaves the nucleus, entering the cytoplasm, where ribosomes bind to it. Each set of three nucleotides on the mRNA is a codon. tRNA with the complementary anticodon bind to the mRNA in the ribosome. Each tRNA has a specific amino acid attached to it, that corresponds to its anticodon. The ribosome forms a peptide bond between this amino acid and the existing polypeptide chain. As such, the sequence of amino acids added to the growing polypeptide chain corresponds to the sequence of codons on the mRNA. A specific codon on the mRNA signals the translation to stop, completing the polypeptide enzyme.

Question 29

Describe the role of tRNA in protein synthesis.

Answer 29

tRNA molecules consist of nucleic acid, and they have an anticodon and an amino acid binding region. The amino acid binding region is specific to only one type of amino acid.

It binds to its amino acid in the cytoplasm. Then, the anticodon recognises and binds to its complementary codon sequence on the mRNA as it is being translated at the ribosome. The ribosome attaches the amino acid to the growing polypeptide chain, in accordance with the mRNA sequence.

Question 30

Compare proteins and polypeptides.

Answer 30

Both proteins and polypeptides are composed of amino acids. Polypeptides, however, are single amino acid chains, whereas proteins are made up of one or more polypeptides.

Question 31

Describe the roles of two types of RNA in protein synthesis.

Answer 31

Two types of RNA are mRNA and tRNA. The code contained in the DNA molecule is copied into an mRNA molecule. The mRNA molecule is then able to move out of the nucleus and into the cytoplasm where it binds with a ribosome. In the ribosome, tRNA (which carries specific amino acids) interacts with the mRNA by binding with the complementary triplet codon on the mRNA molecule. In this way, successive amino acids bind to each other to produce a polypeptide.

Question 32

Genes influence proteins and proteins influence genes.

Evaluate this statement with reference to the structure and function of genes and proteins.

Answer 32

Genes are lengths of DNA. The building blocks of DNA are four different types of nucleotides with the bases adenine, thymine, cytosine and guanine. Within the DNA triplets of bases form codons that represent the genetic code for the production of a polypeptide. Genes provide the template for the production of mRNA which carries a copy of the genetic information to the ribosomes.

Ribosomes translate the mRNA codons into a chain of amino acids called a polypeptide. This polypeptide folds into a specific shape, depending on the amino acids present in the chain. Several polypeptide chains may combine to form a larger protein structure such as haemoglobin.

If the nucleotide sequence of a gene changes this may or may not change the polypeptide/protein that it codes for as there are various codons that code for each amino acid.

The sequence and types of amino acids determine the function of a protein. This is because the amino acids determine the shape and the chemical properties of a protein. Both the shape and the chemical properties of proteins determine their function. For example, enzymes are globular proteins with an active site that has specific physical and chemical properties determined by the amino acids present.

Proteins can interact with DNA and promote or inhibit the expression of genes. In this way, a protein produced as a result of a gene may influence the transcription of other genes. One example of this is a gene cascade where the protein product of a gene results in a cascade of effects on a series of other genes that control the development of a limb in mammals.

While proteins have some influence on genes, genes are a more powerful influence on proteins and therefore evolution.

Question 33

describe the order of amino acids

Answer 33

the order of animo acids in each protein follows a sequence which is coded for by its gene

Question 34

define gene

Answer 34

a section of DNA that contains instructions for making an RNA molecule or a protein.

Question 35

what are telomeres

Answer 35

telomeres are the structural components of DNA found at the ends of chromosomes.

Question 36

describe functional RNA

Answer 36

functional RNA includes any RNA that isn’t mRNA. e.g. tRNA is used to carry amino acids to the ribosome during protein synthesis, & rRNA which plays an important role in the ribosome.

Question 37

are introns transcribed?

Answer 37

these sections of DNA are transcribed but are removed from mRNA before translation. introns are removed from DNA by splicing.

Question 38

define coding-DNA

Answer 38

coding dna is dna that is transcribed and then translated into proteins

Question 39

define non-coding DNA

Answer 39

Non-coding DNA is DNA that does not code for proteins.

Question 40

Distinguish between coding and non-coding DNA.

Answer 40

DNA is found in the nucleus of all cells, where its nucleotide sequence controls the survival, growth and reproduction of the organism. Coding DNA is DNA that is transcribed and translated into amino acids. Non-coding DNA is DNA that is not translated into mRNA, so it does not code for a protein.

Question 41

Identify three types of non-coding DNA, and whether or not they are transcribed.

Answer 41

Structural DNA, not transcribed
Functional RNA, transcribed
Introns, transcribed
Telomeres, not transcribed
Centromeres, not transcribed

Question 42

Compare introns and exons.

Answer 42

Both introns and exons are nucleotide sequence regions in genes
Both introns and exons in a gene are transcribed into mRNA
Introns are spliced out and removed from the mRNA during modification, the exons remain
Exons leave the nucleus and are translated into a polypeptide sequence, introns are not

Question 43

Explain the role of introns and exons in protein synthesis.

Answer 43

Introns and exons are terms given to specific regions within a gene. During protein synthesis, both introns and exons are transcribed into mRNA. The mRNA is then modified by splicing, in which the intron regions are cut out of the mRNA. The exon regions join together, and the modified mRNA leaves the nucleus. In the cytoplasm, the mRNA is translated into a polypeptide. Therefore, in protein synthesis both introns and exons are transcribed, but only the exons are translated.

Question 44

Not all DNA codes for proteins, some of it is “junk DNA”.

Evaluate this statement, using examples.

Answer 44

This statement is correct in saying “not all DNA encodes for proteins”. DNA that encodes for proteins is coding DNA. DNA that doesn’t encode for proteins is known as non-coding DNA. However, it is incorrect to call non-coding DNA “junk DNA”. Non-coding DNA has numerous other cellular functions. An example of this is the DNA that codes for functional RNA, which is RNA that has its own role and isn’t translated into a polypeptide, like tRNA which is involved in translation during protein synthesis. Another example is structural DNA, like centromeres and telomeres. These are regions of DNA that are not transcribed, but have a role in maintaining chromosomal structure.

Question 45

define genotype

Answer 45

genotype refers to the combination of alleles in an organism’s DNA which code for a trait.

Question 46

how can mutations that affect of offsping be inherited

Answer 46

1. the parent of the offspring would have a germ-line mutation, i.e., a. mutation in a sperm or egg cell
2. this would create a new inheritable allele (variation of a gene)
3. if the sperm of egg cell was fertilised, a zygote containing the mutated gene would be formed, causing the mutation to be present in the genotype of the offspring.

Question 47

Using examples, explain how genes and the environment affect phenotypic expression.

Answer 47

Genes are segments of DNA that code for a particular characteristic. When genes are expressed through protein synthesis, the phenotype of the organism is affected. For example, an individual who has a blue-eyed phenotype is homozygous for the blue-eyed gene, and therefore produces proteins that contribute to having blue eyes.

Features of the environment such as temperature and the availability of nutrients can alter the expression of genes and therefore phenotype. For example, temperature affects coat color in Siamese cats. This is because the dark patches on Siamese cats are caused by a gene that codes for a temperature-sensitive enzyme.

Question 48

Deforestation destroys the natural habitats of many species, leaving them in unnatural surroundings. This often results in the decline of native species.

Describe how genotype and phenotype play a role in the survival of these species.

Answer 48

Genotype refers to the set of genes an organism inherits from its parents. Phenotype refers to the observable characteristics of an organism (how it develops, looks, reacts and behaves) and is often the result of its genotype and exposure to environmental factors. The genotypes and phenotypes of a species have typically evolved to suit their native habitats. Destruction of these habitats, such as the removal of trees by deforestation, will mean the native species’ genotypes and phenotypes will no longer be suited to their surroundings, dramatically reducing their likelihood of survival. If individuals in the population exist with genotypes and phenotypes suitable to the new environment, they will be naturally selected and the species will evolve.

Question 49

mitosis

Answer 49

DNA replication and segregation occurs in
separate steps
Occurs in eukaryotes
Far more complex
* Have lots of DNA and
organelles to coordinate
Steps in mitosis are more
distinct to ensure correct
division

Question 49

binary fission

Answer 49

DNA replicates and segregates at the same
time, while the cytoplasm divides
Occurs in prokaryotes
Much simpler
Able to split in one go

Question 49

which groups of animals are endotherms

Answer 49

birds and mammals